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19-3017; Rev 0; 10/03
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
General Description
The MAX8553 is a 4.5V to 28V input-voltage, synchronous step-down controller that provides a complete power-management solution for DDR memory. The MAX8553 generates 1/2 VREFIN voltage for VTT and VTTR. The VTT and VTTR tracking voltages are maintained within 1% of 1/2 VREFIN. The MAX8554 is a 4.5V to 28V input voltage, nontracking step-down controller with a low 0.6V feedback threshold voltage. The MAX8553/ MAX8554 use Maxim's proprietary Quick-PWMTM architecture for fast transient response and operate with selectable pseudo-fixed frequencies. Both controllers can operate without an external bias supply. The controllers operate in synchronous-rectification mode to ensure balanced current sourcing and sinking capability of up to 25A. The MAX8553/MAX8554 also provide up to 95% efficiency, making them ideal for server and pointof-load applications. Additionally, a low 5A shutdown current allows for longer battery life in notebook applications. Lossless current monitoring is achieved by monitoring the low-side MOSFET's drain-to-source voltage. The MAX8553/MAX8554 have an adjustable foldback current limit to withstand a continuous output overload and short circuit. Digital soft-start provides control of inrush current during power-up. Overvoltage protection shuts the converter down and discharges the output capacitor. The MAX8553/MAX8554 come in space-saving 16-pin QSOP packages. o Up to 25A Output-Current Capability o Quick-PWM Control for Fast Loop Response o Up to 95% Efficiency o 4.5V to 28V Input Voltage Range o No External Bias Supply Required o 0 to 3.6V Input REFIN Range (MAX8553) o Automatically Sets VTT and VTTR to within 1% of 1/2 VREFIN- (MAX8553) o Low 0.6V Feedback Threshold (MAX8554) o 200kHz/300kHz/400kHz/550kHz Selectable Switching Frequencies o Adjustable Foldback Current Limit o Overvoltage Protection o Digital Soft-Start
Features
MAX8553/MAX8554
Typical Operating Circuit
VIN +5V EN/HSD DH
Applications
Wide-Input Power Supplies Servers and Storage Applications ASIC and CPU Core Voltages Notebook and LCD-PC Power Supplies DDR I and DDR II Memory Power Supplies AGTL Bus Termination Supplies
POK POK REFIN
MAX8553
VL
REFIN V+
BST
VOUT
V+
LX
REF
DL
Ordering Information
PART MAX8553EEE MAX8554EEE TEMP RANGE -40C to +85C -40C to +85C PIN-PACKAGE 16 QSOP 16 QSOP
ILIM FSEL GND
PGND VTT VTTR VTTR
Pin Configurations appear at end of data sheet. Quick-PWM is a trademark of Maxim Integrated Products, Inc. ________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
ABSOLUTE MAXIMUM RATINGS
V+, EN/HSD, EN, HSD to GND...............................-0.3V to +30V PGND to GND .......................................................-0.3V to +0.3V VTT, REFIN, POK, OUT, FB, VL to GND...................-0.3V to +6V REF, VTTR, DL, ILIM, FSEL to GND ............-0.3V to (VVL + 0.3V) LX to PGND ...............................................................-2V to +30V BST to GND ............................................................-0.3V to +36V DH to LX ...................................................................-0.3V to +6V LX to BST..................................................................-6V to +0.3V REF Short Circuit to GND ...........................................Continuous Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derated 8.3mW/C above +70C) ........667mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VV+ = VHSD = +12V, VEN/HSD = VREFIN = +2.5V, VEN = +5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0V, ILIM = VL, PGND = LX = GND, BST = VL, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise specified.)
PARAMETER V+ Input Voltage Range V+ Input Voltage Range EN/HSD Input Voltage Range EN Input Voltage Range EN Input Current HSD Input Voltage Range HSD Input Current REFIN Input Voltage Range V+ Supply Current (MAX8553) V+ Supply Current (MAX8554) REFIN Supply Current EN/HSD Supply Current VL Supply Current V+ Shutdown Supply Current REFIN Shutdown Supply Current VL Shutdown Supply Current VL Undervoltage-Lockout Threshold VTT VTT Input Bias Current VTT Feedback Voltage Range VTT Feedback Voltage Accuracy FB Input Bias Current VREFIN = VEN/HSD = +1.8V VREFIN = VEN/HSD = +3.6V MAX8554, VFB = +600mV VVTT = +1.25V -0.15 0 49.5 49.5 -0.15 50 50 0 1.8 50.5 50.5 0 A V % VREFIN A VVL = VV+ = 5.5V, VVTT = +1.35V EN/HSD = GND EN/HSD = GND VVL = VV+ = +5.5V, VEN/HSD = 0V Rising edge, typical hysteresis = 40mV 4.05 5 4.25 VVTT = +1.35V VFB = 630mV 0 0.8 0.62 125 5 0.8 3 MAX8554 enabled 1.5 20 VL connected to V+ MAX8553 enabled MAX8554 enabled CONDITIONS VL not connected to V+ MIN 6 4.5 1.5 1.5 2 TYP MAX 28 5.5 28.0 28.0 3 28.0 40 3.6 1.2 0.90 250 10 1.2 5 1 12 4.40 UNITS V V V V A V A V mA mA A A mA A A A V
2
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
ELECTRICAL CHARACTERISTICS (continued)
(VV+ = VHSD = +12V, VEN/HSD = VREFIN = +2.5V, VEN = +5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0V, ILIM = VL, PGND = LX = GND, BST = VL, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise specified.)
PARAMETER FB Regulation Voltage Output Adjust Range VTT Line Regulation FB Line Regulation VTT Load Regulation FB Load Regulation REFERENCE Reference Output Voltage Reference Load Regulation Reference UVLO VTTR VTTR Output Voltage Range IVTTR = -5mA to +5mA VTTR Output Accuracy Thermal Shutdown SOFT-START ILIM Ramp Period Output Predischarge Period OSCILLATOR FSEL = VL Oscillator Frequency FSEL = unconnected FSEL = REF FSEL = GND FSEL = VL On-Time MAX8553, VVTT = +1.25V (Note 2) FSEL unconnected FSEL = REF FSEL = GND FSEL = VL On-Time MAX8554, VOUT = +2.5V (Note 2) (Note 2) FSEL unconnected FSEL = REF FSEL = GND Off-Time 2.18 1.45 1.09 0.82 0.89 0.61 0.43 0.33 200 300 400 550 2.5 1.67 1.25 0.91 1.02 0.71 0.49 0.37 350 2.83 1.89 1.41 1.00 1.16 0.80 0.56 0.41 400 ns s s kHz Ramps the ILIM trip threshold from 20% to 100% in 20% increments Rising edge of EN/HSD to the start of internal digital soft-start 0.8 0.8 1.7 1.7 3.0 3.0 ms ms IVTTR = -25mA to +25mA, VREFIN = +1.8V IVTTR = -25mA to +25mA, VREFIN = +3.6V Rising temperature, typical hysteresis = 15C 0 49.5 49 49.5 50 50 50 +160 1.8 50.5 51 50.5 C V % VREFIN VV+ = VVL = +4.5 to +5.5V, IREF = 0 VV+ = VVL = +5V, IREF = 0 to 50A VV+ = VVL = +5V, reference rising, hysteresis = 27mV 1.5 1.6 1.97 2.00 2.03 10 1.7 V mV V MAX8554 (Note 1) VEN/HSD 10%, VVTT = +1.25V, IOUT = 0A MAX8554, VHSD 10%, VOUT = +2.5V, IOUT = 0A, FSEL unconnected 0 < IOUT < +7A, VVTT = +1.25V MAX8554, 0 < IOUT < +7A, VOUT = +2.5V, FSEL unconnected CONDITIONS MAX8554, VOUT = +2.5V, FSEL unconnected MIN 0.598 0.6 0.325 0.325 0.2 0.2 TYP 0.607 MAX 0.616 3.5 UNITS V V % % % %
MAX8553/MAX8554
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3
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
ELECTRICAL CHARACTERISTICS (continued)
(VV+ = VHSD = +12V, VEN/HSD = VREFIN = +2.5V, VEN = +5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0V, ILIM = VL, PGND = LX = GND, BST = VL, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise specified.)
PARAMETER CURRENT LIMIT LX to PGND, ILIM = VL Current-Limit Threshold (Positive Direction) Current-Limit Threshold (Negative Direction) ILIM Input Current FAULT DETECTION MAX8553 (VREFIN > +1V) Overvoltage Threshold VL REGULATOR Output Voltage Line Regulation RMS Output Current Bypass Capacitor DRIVER DH Gate-Driver On-Resistance DL Gate-Driver On-Resistance (Source) DL Gate-Driver On-Resistance (Sink) Dead Time FSEL LOGIC Logic Input Current Logic Low (GND) Logic REF Level Logic Float Level Logic VL Level EN/HSD OR EN LOGIC EN/HSD or EN Shutdown Current Logic High Logic Low Max IEN/HSD for VEN/HSD < +0.8V or VEN < +0.8V VVL = VV+ = +4.5 to +5.5V, 100mV hysteresis VVL = VV+ = +4.5 to +5.5V 0.5 1.5 0.8 3.0 A V V FSEL = REF FSEL unconnected FSEL = VL 1.65 3.15 VVL - 0.4 -3 +3 0.5 2.35 3.85 A V V V V VBST - VLX = +5V DL high state DL low state DL rising DL falling 1.4 1.6 0.75 32 30 2.5 3.0 1.25 ns ESR < 100m 2.2 +6V < VV+ <+28V, 1mA < IVL < 35mA +6V < VV+ < +28V, IVL = 10mA 4.80 5.0 0.2 35 5.33 V % mA F MAX8553 (VREFIN +1V) MAX8554 57 0.576 0.696 60 0.600 0.720 63 0.624 0.744 % VREFIN V LX to PGND, RILIM = 100k LX to PGND, RILIM = 400k LX to PGND, ILIM = VL, with respect to positive current-limit threshold 80 35 160 -130 100 50 200 -110 5 115 65 230 -90 % A mV CONDITIONS MIN TYP MAX UNITS
4
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
ELECTRICAL CHARACTERISTICS (continued)
(VV+ = VHSD = +12V, VEN/HSD = VREFIN = +2.5V, VEN = +5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0V, ILIM = VL, PGND = LX = GND, BST = VL, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise specified.)
PARAMETER POWER-OK OUTPUT Upper VTT and VTTR Threshold Lower VTT and VTTR Threshold Upper Threshold Lower Threshold POK Output Low Level POK Output High Leakage MAX8553 MAX8553 MAX8554 MAX8554 ISINK = 2mA VPOK = +5V 55 43 0.646 0.504 56 44 0.672 0.528 57 45 0.698 0.552 0.4 5 % VREFIN % VREFIN V V V A CONDITIONS MIN TYP MAX UNITS
MAX8553/MAX8554
ELECTRICAL CHARACTERISTICS
(VV+ = VHSD = +12V, VEN/HSD = VREFIN = +2.5V, VEN = +5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0, ILIM = VL, PGND = LX = POK = GND, BST = VL, TA = -40C to +85C, unless otherwise specified.) (Note 3)
PARAMETER V+ Input Voltage Range V+ Input Voltage Range EN/HSD Input Voltage Range EN Input Voltage Range EN Input Current HSD Input Voltage Range HSD Input Current REFIN Input Voltage Range V+ Supply Current (MAX8553) V+ Supply Current (MAX8554) REFIN Supply Current EN/HSD Supply Current VL Supply Current V+ Shutdown Supply Current REFIN Shutdown Supply Current VL Shutdown Supply Current VL Undervoltage-Lockout Threshold VTT VTT Input Bias Current VTT Feedback Voltage Range VTT Feedback Voltage Accuracy FB Input Bias Current FB Regulation Voltage Output Adjust Range VREFIN = VEN/HSD = +1.8V VREFIN = VEN/HSD = +3.6V MAX8554, VFB = +600mV MAX8554, VOUT = +2.5V, FSEL unconnected MAX8554 (Note 1) VVTT = +1.25V -0.2 0 49.5 49.5 -0.2 0.598 0.6 0 1.8 50.5 50.5 0 0.616 3.5 A V % VREFIN A V V VVL = VV+ = 5.5V, VVTT = +1.35V EN/HSD = GND EN/HSD = GND VVL = VV+ = +5.5V, VEN/HSD = 0V Rising edge, typical hysteresis = 40mV 4.05 VVTT = +1.35V VFB = 630mV 0 MAX8554 enabled 1.5 VL connected to V+ MAX8553 enabled MAX8554 enabled CONDITIONS VL not connected to V+ MIN 6 4.5 1.5 1.5 MAX 28 5.5 28.0 28.0 3 28.0 40 3.6 1.2 0.90 250 10 1.2 5 1 12 4.40 UNITS V V V V A V A V mA mA A A mA A A A V
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5
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
ELECTRICAL CHARACTERISTICS (continued)
(VV+ = VHSD = +12V, VEN/HSD = VREFIN = +2.5V, VEN = +5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0, ILIM = VL, PGND = LX = POK = GND, BST = VL, TA = -40C to +85C, unless otherwise specified.) (Note 3)
PARAMETER REFERENCE Reference Output Voltage Reference Load Regulation Reference UVLO VTTR VTTR Output Voltage Range IVTTR = -5mA to +5mA VTTR Output Accuracy SOFT-START ILIM Ramp Period Output Predischarge Period OSCILLATOR FSEL = VL On-Time MAX8553, VVTT = +1.25V (Note 2) FSEL unconnected FSEL = REF FSEL = GND FSEL = VL On-Time MAX8554, VOUT = +2.5V (Note 2) (Note 2) LX to PGND, ILIM = VL Current-Limit Threshold (Positive Direction) Current-Limit Threshold (Negative Direction) ILIM Input Current FAULT DETECTION MAX8553 (VREFIN > +1V) Overvoltage Threshold VL REGULATOR Output Voltage RMS Output Current Bypass Capacitor ESR < 100m 2.2 +6V < VV+ < +28V, 1mA < IVL < 35mA 4.80 5.33 35 V mA F MAX8553 (VREFIN +1V) MAX8554 57 0.576 0.696 63 0.624 0.744 % V LX to PGND, RILIM = 100k LX to PGND, RILIM = 400k LX to PGND, ILIM = VL, with respect to positive current-limit threshold 80 30 150 -130 FSEL unconnected FSEL = REF FSEL = GND Off-Time CURRENT LIMIT 115 65 230 -90 % A mV 2.18 1.45 1.09 0.82 0.89 0.61 0.43 0.33 2.83 1.89 1.41 1.00 1.16 0.80 0.56 0.41 420 ns s s Ramps the ILIM trip threshold from 20% to 100% in 20% increments Rising edge of EN/HSD to the start of internal digital soft-start 0.8 0.8 3.0 3.0 ms ms IVTTR = -25mA to +25mA, VREFIN = +1.8V IVTTR = -25mA to +25mA, VREFIN = +3.6V 0 49.5 49 49.5 1.8 50.5 51 50.5 % VREFIN V VV+ = VVL = +4.5 to +5.5V, IREF = 0 VV+ = VVL = +5V, IREF = 0 to 50A VV+ = VVL = +5V, reference rising, hysteresis = 27mV 1.5 1.97 2.03 10 1.7 V mV V CONDITIONS MIN MAX UNITS
6
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
ELECTRICAL CHARACTERISTICS (continued)
(VV+ = VHSD = +12V, VEN/HSD = VREFIN = +2.5V, VEN = +5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0, ILIM = VL, PGND = LX = POK = GND, BST = VL, TA = -40C to +85C, unless otherwise specified.) (Note 3)
PARAMETER DRIVER DH Gate-Driver On-Resistance DL Gate-Driver On-Resistance (Source) DL Gate-Driver On-Resistance (Sink) FSEL LOGIC Logic Input Current Logic Low (GND) Logic REF Level Logic Float Level Logic VL Level EN/HSD OR EN LOGIC EN/HSD or EN Shutdown Current Logic High Logic Low POWER-OK OUTPUT Upper VTT, and VTTR Threshold Lower VTT, and VTTR Threshold Upper Threshold Lower Threshold POK Output Low Level POK Output High Leakage MAX8553 MAX8553 MAX8554 MAX8554 ISINK = 2mA VPOK = +5V 55 43 0.646 0.504 57 45 0.698 0.552 0.4 5 % VREFIN % VREFIN V V V A Max IEN/HSD for VEN/HSD < +0.8V or VEN < +0.8V VVL = VV+ = +4.5 to +5.5V, 100mV hysteresis VVL = VV+ = +4.5 to +5.5V 0.5 1.5 0.8 3.0 A V V FSEL = REF FSEL unconnected FSEL = VL 1.65 3.15 VVL - 0.4 -3 +3 0.5 2.35 3.85 A V V V V VBST - VLX = +5V DL high state DL low state 2.5 3.0 1.25 CONDITIONS MIN MAX UNITS
MAX8553/MAX8554
Note 1: Consult factory for applications that require higher than 3.5V output. Note 2: On-time and off-time specifications are measured from 50% point to 50% point at the DH pin with LX forced to 0V, BST forced to 5V, and a 250pF capacitor connected from DH to LX. Actual in-circuit times may differ due to MOSFET switching speeds. Note 3: Specifications to -40C are guaranteed by design and are not production tested.
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7
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
Typical Operating Characteristics
(VV+ = 12V, VOUT = 1.8V, circuit of Figure 1, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (CIRCUIT OF FIGURE 2)
MAX8553/4 toc01
EFFICIENCY vs. LOAD CURRENT (CIRCUIT OF FIGURE 1)
VOUT = 2.5V 90 80 EFFICIENCY (%) 70 60 50 40 30 400 0.1 1.0 10.0 100.0 VOUT = 1.8V
MAX8553/4 toc02
SWITCHING FREQUENCY vs. OUTPUT CURRENT (CIRCUIT OF FIGURE 2)
MAX8553/4 toc03
100 90 80 EFFICIENCY (%) 70 60 50 40 30 0.1 1.0 LOAD CURRENT (A) VIN = 1.8V VOUT = 0.9V VIN = 2.5V VOUT = 1.25V
100
650
SWITCHING FREQUENCY (kHz)
600
550
500
450
10.0
-8
-6
-4
-2
0
2
4
6
8
LOAD CURRENT (A)
OUTPUT CURRENT (A)
SWITCHING FREQUENCY vs. INPUT VOLTAGE (CIRCUIT OF FIGURE 3)
MAX8553/4 toc04
FREQUENCY vs. TEMPERATURE
MAX8553/4 toc05
MAX8554 SHUTDOWN SUPPLY CURRENT vs. INPUT VOLTAGE
4.5 SHUTDOWN SUPPLY CURRENT (A) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 4 V+ = HSD, EN = GND 8 12 16 20 24 28
MAX8553/4 toc06
320 300 SWITCHING FREQUENCY (kHz) 280 260 240 220 200 6 8
220 210 FREQUENCY (kHz) 200 190 180 170 160
5.0
10 12 14 16 18 20 22 24 26 28 INPUT VOLTAGE (V)
-40
-15
10
35
60
85
TEMPERATURE (C)
INPUT VOLTAGE (V)
MAX8553 SHUTDOWN SUPPLY CURRENT vs. INPUT VOLTAGE
MAX8553/4 toc07
LOAD REGULATION (CIRCUIT OF FIGURE 1)
MAX8553/4 toc08
LOAD REGULATION (CIRCUIT OF FIGURE 2)
MAX8553/4 toc09
5.0 4.5 SHUTDOWN SUPPLY CURRENT (A) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 4 EN = GND
1.86 1.84 OUTPUT VOLTAGE (V) 1.82 1.80 1.78 1.76 1.74
1.31 1.29 OUTPUT VOLTAGE (V) 1.27 1.25 1.23 1.21 1.19 VTT CONNECTED AS SHOWN IN FIGURE 2 VTT CONNECTED TO THE OUTPUT
8
12
16
20
24
28
0
4.0
8.0
12.0
16.0
20.0
0
2
4 LOAD CURRENT (A)
6
8
INPUT VOLTAGE (V)
LOAD CURRENT (A)
8
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
Typical Operating Characteristics (continued)
(VV+ = 12V, VOUT = 1.8V, circuit of Figure 1, TA = +25C, unless otherwise noted.)
MAX8553/MAX8554
LOAD TRANSIENT 0A TO 20A TO 0A (CIRCUIT OF FIGURE 1)
MAX8553/4 toc10
LOAD TRANSIENT -8A TO +8A TO -8A (CIRCUIT OF FIGURE 2)
MAX8553/4 toc11
IOUT
10A/div
VOUT
20mV/div AC-COUPLED
VOUT
100mV/div AC-COUPLED
IOUT
0 5A/div
40s/div
40s/div VTT CONNECTED TO THE OUTPUT
LOAD TRANSIENT -8A TO +8A TO -8A (CIRCUIT OF FIGURE 2)
MAX8553/4 toc12
SWITCHING WAVEFORMS WITH 20A LOAD (CIRCUIT OF FIGURE 1)
MAX8553/4 toc13
VOUT
20mV/div AC-COUPLED
VLX
5V/div 0
VOUT
IOUT 0 5A/div
20mV/div AC-COUPLED 10A/div
IL
40s/div VTT CONNECTED AS SHOWN IN FIGURE 2
2s/div
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
Typical Operating Characteristics (continued)
(VV+ = 12V, VOUT = 1.8V, circuit of Figure 1, TA = +25C, unless otherwise noted.)
POWER-UP WAVEFORMS WITH 20A LOAD (CIRCUIT OF FIGURE 1)
MAX8553/4 toc14
POWER-DOWN WAVEFORMS WITH 20A LOAD (CIRCUIT OF FIGURE 1)
MAX8553/4 toc15
VIN
VIN 10V/div VOUT 10V/div
VOUT VPOK IL
1V/div 5V/div VPOK
1V/div 5V/div
10A/div
IL
10A/div
0 2ms/div 1ms/div
0
STARTUP/SHUTDOWN WAVEFORMS WITH 20A LOAD (CIRCUIT OF FIGURE 1)
MAX8553/4 toc16
SHORT CIRCUIT AND RECOVERY (CIRCUIT OF FIGURE 1)
MAX8553/4 toc17
VEN
5V/div
VLX
20V/div
VOUT VPOK
1V/div 5V/div
IL
5A/div
1V/div 2A/div IIN 0 0A 0V VOUT 200s/div
2ms/div
10
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
Pin Description
PIN MAX8553 EN/HSD -- REFIN 2 -- 3 POK VTT 4 -- ILIM FSEL REF GND VTTR 9 -- 10 11 12 13 14 15 16 V+ VL DL PGND BST DH LX OUT V+ VL DL PGND BST DH LX Output Voltage. Connect directly to the output. OUT senses the output voltage to determine the on-time for the high-side switching MOSFET. Input Supply Voltage. Supply input for the VL regulator. Bypass with a 0.22F or greater capacitor. Internal Regulator Output. Connect a 2.2F or greater capacitor from VL to GND. VL can be connected to V+ if the operating range is from +4.5V to +5.5V. Low-Side MOSFET Gate Drive. Connect to the gate of the low-side N-channel MOSFET. DL is low in shutdown or in undervoltage lockout. Power Ground Bootstrapped Supply. Drives high-side N-channel MOSFET. Connect a 0.1F or greater capacitor from BST to LX. High-Side MOSFET Gate Drive. Connect to the high-side N-channel MOSFET gate. DH is low in shutdown or in undervoltage lockout. Inductor Switching Node EN POK -- FB ILIM FSEL REF GND -- Enable. Drive EN high to enable the output. Drive EN low to shut down the IC. If the enable function is not used, connect EN to V+. Power-OK Output. POK is an open-drain output and is logic high when both VTT and VTTR are within 12% of regulation. POK is pulled low in shutdown. VTT Feedback Input. Connect to VTT output. Output Feedback. Connect to the center of a resistor-divider between the output and ground to set the output voltage. FB threshold is 0.6V. Current-Limit Threshold Adjustment. Connect a resistor from ILIM to GND to set the current-limit threshold, or connect ILIM to VL for the default setting. See the Setting the Current Limit section. Frequency Select. Selects the switching frequency. See Tables 1 and 2 for configuration of FSEL. Reference. Connect a 0.22F or greater capacitor from REF to GND. Ground VTTR Reference Output. Connect a 1F or greater capacitor from VTTR to GND. VTTR is capable of sourcing and sinking up to 25mA. MAX8554 -- HSD -- FUNCTION Enable/High-Side Drain. Connect to the high-side N-channel MOSFET drain through a 5.1k resistor for normal operation. Connect to GND for low-power shutdown (Figure 2). If the enable function is not used, connect EN/HSD directly to the high-side N-channel MOSFET drain. High-Side Drain. Connect to the high-side N-channel MOSFET drain for normal operation. Reference Input. An applied voltage at REFIN sets VVTT and VVTTR to 1/2 VREFIN. REFIN voltage range is from 0 to +3.6V.
MAX8553/MAX8554
1
5 6 7 8
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11
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
Functional Diagrams
ILIM 5A Q ONE-SHOT /10 S TON Q TRIG ONE-SHOT Q R DH TRIG BST
FSEL
VTT ON-TIME COMPUTE
LX
POK POK
VTTR VTT Q R
LATCH
S
VL REFIN DL EN/HSD OVP 60% 56% 50% 44% 0.6V ERROR COMP PGND VTT V+
5V REG
VL
MAX8553
2V REF REF
GND
VTTR
12
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
Functional Diagrams (continued)
OUT ILIM
MAX8553/MAX8554
5A FSEL ON-TIME COMPUTE Q ONE-SHOT /10 S TON Q TRIG ONE-SHOT Q R DH BST TRIG
HSD
LX
POK POK
FB Q R
LATCH
S
EN
VL DL OVP ERROR COMP 0.600V 0.720V 0.672V V+ PGND FB
0.528V 5V REG 2V REF VL
REF
MAX8554
GND
______________________________________________________________________________________
13
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
Typical Application Circuits
VL R1 20k POK POK V+ ON OFF C7 0.22F 10V EN HSD DH FSEL VL BST C4 0.47F 10V C2 2x470F 16V L1 1.0H 30A C1 10F 16V 2.5V AT 20A VOUT D1 CMPSH-3
C3 4.7F 6.3V 12V
VIN
Q1 IRF7832
REF
MAX8554
LX R6 1 C8 4700pF R2 19.1k C6 3x470F 4V R3 6.04k PGND C5 10F 6.3V
DL R4 25.5k ILIM R5 110k GND PGND OUT
Q2 2xIRF7832
OPTIONAL
FB
Figure 1. Typical Application Circuit 1: 12V Input, 2.5V Output at Up to 20A with 200kHz Switching Frequency
VL R3 20k POK POK V+ REFIN 6V TO 28V R8 20 C9 0.47F 25V C11 1F EN/HSD SDN C8 0.22F 10V REF R1 143k ILIM FSEL GND R6 54.9k TO VOUT VTTR C7 1F 6.3V DL PGND VTT Q2 IRF7821 DH LX R7 3 C10 2200F C5 10F 6.3V C6 4x470F 2V V+ REFIN VL BST C4 0.47F 6.3V Q1 IRF7821 L1 0.68H 9A R4 0.002 C2 2x330F 6V C1 10F 6.3V D1 CMPSH-3 C3 4.7F 6.3V 2.5V
R2 5.1k
VIN
1.25V AT VOUT 8A
Q3 2N7002K
MAX8553
R5 100k
PGND
VTTR
Figure 2. Typical Application Circuit 2: 2.5V Input, 1.25V VTT at Up to 8A, and 1.25V VTTR at Up to 25mA with 550kHz Switching 14 ______________________________________________________________________________________
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
VL R1 20k POK POK V+ ON OFF C7 0.22F 10V EN HSD DH FSEL VL BST C4 0.47F 10V C2 470F 16V L1 2.5H 10A C1 10F 16V 1.8V AT VOUT 8A D1 CMPSH-3
C3 4.7F 6.3V 17V TO 21V
VIN
Q1 IRF7807V
REF
MAX8554
LX R6 3 C8 2200pF R2 12.1k C6 2x470F 4V R3 6.04k PGND C5 10F 6.3V
DL R4 61.9k ILIM R5 80.6k GND PGND OUT
Q2 IRF7821
FB
Figure 3. Typical Application Circuit 3: 19V Input, 1.8V Output at Up to 8A with 300kHz Switching Frequency
Detailed Description
Internal Linear Regulator
An internal regulator produces the +5V supply (VL) that powers the PWM controller, MOSFET driver, logic, reference, and other blocks within the IC. This +5V lowdropout (LDO) linear regulator supplies up to 35mA for MOSFET gate-drive and external loads. For supply voltages between +4.5V and +5.5V, connect VL to V+. This bypasses the VL regulator, which improves efficiency and allows the IC to function at lower input voltages.
the 455kHz IF band. Also, with a constant switching frequency, the inductor ripple-current operating point remains relatively constant, resulting in easy design methodology and predictable output voltage ripple. The general formula for the MAX8553 on-time (tON) is: t ON = K x N x 1 VEN / HSD x VOUT
On-Time One-Shot and Switching Frequency
The heart of the PWM is the one-shot that sets the highside switch on-time. This fast, low-jitter, adjustable oneshot includes circuitry that varies the on-time in response to both input and output voltages. The highside switch on-time is inversely proportional to the input voltage as measured by the EN/HSD (HSD for the MAX8554) input, and is directly proportional to the output voltage. This algorithm results in a nearly constant switching frequency despite the lack of a fixed-frequency clock generator. The switching frequency can be selected to avoid noise-sensitive regions such as
where VEN/HSD and VOUT are the voltages measured at EN/HSD and the output, respectively, and K = 1.7s. The value of N depends on the configuration of FSEL and is listed in Table 1. For the MAX8554, the general formula for on-time (tON) is: t ON = K x N x 1 VHSD x VOUT
where VHSD and VOUT are the voltages measured at HSD and the output, respectively, and K = 1.7s. The value of N depends on the configuration of FSEL and is listed in Table 2.
______________________________________________________________________________________
15
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
Table 1. Configuration of FSEL (MAX8553)
FSEL CONNECTED TO Ground REF Floating VL N 1.07 1.33 2.00 3.00 tON (s) 0.91 1.15 1.70 2.55 FREQUENCY (kHz) 550 400 300 200 CONDITION VOUT / VEN/HSD = 0.5 VOUT / VEN/HSD = 0.5 VOUT / VEN/HSD = 0.5 VOUT / VEN/HSD = 0.5
Table 2. Configuration of FSEL (MAX8554)
FSEL CONNECTED TO Ground REF Floating VL N 1.07 1.33 2.00 3.00 tON (s) 0.37 0.49 0.71 1.02 FREQUENCY (kHz) 550 400 300 200 CONDITION VHSD = 12V, VOUT = 2.5V VHSD = 12V, VOUT = 2.5V VHSD = 12V, VOUT = 2.5V VHSD = 12V, VOUT = 2.5V
This algorithm results in a nearly constant switching frequency despite the lack of a fixed-frequency clock generator. The actual switching frequency, which is given by the following equation, varies slightly due to the voltage drop across the on-resistance of the MOSFETs and the DC resistance of the output inductor: fS = 1 D t ON KxN
VTTR Reference (MAX8553 Only)
The MAX8553's VTTR output is capable of sourcing or sinking up to 25mA of current. The VTTR output voltage is one-half of the voltage applied to REFIN. Bypass VTTR with at least a 1F ceramic capacitor.
Voltage Reference
The voltage at REF is nominally 2.00V. Connect a 0.22F ceramic bypass capacitor between REF and GND.
EN and HSD (MAX8554 Only)
EN is a logic input used to enable or shut down the MAX8554. Drive EN high or connect to V+ to enable the output. Drive EN low to place the MAX8554 in lowpower shutdown mode, reducing input current to less than 5A (typ). HSD senses the input voltage at the drain of the high-side MOSFET, which is used to set the high-side MOSFET ontime. For normal operation, connect HSD to the drain of the high-side MOSFET.
where D is the duty cycle: D= VOUT + IO (RDSONL + RDC ) VHSD + IO (RDSONL - RDSONH )
where IO is the output current, RDSONL is the on-resistance of the low-side MOSFET, RDSONH is the on-resistance of the high-side MOSFET, and RDC is the DC resistance of the output inductor. The ideal switching frequency for VREFIN = 2.5V is about 550kHz. Switching frequency increases for positive (sourcing) load current and decreases for negative (sinking) load current, due to the changing voltage drop across the low-side MOSFET, which changes the inductor-current discharge ramp rate. The on-times guaranteed in the Electrical Characteristics are also influenced by switching delays caused by the loading effect of the external power MOSFETs. The switching frequency can also be adjusted to a value other than the preset frequencies by adding a resistor voltage-divider at HSD. See the Adjusting the Switching Frequency section.
EN/HSD Function (MAX8553 Only)
In order to reduce pin count and package size, the MAX8553 features a dual-function input pin, EN/HSD. When EN/HSD is pulled to ground, the internal circuitry powers off, reducing current consumption to less than 5A (typ). To enable normal operation, connect EN/HSD to the drain of the high-side MOSFET through a 5.1k resistor (Figure 2). In this configuration, EN/HSD becomes an input that monitors the high-side MOSFET drain voltage (converter input voltage) and uses that measurement to calculate the appropriate on-time for the converter. If the enable function is not used, connect EN/HSD directly to the high-side MOSFET drain.
16
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
Predischarge Mode
The MAX8553/MAX8554 discharge the output to GND before the digital soft-start begins. When EN/HSD (EN) is pulled high, the MAX8553 (MAX8554) starts an internal counter, and forces VDL to VVL. This discharges the output to GND through the low-side MOSFET. If the output voltage is above ground before enable, the output voltage goes slightly negative due to energy stored in the output LC. If the load cannot tolerate a negative voltage, place a power Schottky diode from the output to PGND (anode to PGND) to act as a reverse-polarity clamp. The period for this discharge mode is 1.7ms. Both the buck controller and the VTTR buffer are turned off during this period. After the predischarge period, both the buck controller and the VTTR buffer are turned on and go through soft-start.
Overvoltage Protection (OVP)
When the buck output voltage rises above 120% of the nominal regulation voltage, the OVP circuit sets the fault latch, shuts down the PWM controller, and immediately pulls DH low and forces DL high. The negative current limit is also disabled. This turns on the low-side MOSFET, which rapidly discharges the output capacitors and clamps the output to ground. Note that immediately latching DL high can cause the output voltage to go slightly negative due to energy stored in the output LC at the instant the OVP occurs. If the load cannot tolerate a negative voltage, place a power Schottky diode from the output to PGND (anode to PGND) to act as a reverse-polarity clamp. Cycle EN or input power to reset the latch.
MAX8553/MAX8554
Overcurrent Protection
The current-limit circuit employs a unique "valley" current-sensing algorithm that uses the on-resistance of the low-side MOSFET as a current-sensing element. If the current-sense signal is above the current-limit threshold, the PWM is not allowed to initiate a new cycle. The actual peak current is greater than the current-limit threshold by an amount equal to the inductor ripple current (Figure 4). Therefore, the exact currentlimit characteristic and maximum load capability are a function of the MOSFET on-resistance, the inductor value, and the input voltage. The reward for this uncertainty is robust, lossless overcurrent sensing. There is also a negative current limit that prevents excessive reverse inductor currents when VOUT is sinking current. The negative current-limit threshold is set to approximately 110% of the positive current limit, and therefore tracks the positive current limit when ILIM is adjusted. The current-limit threshold can be adjusted with an external resistor (RILIM) at ILIM. A precision 5A pullup current source at ILIM sets a voltage drop on this resistor, adjusting the current-limit threshold from approximately 50mV to 200mV. In the adjustable mode, the current-limit threshold voltage is precisely 1/10 the voltage seen at ILIM. Therefore, choose RILIM equal to 2k/mV of the current-limit threshold. The threshold defaults to 100mV when ILIM is connected to VL. The logic threshold for switchover to the 100mV default value is approximately VVL - 1V. The adjustable current limit can accommodate various MOSFETs. Alternately, foldback current limit can also be implemented by adding a resistor from ILIM to VOUT. See the Setting the Current Limit section.
Digital Soft-Start
The digital soft-start allows a gradual increase of the internal current-limit level during startup to reduce the input surge current. The MAX8553/MAX8554 divide the soft-start period into five phases. During the first phase, the controller limits the current limit to only 20% of the full current limit. If the output does not reach the regulation within 425s, soft-start enters the second phase and the current limit is increased by another 20%. This process repeats until the maximum current limit is reached (after 1.7ms) or when the output reaches the nominal regulation voltage, whichever occurs first. Adding a capacitor in parallel with the external ILIM resistor creates a continuously adjustable analog softstart function. If the foldback current-limiting feature is implemented in the application circuit, the maximum current limit is also a function of the output voltage and the resistors connected to ILIM.
Power-Good Output (POK)
POK is the open-drain output of the internal window comparators that continuously monitor VTT and VTTR for the MAX8553 and FB for the MAX8554. POK is actively held low in shutdown, and becomes high impedance when the outputs are within 12% of their respective nominal regulation voltage.
______________________________________________________________________________________
17
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
erly. Otherwise, the sense circuitry in the MAX8553/ MAX8554 can interpret the MOSFET gate as "off" while there is actually still charge left on the gate. Use very short, wide traces measuring 10 squares to 20 squares (50 mils to 100 mils wide if the MOSFET is 1in from the MAX8553/MAX8554). This adaptive dead-time delay is in addition to a fixed delay of 30ns (typ). The dead time at the other edge (DH turning off) is determined by a fixed 32ns (typ) internal delay.
IPEAK
ILOAD INDUCTOR CURRENT
IVALLEY
Design Procedure
Setting the Output Voltage
For the MAX8553, the output voltage, VVTT, is always 50% of VREFIN. For the MAX8554, the output voltage can be adjusted from 600mV to 3.5V using a resistive voltage-divider (R2 and R3 in Figures 1 and 3). To set the voltage, choose a value for R3 in the range of 1k to 10k, then solve for R2 using the following equation: V R2 = R3 OUT - 1 VFB where VFB is 0.6V.
TIME
Figure 4. Inductor-Current Waveform
Carefully observe the PC board layout guidelines to ensure that noise and DC errors do not corrupt the current-sense signals seen by LX and PGND. The IC must be mounted close to the low-side MOSFET with short, direct traces making a Kelvin-sense connection to the source and drain terminals. See the PC Board Layout section.
Voltage Positioning
The Quick-PWM control architecture responds virtually instantaneously to transient load changes and eliminates the control loop delay of conventional PWM controllers. Therefore, a large portion of the voltage deviation during a step load change is from the ESR (equivalent series resistance) of the output capacitors. For DDR termination applications, the maximum allowed voltage deviation is 40mV for any output load transition from sourcing current to sinking current. Passive voltage positioning adjusts the converter's output voltage based on its load current to optimize transient response and minimize the required output capacitance. Voltage positioning is implemented by connecting a low ohmic resistor (R4) as shown in Figure 2.
Inductor Selection
Three key inductor parameters must be specified: inductance value (L), peak inductor current (IPEAK), and DC resistance (R DC ). A good compromise between size and efficiency is to set the inductor peakto-peak ripple current equal to 30% of the maximum load current, thus LIR = 0.3. The switching frequency, input voltage, output voltage, and selected LIR determine the inductor value as follows: L= VOUT(VIN
-
VOUT)
VIN x fS x ILOAD(MAX) x LIR
MOSFET Drivers
The DH and DL drivers are optimized to drive MOSFETs that can deliver up to 25A output current. An adaptive dead-time circuit monitors the DL output and prevents the high-side MOSFET from turning on until DL is fully off. There must be a low-resistance, lowinductance path from the DL driver to the MOSFET gate in order for the adaptive dead-time circuit to work prop-
where fS is the switching frequency. The exact inductor value is not critical and can be adjusted in order to make trade-offs among size, cost, and efficiency. Lower inductor values minimize size and cost and also improve transient response but reduce efficiency and increase output voltage ripple due to higher peak currents. Higher inductance increases efficiency by reducing the RMS current. Find a low-loss inductor with the lowest possible DC resistance that fits in the allotted dimensions. The inductor's current saturation rating must exceed the
18
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
peak inductor current at the maximum-defined load current (ILOAD(MAX)): LIR IPEAK = ILOAD(MAX) + xI 2 LOAD(MAX) Output-Capacitor Stability Consideration Stability is determined by the value of the ESR zero relative to the switching frequency. To ensure stability, the following condition must be met: f fESR < S where fS is the switching frequency and: fESR = 1 2 x RESR x COUT
MAX8553/MAX8554
Output-Capacitor Selection
The key selection parameters for the output capacitor are the actual capacitance value, the ESR, the equivalent series inductance (ESL), and the voltage-rating requirements, which affect the overall stability, output ripple voltage, and transient response. The worst-case output ripple has three components: variations in the charge stored in the output capacitor, the voltage drop across the capacitor's ESR, and ESL caused by the current into and out of the capacitor. This can be approximated by: VRIPPLE = VRIPPLE(ESR) + VRIPPLE(C) + VRIPPLE(ESL) The output voltage ripple due to the ESR is: VRIPPLE(ESR) = IP-P x ESR The output voltage ripple due to the output capacitance is: VRIPPLE(C) = IP-P 8 x COUT x fS
For a typical 300kHz application, the ESR zero frequency must be well below 95kHz, preferably below 50kHz. Do not put high-value ceramic capacitors directly across the feedback sense point without taking precautions to ensure stability. Large ceramic capacitors can have a high-ESR zero frequency and cause erratic, unstable operation. However, it is easy to add enough series resistance by placing the capacitors a couple of inches downstream from the feedback sense point, which should be as close as possible to the inductor. The easiest method for checking stability is to apply a very fast zero-to-max load transient and carefully observe the output voltage-ripple envelope for overshoot and ringing. It can help to simultaneously monitor the inductor current with an AC current probe. Do not allow more than one cycle of ringing after the initial step-response under- or overshoot.
The output voltage ripple due to the ESL of the output capacitor is: VRIPPLE (ESL) = (VIN x ESL) / (L+ESL) IP-P is the peak-to-peak inductor current: IP-P = VIN - VOUT VOUT x VIN fS x L
Input-Capacitor Selection
The input capacitor (CIN) reduces the current peaks drawn from the input supply and reduces noise injection. The source impedance to the input supply largely determines the value of CIN. High source impedance requires high input capacitance. The input capacitor must meet the ripple current requirement (I RMS ) imposed by the switching currents. The RMS input ripple current is given by: IRMS = ILOAD x VOUT x (VIN - VOUT ) VIN
After a load transient, the output voltage instantly changes by ESR x ILOAD + ESL x di/dt and the controllers respond within 100ns and try to regulate back to the nominal output value. Solid polymer or OSCON electrolytic capacitors are recommended due to their low ESR and ESL at the switching frequency. Higher output-current applications require multiple output capacitors connected in parallel to meet the output ripple-voltage requirements. Do not exceed the capacitor's voltage or ripple-current ratings.
IRMS has a maximum value of 1/2 ILOAD, which occurs when VIN is twice VOUT. For optimal circuit reliability, choose a capacitor that has less than 10C temperature rise at the peak ripple current.
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19
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
Setting the Current Limit
Constant Current Limit The adjustable current limit accommodates MOSFETs with a wide range of on-resistance values. The currentlimit threshold is adjusted with an external resistor connected from ILIM to GND (R ILIM_ ). The adjustment range is 50mV to 200mV measured across the low-side MOSFET. The value of RILIM is calculated using the following formula: I RILIM = 10 x VALLEY x RDS(ON) 5A
DL DH
VOUT
MAX8553
LX
where IVALLEY is the valley current limit and RDS(ON) is the on-resistance of the low-side MOSFET. To avoid reaching the current at a lower current than expected, use the maximum value for RDS(ON) at elevated junction temperature. Refer to the MOSFET manufacturer's data sheet for maximum values. Foldback Current Limit Foldback current limit is used to reduce power dissipation during overload and short-circuit conditions. This is accomplished by lowering the current-limit threshold as the output voltage drops due to the overload. To use foldback current limit, connect a resistor (RFOBK) from ILIM to the output, and connect a resistor (RILIM) from ILIM to GND (Figure 5). The values of RILIM and RFOBK are calculated as follows: First, select the percentage of foldback, PFB. This percent corresponds to the current limit when VOUT equals zero divided by the current limit when VOUT equals its nominal voltage. Typical values range from 15% to 30%. To solve for the resistor values, use the following equations: RFOBK = PFB x VOUT 5A 1 - PFB
RILIM ILIM RFOBK PGND
Figure 5. Setting the Foldback Current Limit with Two Resistors, RILIM and RFOBK
Adjusting the Switching Frequency
The switching frequency of the MAX8553/MAX8554 can be lowered from the value set by FSEL by adding a resistor voltage-divider to EN/HSD (HSD) as shown in Figure 6. This voltage-divider lowers the voltage the IC measures on EN/HSD (HSD), which increases the ontime. The switching frequency with the added resistordivider is calculated as follows: fS R2 1 x KxN R1 + R2
(
RILIM =
10 x RDS(ON) x IVALLEY x 1 - PFB x RFOBK VOUT - 10 x RDS(ON) x IVALLEY x 1 - PFB
(
(
)
)
where K = 1.7s and N is given in Tables 1 and 2. To set the frequency, select a value for R2 between 10k and 100k, then calculate R1 from the following equation:
(
))
R1 =
R2 1 x - R2 K x N fS
If RILIM results in a negative number, select another lowside MOSFET with lower RDS(ON) or increase PFB or a combination of both for the best compromise of cost, efficiency, and lower short-circuit power dissipation.
With the minimum input voltage, make sure that the voltage present at EN/HSD (HSD) is greater than 1.5V when the resistor-divider is used: VIN(MIN) x R2 R1 + R2 > 1.5V
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
Setting Voltage Positioning
The droop resistor, RDRP (R4) in Figure 2, in series with the output inductor before the output capacitor, sets the droop voltage, VDRP. Choose RDRP such that the output voltage at the maximum load current, including ripple, is just above the lower limit of the output tolerance: RDRP < VOUT(TYP) - VOUT(MIN) - VRIPPLE / 2 IOUT(MAX) Choose the MOSFETs with rated RDS(ON) at VGS = 4.5V. For a good compromise between efficiency and cost, choose the high-side MOSFET that has a conduction loss equal to switching loss at nominal input voltage and maximum output current (see below). For the low-side MOSFET, make sure that it does not spuriously turn on because of the dV/dt caused by the high-side MOSFET turning on, as this would result in shoot-through current degrading the efficiency. MOSFETs with a lower QGD to QGS ratio have higher immunity to dV/dt. For proper thermal-management design, calculate the power dissipation at the desired maximum operating junction temperature, maximum output current, and worst-case input voltage (for low-side MOSFET, worst case is at VIN(MAX); for high-side MOSFET, it could be either at VIN(MIN) or VIN(MAX)). The high-side MOSFET and low-side MOSFET have different loss components due to the circuit operation. The low-side MOSFET operates as a zero voltage switch; therefore, major losses are: the channel conduction loss (PLSCC), the body-diode conduction loss (PLSDC), and the gatedrive loss (PLSDR): V 2 PLSCC = 1- OUT x (ILOAD ) x RDS(ON) VIN Use RDS(ON) at TJ(MAX): PLSDC = 2 x ILOAD x VF x tDT x fS where VF is the body-diode forward-voltage drop, tDT is the dead time (~30ns), and fS is the switching frequency. Because of the zero-voltage switch operation, low-side MOSFET gate-drive loss occurs as a result of charging and discharging the input capacitance (CISS). This loss is distributed among the average DL gate driver's pullup and pulldown resistance, RDL (~1.2), and the internal gate resistance (RGATE) of the MOSFET (~2). The drive power dissipated is given by: PLSDR = CISS x (VGS ) x fS x
2
MAX8553/MAX8554
R DRP introduces some power dissipation, which is given by: PD(DRP) = RDRP x IOUT(MAX)
(
)2
RDRP should be chosen to handle this power dissipation.
Power MOSFET Selection
The MAX8553/MAX8554 drive external, logic-level, Nchannel MOSFETs as the circuit-switch elements. The key selection parameters are: On-resistance (RDS(ON)): The lower, the better. Maximum drain-to-source voltage (V DSS ): This should be at least 20% higher than the input supply rail at the high-side MOSFET's drain. Gate charges (QG, QGD, QGS): The lower, the better.
R1 VIN POK V+ REFIN VL BST DH
Q3 2N7002 SDN R2 R6 10k
MAX8553
EN/HSD REF ILIM FSEL GND LX DL PGND VTT VTTR
RGATE RGATE + RDL
The high-side MOSFET operates as a duty-cycle control switch and has the following major losses: the channel conduction loss (PHSCC), the VI overlapping switching loss (PHSSW), and the drive loss (PHSDR). The high-side MOSFET does not have body-diode conduction loss because the diode never conducts current.
Figure 6. A resistor-divider (R1 and R2) is used to lower the switching frequency. ______________________________________________________________________________________ 21
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
V 2 PHSCC = OUT x (ILOAD ) x RDS(ON) VIN Use RDS(ON) at TJ(MAX): PHSSW = VIN x ILOAD x fS x QGS + QGD IGATE
Control IC Power Dissipation
Power dissipation in the MAX8553/MAX8554 IC is primarily due to the on-chip MOSFETs' gate drivers (DH and DL). This power dissipation depends on the gate charge of the external MOSFETs used. Power dissipation in the MAX8553 also depends on the VTTR load current (IVTTR). Use the following equation to calculate the power dissipation: PD = (VV + ) x fS x (QGH + QGL ) + IVTTR
where IGATE is the average DH driver output current determined by: IGATE = 2.5V RDH + RGATE
[
]
where RDH is the high-side MOSFET driver's on-resistance (1.4 typ) and RGATE is the internal gate resistance of the MOSFET (~2): PHSDR = QG x VGS x fS x RGATE RGATE + RDH
where QGH and QGL are the total gate charge of the high-side and low-side MOSFETs, respectively. Select the switching frequency and VV+ correctly to ensure the power dissipation does not exceed the package power-dissipation requirement.
Applications Information
PC Board Layout
A properly designed PC board layout is important in any switching regulator. The switching power stage requires particular attention. If possible, mount all the power components on the top-side of the board with their ground terminals flush against one another. Follow these guidelines for good PC board layout: 1) Keep the high-current paths short, especially at the ground terminals. This practice is essential for stable, low-jitter operation. 2) Connect GND and PGND together at a single point. 3) Keep the power traces and load connections short. This practice is essential for high efficiency. The use of thick copper PC boards (2oz vs. 1oz) can noticeably enhance full-load efficiency. Correctly routing PC board traces is a difficult task that must be approached in terms of fractions of centimeters, where a single m of excess trace resistance causes a measurable efficiency penalty. 4) LX and PGND connections to the low-side MOSFET for current limiting must be made using Kelvinsense connections in order to guarantee the current-limit accuracy. With 8-pin SO MOSFETs, this can be done by routing power to the MOSFETs from
where VGS = VVL = 5V. When the MAX8553 is sinking current, the high-side MOSFET operates as a zero-voltage switch and the low-side MOSFETs operate as a nonzero-voltage switch. In addition to the losses above, allow about 20% more for additional losses due to MOSFET output capacitances and low-side MOSFET body-diode reverse recovery charge dissipated in the high-side MOSFET that is not well defined in the MOSFET data sheet. Refer to the MOSFET data sheet for thermal-resistance specifications to calculate the PC board area needed to maintain the desired maximum operating junction temperature with the above calculated power dissipations. To reduce EMI caused by switching noise, add a 0.1F ceramic capacitor from the high-side switch drain to the low-side switch source, or add resistors in series with DH and DL to slow down the switching transitions. Adding series resistors increases the power dissipation of the MOSFET, so ensure that this does not overheat the MOSFET.
22
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4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
outside using the top copper layer, while tying in PGND and LX inside (underneath) the 8-pin SO package. 5) When trade-offs in trace lengths must be made, it is preferable to allow the inductor charging path to be made longer than the discharge path. For example, it is better to allow some extra distance between the input capacitors and the high-side MOSFET than to allow distance between the inductor and the lowside MOSFET or between the inductor and the output filter capacitor. 6) It may be desirable to deliberately introduce some trace length (droop resistance) between the FB inductor node and the output filter capacitor to meet the stability criteria (fESR < fS / ). 7) Place feedback resistors as close as possible to the IC. 8) Route high-speed switching nodes away from sensitive analog nodes. 9) Make all pin-strap control input connections (ILIM, etc.) to GND or VL close to the chip, and do not connect to PGND.
MAX8553/MAX8554
Pin Configurations
TOP VIEW
EN/HSD 1 REFIN 2 POK 3 VTT 4 ILIM 5 FSEL 6 REF 7 GND 8 16 LX 15 DH 14 BST HSD 1 EN 2 POK 3 FB 4 ILIM 5 FSEL 6 REF 7 GND 8 16 LX 15 DH 14 BST
MAX8553
13 PGND 12 DL 11 VL 10 V+ 9 VTTR
MAX8554
13 PGND 12 DL 11 VL 10 V+ 9 OUT
QSOP
QSOP
Chip Information
TRANSISTOR COUNT: 2827 PROCESS: BiCMOS
______________________________________________________________________________________
23
4.5V to 28V Input, Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications MAX8553/MAX8554
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
QSOP.EPS
PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH
21-0055
E
1 1
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
24 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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