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19-2830; Rev 0; 4/03
KIT ATION EVALU E AILABL AV
Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies
General Description
The MAX8540/MAX8541 pulse-width-modulated (PWM) controllers for forward/flyback isolated DC-to-DC converters provide maximum flexibility to power-supply designers and reduce external component count. These controllers allow common PC board layout for currentmode (MAX8540) and voltage-mode (MAX8541) designs. Both controllers feature adjustable switching frequency and external synchronization from 200kHz to 1MHz. Active-high and active-low enable, undervoltage protection (UVP), and overvoltage protection (OVP) reduce external component count. Maximum duty cycle is adjustable, and the feed-forward function scales the maximum duty cycle with input voltage to limit the maximum volt-seconds applied to the transformer primary. The MAX8540 allows the user to select the value of slope compensation to further optimize magnetics design. The MAX8541 features useradjustable ramp magnitude for the PWM comparator. A cycle-by-cycle current-limit function controls the peak primary current during overload and short circuit. Both controllers can be set to latch off or to hiccup when a short circuit is detected. The number of current-limited cycles to initiate the hiccup mode and number of cycles skipped are user selectable to allow startup with high-capacitance loads and protect the converter against sustained short circuits. The MAX8540/MAX8541 feature 9 internal gate drivers for low-power applications and are compatible with external gate drivers for high-power applications. Both devices are available in the space-saving 16-pin QSOP package. The MAX8541 EV kit evaluates the MAX8540 and the MAX8541 to speed designs. Design examples are also available in application notes: 50W Voltage-Mode Forward Converter Design with the MAX8541 and 50W Current-Mode Forward Converter Design with the MAX8540. o Synchronization to External Clock o Programmable, Constant Maximum Volt-Second Simplifies Transformer Design o Programmable Hiccup/Skip Cycles or Latch-Mode Protection o Clean Startups with High-Capacitance Loads o Programmable Under/Overvoltage Protection o Current-Mode, Adjustable Slope Compensation (MAX8540) o Voltage-Mode, Adjustable Ramp Magnitude (MAX8541) o 10mA, 5V Regulator o 75mV to 1.25mV Adjustable Current Limit Reduces External Components o Internal Gate Driver for Low-Power Applications o External Gate Driver for High-Power Applications
Features
o 200kHz to 1MHz Adjustable Switching Frequency
MAX8540/MAX8541
Ordering Information
PART MAX8540EEE MAX8541EEE TEMP RANGE -40C to +85C -40C to +85C PIN-PACKAGE 16 QSOP 16 QSOP
Pin Configurations
TOP VIEW
REF 1 OV 2 UV 3 FREQ/SYNC 4 SS 5 MAXDTY 6 EN 7 SCOMP 8 16 VCC 15 DRV 14 GND
Applications
Isolated DC-to-DC Modules (Bricks) Cellular Base Stations Telecom and Network Systems High-Performance Off-Line AC/DC Converters
MAX8540EEE
13 SKTON 12 CS 11 SKTOFF 10 ILIM 9 OPTO
QSOP Functional Diagrams and Typical Operating Circuit appear at end of data sheet. Pin Configurations continued at end of data sheet. 1
________________________________________________________________ Maxim Integrated Products
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Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies MAX8540/MAX8541
ABSOLUTE MAXIMUM RATINGS
VCC to GND ............................................................-0.3V to +20V OPTO, UV, OV, EN, PRAMP, ILIM, REF to GND ......-0.3V to +6V SCOMP, FREQ, CS, SKTON, SKTOFF, SS, MAXDTY to GND .....................................-0.3V to VREF + 0.3V DRV to GND ................................................-0.3V to VVCC + 0.3V Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate 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
(VVCC = 12V, CREF = 1F, VUV = VOV = VOPTO, VILIM = 2V, RFREQ/SYNC = 32.4k, CSS = 0.1F, RMAXDTY = 97.6k, V EN = 0, RSCOMP = 68k, RPRAMP = 25k, CSKTON = 0.01F, CSKTOFF = 0.1F, VCS = 0.6V, CDRV = 10pF, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER VCC Operating Voltage Range Undervoltage Lockout Undervoltage Lockout Hysteresis Supply Current Shutdown Current REF Output Voltage Load Regulation Line Regulation Pulldown Resistor in Shutdown IREF = 0 IREF = 0 to 10 mA VVCC = 8.4V to 18V V EN = 3V VUV rising, 120mV typical hysteresis VUV falling VOV rising, 130mV typical hysteresis VOV falling VIL VIH V EN = 2.6V 2.6 5 0.075 110 10 1.175 1.250 1.330 17 1.250 150 1.200 1.083 -0.5 2.9 2.778 4.9 5.0 20 5 10 1.250 1.128 +0.01 3.021 2.894 5.1 50 20 30 1.300 1.173 +0.5 3.1 3.010 1.2 V mV mV VREF is active, IREF = 0 V EN = 3V No switching Switching with no load (Note 1) VCC rising VCC falling 7.6 7.6 6.6 0.9 8.0 7.0 1.0 3.5 5 0.5 4.2 6.3 0.9 18.0 8.4 7.4 V V V mA mA CONDITIONS MIN TYP MAX UNITS
UNDERVOLTAGE/OVERVOLTAGE (UV/OV) UVLO Threshold Input Bias Current OVLO Threshold EN Input Low Threshold Input High Threshold Pullup Current CURRENT LIMIT VILIM Range CS Current-Limit to DRV Turn-Off Includes leading-edge blanking time Delay VCS Cycle-by-Cycle Threshold Voltage Accuracy when VILIM = 0.075V VILIM = 1.250V V ns % V V V A V A V
2
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Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies
ELECTRICAL CHARACTERISTICS (continued)
(VVCC = 12V, CREF = 1F, VUV = VOV = VOPTO, VILIM = 2V, RFREQ/SYNC = 32.4k, CSS = 0.1F, RMAXDTY = 97.6k, V EN = 0, RSCOMP = 68k, RPRAMP = 25k, CSKTON = 0.01F, CSKTOFF = 0.1F, VCS = 0.6V, CDRV = 10pF, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER ILIM Leakage Current Hiccup Skip Off-Time Hiccup Skip Off-Charging Hiccup Skip On-Time Hiccup Skip On-Charging Current Hiccup SKTOFF Voltage Threshold OPTO Input Bias Current Minimum Input Voltage Maximum Input Voltage DUTY CYCLE RMAXDTY = 24.3k, VUV = 1.3V RMAXDTY = 130k, VUV = 1.3V Maximum Duty Cycle VUV = 1.3V, RMAXDTY = 97.6k Internally programmed limit for maximum duty cycle at all frequencies SOFT-START Source Current Time FREQUENCY Initial Accuracy Switching Frequency Range SCOMP (MAX8540) Slope Compensation PRAMP (MAX8541) RPRAMP = 12.4k Ramp Voltage Amplitude DRV (MOSFET DRIVER) Rise Time CDRV = 500pF, 10% to 90% of V VCC CDRV = 2000pF, 10% to 90% of V VCC 30 57 ns RPRAMP = 21k RPRAMP = 42k 2.5 1 0.5 V RSCOMP = 14k RSCOMP = 204k 1.5 0.1 V RFREQ = 32.4k RFREQ = 48.7k RFREQ = 8.87k 280 300 200 1000 320 kHz kHz VSS = 1V 4.5 5.5 10 440 6.5 30 A ms/F Pulldown Resistance in Shutdown ISS = 10mA, V E N = 3V 79 15 80 60 80 82 % VOPTO = 3.5V Minimum voltage for duty-cycle control Maximum voltage for duty-cycle control 30 0.6 2.6 A V V CSKTOFF = 0.1F (Note 3) 0.9 CSKTON = 0.01F (Note 3) 1.5 2.3 CONDITIONS MIN -1 90 1.5 9 2.5 2.5 3.5 2.7 2.1 TYP MAX +1 UNITS A ms A ms A V
MAX8540/MAX8541
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Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies MAX8540/MAX8541
ELECTRICAL CHARACTERISTICS (continued)
(VVCC = 12V, CREF = 1F, VUV = VOV = VOPTO, VILIM = 2V, RFREQ/SYNC = 32.4k, CSS = 0.1F, RMAXDTY = 97.6k, V EN = 0, RSCOMP = 68k, RPRAMP = 25k, CSKTON = 0.01F, CSKTOFF = 0.1F, VCS = 0.6V, CDRV = 10pF, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Fall Time On-Resistance Pullup On-Resistance Pulldown SYNC (EXTERNAL FREQUENCY SYNCHRONIZATION) Frequency Range FREQ Input Threshold THERMAL SHUTDOWN Thermal Shutdown TA rising, 20C hysteresis 170 C Frequency syncs 30% of frequency set by external R on FREQ pin (Note 4) 0.2 2 1.0 3 MHz V CONDITIONS CDRV = 500pF, 90% to 10% of V VCC CDRV = 2000pF, 90% to 10% of V VCC MIN TYP 27 53 9 7 MAX UNITS ns
ELECTRICAL CHARACTERISTICS
(VVCC = 12V, CREF = 1F, VUV = VOV = 2V, RFREQ/SYNC = 32.4k, CSS = 0.1F, RMAXDTY = 97.6k, V EN = 0, RSCOMP = 68k, RPRAMP = 25k, CSKTON = 0.01F, CSKTOFF = 0.1F, VOPTO = 2V, VILIM = 4V, VCS = 0.6V, CDRV = 10pF, TA = -40C to +85C, unless otherwise noted.) (Note 5)
PARAMETER VCC Operating Voltage Range Undervoltage Lockout Undervoltage Lockout Hysteresis Supply Current Shutdown Current REF Output Voltage Load Regulation Line Regulation Shutdown Resistor IREF = 0 IREF = 0 to 10mA VVCC = 8.4V to 18V V EN = 3V VUV falling, 120mV typical hysteresis VUV rising VOV falling, 130 mV typical hysteresis VOV rising 1.200 1.083 -0.5 2.901 2.778 4.9 5.1 50 20 30 1.300 1.173 +0.5 3.142 3.010 V mV mV VREF = is active, IREF = 0 V EN = 3V No switching Switching with no load (Note 1) VCC rising VCC falling 7.6 7.6 6.6 0.9 4.2 6.3 0.9 18.0 8.4 7.4 V V V mA mA CONDITIONS MIN TYP MAX UNITS
UNDERVOLTAGE/OVERVOLTAGE (UV/OV) UVLO Threshold Input Bias Current OVLO Threshold V A V
4
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Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies
ELECTRICAL CHARACTERISTICS (continued)
(VVCC = 12V, CREF = 1F, VUV = VOV = 2V, RFREQ/SYNC = 32.4k, CSS = 0.1F, RMAXDTY = 97.6k, V EN = 0, RSCOMP = 68k, RPRAMP = 25k, CSKTON = 0.01F, CSKTOFF = 0.1F, VOPTO = 2V, VILIM = 4V, VCS = 0.6V, CDRV = 10pF, TA = -40C to +85C, unless otherwise noted.) (Note 5)
PARAMETER EN Input Threshold Pullup Current CURRENT LIMIT VILIM Range CS Current Limit to DRV Delay VCS Cycle-by-Cycle Threshold Voltage ILIM Leakage Current Hiccup Skip Off-Charging Current Hiccup Skip On-Charging Current Hiccup SKTOFF Voltage Threshold DUTY CYCLE Duty-Cycle Range Maximum Duty Cycle SOFT-START Source Current Pulldown Resistance in Shutdown FREQUENCY Initial Accuracy RFREQ = 32.4k Frequency syncs 30% of frequency set by external R on FREQ pin (Note 4) 280 320 kHz SYNC (EXTERNAL FREQUENCY SYNCHRONIZATION) Frequency Range FREQ Input Threshold 0.2 2 1.0 3 MHz V VSS = 1V ISS = 10mA, V EN = 3V 4.5 6.5 30 A VUV = 1.3V Internal programmed max duty at all frequencies 15 79 80 82 % % Includes leading-edge blanking time Accuracy when VILIM = 0.075V VILIM = 1.250V 1.130 -1 0.9 1.5 2.3 10 1.375 +1 2.1 3.5 2.7 0.075 1.25 150 V ns % V A A A V VIL VIH V EN = 2.6V 2.6 5 1.2 V A CONDITIONS MIN TYP MAX UNITS
MAX8540/MAX8541
This is the VCC operating range after clearing the rising UVLO. Guaranteed by design. See the SKTON and SKTOFF section. The minimum FREQ pulse must be 200ns in duration with a minimum magnitude of 3V plus VF of the external diode and have a maximum duty cycle of 55%. Note 5: Specifications to -40C are guaranteed by design and not production tested. Note 1: Note 2: Note 3: Note 4:
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Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies MAX8540/MAX8541
Typical Operating Characteristics
(VCC = 12V, TA = +25C, unless otherwise specified.)
VCC NO-LOAD SWITCHING CURRENT vs. TEMPERATURE
MAX8540 toc01
SHUTDOWN VCC CURRENT vs. TEMPERATURE
MAX8540 toc02
REFERENCE VOLTAGE vs. TEMPERATURE
MAX8540 toc03
6.0 5.8 5.6 5.4 IVCC (mA)
600 580 560 540 IVCC (A)
5.10
5.05 VREF (V) VEN = 5V 4.90 -40 -20 0 20 40 60 80 100 -40 -20 0 20 40 60 80 100 TA (C) TA (C)
5.2 5.0 4.8 4.6 4.4 4.2 4.0 -40 -20 0 20 40 60 80 100 TA (C) VEN = 0
520 500 480 460 440 420 400
5.00
4.95
MAXIMUM DUTY CYCLE vs. TEMPERATURE
MAX8540 toc04
FREQUENCY vs. TEMPERATURE
1000 800 600 400 RFREQ = 20k 200 RFREQ = 50k 0 100 -40 -20 0 20 40 60 80 100 -40
MAX8540 toc05
CURRENT-SENSE THRESHOLD vs. TEMPERATURE
MAX8540 toc06
90 85 80
130 125 120 VCS (mV) 115 110 105
DMAX (%)
75 70 65 60 55 50 -40 -20 0
RFREQ = NONE
RFREQ = 36.5k
FREQUENCY (kHz)
RFREQ = 10k
20
40
60
80
100
-20
0
20
40
60
80
100
TA (C)
TA (C)
TA (C)
CURRENT-LIMIT DELAY TIME vs. TEMPERATURE
MAX8540 toc07
OV/UV THRESHOLD VOLTAGE vs. TEMPERATURE
3.5 3.0 OV/UV VOLTAGE (V) 2.5 2.0 1.5 1.0 0.5 0 VUV VOV
MAX8540 toc08
200 190 180 DELAY TIME (ns) 170 160 150 140 130 120 110 100 -40 -20 0 20 40 60 80
4.0
100
-40
-20
0
20
40
60
80
100
TA (C)
TA (C)
6
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Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies
Typical Operating Characteristics (continued)
(VCC = 12V, TA = +25C, unless otherwise specified.)
FREQUENCY vs. INPUT VOLTAGE
1000 800 600 400 RFREQ = 20k 200 RFREQ = 50k 0 7.6 9.7 11.8 13.8 15.9 18.0 INPUT VOLTAGE (V) 0 0 0.2 0.4 0.6 0.8 1.0 4ms/div CAPACITANCE (F) 200 CSKTON = 0.01F
MAX8540 toc10
MAX8540/MAX8541
SKTON/SKTOFF TIME vs. CAPACITANCE
1000 800 TIME (ms) 600 400
MAX8540 toc11
PROGRAMABLE LATCH-OFF DELAY
MAX8540 toc12
VDRV
FREQUENCY (kHz)
RFREQ = 10k
VCS
2V/div
REFERENCE VOLTAGE vs. LOAD CURRENT
MAX8540 toc13
REFERENCE VOLTAGE vs. VCC
MAX8540 toc14
5.10 5.08 5.06 5.04 VREF (V)
5.06 5.04 5.02 VREF (V) 5.00 4.98 4.96 4.94
5.02 5.00 4.98 4.96 4.94 4.92 4.90 0 2 4 6 8 10 12 LOAD CURRENT (mA)
7.6
9.7
11.8
13.8
15.9
18.0
VCC (V)
LATCH-OFF OVERCURRENT WAVEFORMS
MAX8540 toc15
HICCUP-MODE OVERCURRENT WAVEFORMS
MAX8540 toc16
VDRV
VDRV
VSKTON
1V/div
VSKTON
1V/div
VSKTOFF CSKTON = 0.01F 10ms/div
5V
VSKTOFF CSKTON , CSKTFOF = 0.01F 4ms/div
1V/div
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Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies MAX8540/MAX8541
Pin Description
PIN MAX8540 MAX8541 1 1 NAME REF FUNCTION 5V Reference Voltage Output. Bypass to ground with a 1F capacitor. REF is pulled to GND during shutdown. System Input Overvoltage Monitoring. The OV threshold is 3.021V with 130mV hysteresis. Connect to the center of a resistor-divider from the system input to GND to set the overvoltage trip point (see the Overvoltage Threshold section). System Input Undervoltage Monitoring and Feed-Forward Input. The UV threshold is 1.25V with 120mV hysteresis. Connect to the center of a resistor-divider from the system input to GND to set the undervoltage trip point (see the Undervoltage Threshold section). This input can be used as a disable input by pulling it below 1.2V. UV remains active during shutdown. Programmable Frequency and Sync Input. Connect a resistor in parallel with RC (10k, 0.01F) from FREQ/SYNC to GND to set the switching frequency or apply an external clock signal through a diode to synchronize to an external frequency. Soft-Start Capacitor Connection. Connect a capacitor from SS to GND to set the soft-start time period.
2
2
OV
3
3
UV
4 5
4 5
FREQ/ SYNC SS
6
6
Programmable Maximum Duty-Cycle Input. Connect a resistor from MAXDTY to GND to set the maximum duty cycle for minimum system input voltages. The maximum duty cycle is inversely MAXDTY proportional to the system input voltage. The higher the input voltage, the less the maximum duty cycle. The maximum duty cycle is internally limited to 80% at any switching frequency. MAXDTY is high impedance during shutdown. Bypass MAXDTY with a 2200pF capacitor. EN SCOMP PRAMP OPTO ILIM SKTOFF CS SKTON GND DRV Active-Low Enable Input. Drive EN to GND to enable the IC. Drive high to place the IC in shutdown mode. Programmable Slope Compensation Input. Connect a resistor from SCOMP to GND to set the slope compensation. Programmable Voltage-Mode Control-Ramp Input. Connect a resistor from PRAMP to GND to set the magnitude of the voltage ramp. Isolated Feedback Input. Connect to an optocoupler for an isolated power converter or connect to the output of a voltage-error amplifier for a nonisolated power converter for a feedback-error signal. Current-Limit Threshold Input. Connect to the center of a resistor-divider from REF to GND to set the current-limit threshold voltage. Current-Limit Off-Time Input. Connect a capacitor from SKTOFF to GND to set the current-limit off-time. Pull SKTOFF to REF through a 10k resistor to latch off the IC after an overcurrent event. Current-Sense Input. Provides current-sense feedback for cycle-by-cycle current limit and is also the current-mode input for the MAX8540. Connect a current-sense resistor from CS to GND. Current-Limit On-Time Input. Connect a capacitor to ground to set the current-limit on-time. Ground Gate Drive for External N-Channel Power MOSFET. Connect to the gate of an external MOSFET for low-power applications. Connect to the input of an external gate driver for high-power applications. Voltage Supply for the IC. Operating input voltage range is 7.6V to 18V. Bypass VCC with a ceramic capacitor to GND.
7 8 -- 9 10 11 12 13 14 15
7 -- 8 9 10 11 12 13 14 15
16
16
VCC
8
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Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies
Detailed Description
Overvoltage Threshold
The MAX8540/MAX8541 include an overvoltage protection (OVP) feature that turns off the external MOSFET when the input voltage exceeds the user-set threshold. Connect a resistor-divider from the system input to GND with OV connected to the center to set the OVP trip point. The threshold voltage for OV is 3.021V (typ): R1 + R2 VIN(MAX) = x VOV R2 where VOV is the OV threshold, VIN(MAX) is the overvoltage trip point, R1 is the resistor from the system input to OV, and R2 is the resistor from OV to GND.
Table 1. Typical Specifications of UV/OV and the Actual On/Off Hysteresis of Input Voltage
UV VOLTAGE SPECIFICATIONS (V) MIN TYP MAX MIN TYP MAX 1.083 1.128 1.173 1.200 1.250 1.300 VIN OFF WINDOW (UTP) (V) 79.70 83.00 86.32 76.32 -- 79.50 82.68 -- -- VIN OFF WINDOW (LTP) (V) 29.76 31.00 32.24 32.97 34.34 36.00 VIN ON WINDOW (LTP) (V) -- VIN ON WINDOW (UTP) (V)
MAX8540/MAX8541
Undervoltage Threshold
The MAX8540/MAX8541 also include an undervoltage (UV) sensing input. The IC holds the external MOSFET low until UV reaches its threshold (1.25V typ). Once the threshold has been reached, the circuit enters soft-start and brings the output into regulation. Connect a resistordivider from the system input to GND with UV at the center to set the undervoltage protection (UVP) trip point. R3 + R4 VIN(MIN) = x VUV R4 where VUV is the UV threshold, VIN(MIN) is the UV trip point, R3 is the resistor from the system input to UV, and R4 is the resistor from UV to GND. An alternate method used in the application circuits for setting the OV and UV trip points is demonstrated in Figure 1. Use 36.5k for the bottom resistor (RC). RA and RB are calculated as follows: VOV x VIN(MIN) RB = RC x VUV x VIN(MAX) RA = RC x VIN(MIN) VUV RB
-
OV VOLTAGE SPECIFICATIONS (V) MIN TYP MAX MIN TYP MAX 2.901 3.021 3.142 2.778 2.894 3.010
Assume that 1.250V at UV/OV is scaled to 34.34V for a 48V bus.
Table 2. Switching Frequency Selection
RFREQ/SYNC (k) 48.7 19.1 15.8 11.0 8.87 SWITCHING FREQUENCY (kHz) 200 500 600 800 1000
1 RC
Switching Frequency and Synchronization
The MAX8540/MAX8541 oscillator operates in two modes: stand-alone or synchronized (sync). A single input, FREQ/SYNC, doubles as the attachment point for the frequency-programming resistor and as the synchronization input. The mode recognition is automatic, based on the signal applied to FREQ/SYNC. In stand-alone mode, an external resistor connected from FREQ/SYNC to GND sets the operating frequency. A 1.25V source is internally applied to FREQ/SYNC and the oscillator frequency is proportional to the current out of FREQ/SYNC through the programming resistor.
-
-
where VIN(MIN) is the UV trip point, VIN(MAX) is the OV trip point, VUV is the UV threshold (1.25V typ), and VOV is the OV threshold (3.021V typ). RA should consist of two equal-value resistors in series to protect against single-point failure.
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Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies MAX8540/MAX8541
The MAX8540/MAX8541 also synchronize with an external oscillator. Drive FREQ/SYNC with a square wave through a series diode with a positive pulse width of at least 200ns and a minimum pulse amplitude of 3V, plus the VF of the external diode. Alternatively, a 1.5V level-shifted external clock can be applied without the series diode. The maximum duty cycle of the external signal allowed is 55%. The MAX8540/MAX8541 synchronize to frequencies between 200kHz and 1MHz; however, the signal must be within 30% of the frequency set by the external resistor at FREQ/SYNC.
VIN 36V TO 76V C1 C2 C3
RA
Maximum Duty Cycle
Set the maximum duty cycle at the minimum system input voltage (V IN(MIN) ) connecting a resistor from MAXDTY to GND. The maximum duty cycle is inversely proportional to the voltage at UV. As the voltage on UV increases, the duty cycle decreases. The maximum duty cycle is internally limited to 80% at all switching frequencies. The MAXDTY resistor is determined as: RMAXDTY = DMAX x (97.6 x 103 ) 60
RC
RB OV UV
MAX8541
Figure 1. A Method to Set UV and OV Thresholds
where DMAX is the desired maximum duty cycle. The range of valid resistor values for RMAXDTY is from 24.3k to 130k.
N-Channel MOSFET Driver
The DRV output drives an N-channel MOSFET in lowpower applications. In high-power applications, the gate driver internal to the MAX8540/MAX8541 may not be capable of driving the external MOSFET efficiently and an external gate driver may be required. In this situation, connect DRV to the input of the external gate driver.
where dVRAMP/dt = 2.5V(fS), RSUM = 25k, N is the turns ratio of the primary to secondary, L1 is the output inductance, VOUT is the output voltage, fS is the switching frequency, and RCS is the current-sense resistance. For applications where a diode is used in the output instead of the synchronous rectifier, the slope compensation resistor is then equal to: SCF = N VOUT + VF RCS L1 dVRAMP R x SUM 2SCF dt
Slope Compensation (MAX8540)
The MAX8540 is a current-mode device and requires slope compensation for proper operation. To provide slope compensation, connect a resistor from SCOMP to GND (RSCOMP). The value of RSCOMP is determined as follows. For applications using a synchronous rectifier in the output, set the slope compensation equal to the negative slope of the output inductor. RSCOMP is equal to: SCF = N VOUT RCS L1
RSCOMP =
where dVRAMP/dt = 2.5V(fS),RSUM = 25k, N is the turns ratio of the primary to secondary, L1 is the output inductance, VOUT is the output voltage, VF is the diode voltage, fS is the switching frequency, and RCS is the currentsense resistance.
Voltage-Ramp Amplitude (MAX8541)
The MAX8541 is a voltage-mode device and features adjustable voltage ramp. Connect a resistor from PRAMP to GND (R PRAMP ) to set the voltage-ramp magnitude, VM. VM = 1.25V 25k RPRAMP
RSCOMP =
dVRAMP R x SUM 2SCF dt
10
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Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies
where RPRAMP is in k. The ramp-voltage magnitude is independent of frequency. The range of values for RPRAMP is from 12.4k to 42k. the ILIM threshold voltage. Once the voltage on SKTON reaches its threshold voltage, the MAX8540/MAX8541 begin skipping switching cycles for a time determined by the capacitance connected to SKTOFF. Once this time period has elapsed, the IC begins to switch for the time period set by the capacitance connected to SKTON. This process continues until the output short is removed. See the SKTON and SKTOFF section for details on setting the hiccup-mode periods. Connect SKTOFF to REF and SKTON to GND to disable the latched-mode and hiccup-mode protection, and operate continuously in cycle-by-cycle current-limit.
MAX8540/MAX8541
Soft-Start
The soft-start feature allows converters built using the MAX8540/MAX8541 to apply power to the load in a controllable soft ramp, thus reducing startup surges and stresses. It also determines power-up sequencing when several converters are used. Upon power turn-on, the soft-start pin acts as a current sink to discharge any capacitance connected to it. Once the voltage at VCC has exceeded its lockout value, softstart then charges the external capacitor (CSS), allowing the converter output voltage to ramp up. Full output voltage is reached in approximately 440ms/F.
PMW Comparator
The PWM comparator of the MAX8540 transforms the optocoupled error voltage (VOPTO) into a duty cycle by comparing the opto feedback-error voltage with a summed voltage. The summed voltage is the sum of the programmable slope compensation and the current-sense voltages. When the summed voltage exceeds the opto feedback error voltage, the gatedrive logic turns off the external MOSFET. The PWM comparator in the MAX8541 compares VOPTO with the programmable voltage ramp. When the voltage ramp exceeds V OPTO, the gate-driver logic turns off the external MOSFET.
Current Limit
The MAX8540/MAX8541 utilize two current-limit schemes: cycle-by-cycle current limit and short-circuit current limit. Set the current-limit threshold using a resistor-divider from REF to GND with ILIM connected to the center. The current-limit threshold is determined as: R5 VILIM = x VREF R5 + R6 where R5 is the resistor from ILIM to GND and R6 is the resistor from REF to ILIM. Use 240k for R16 and vary R23 to change the threshold. The CS signal provides feedback on the current ramp through the main external MOSFET. The voltage on CS is monitored by the IC. The cycle-by-cycle current limit abbreviates the on-time of the external MOSFET in the event that the voltage at CS is greater than the threshold voltage set by ILIM. The current-limit feature protects against a hard short or overcurrent fault at the output by one of two selected protection modes: by latching off the output, or pulsing the output to reduce the average output current (hiccup mode). To select latched mode, connect SKTOFF to REF. In this mode, if the hard short exists for the time period set by the capacitance at SKTON, the output is latched off. To unlatch the output, toggle EN or cycle the input power to VCC. To select hiccup mode, connect capacitors to SKTON and SKTOFF to program the hiccup mode on- and offtimes. When a cycle-by-cycle event is detected, the IC charges the capacitor at SKTON. The capacitor continues to charge as long as the CS voltage is greater than
SKTON and SKTOFF
The capacitance, CSKTON, determines the time period allowed before the short-circuit current limit initiates. Once the CS voltage exceeds the ILIM threshold, the capacitor at SKTON begins to charge. The capacitor continues to charge until the SKTON threshold voltage is reached or the overcurrent event is removed. This feature allows for the higher currents required during startup with high-capacitance loads. Set CSKTON to allow sufficient time for startup. The required capacitance at SKTON is determined as: CSKTON = tON / 103 where tON is in ms and CSKTON is in F. The allowable range for CSKTON is 100pF to 0.01F. The capacitance at SKTOFF determines the time period that the external MOSFET is turned off during an overcurrent event. Once the SKTON time period is exceeded, the SKTOFF capacitor charges. Once VSKTOFF reaches its threshold, the IC begins to switch again. CSKTOFF is determined as: CSKTOFF = tOFF / 103 where tOFF is in ms and CSKTOFF is in F. The allowable range for CSKTOFF is 1000pF to 1F.
______________________________________________________________________________________________________
11
Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies MAX8540/MAX8541
Pull VSKTOFF to VREF through a 10k pullup resistor to enable the latch-off feature. In this mode, once the SKTON time has elapsed, the IC is latched off. The circuit remains off until EN is toggled, or the input power is toggled.
Pin Configurations (continued)
TOP VIEW
REF 1 OV 2 UV 3 FREQ/SYNC 4 SS 5 MAXDTY 6 EN 7 PRAMP 8 16 VCC 15 DRV 14 GND
Soft-Start Capacitor Selection
During startup, the capacitor at soft-start is charged using a 5A current source. Once the voltage at softstart reaches the threshold voltage (2V typ), the IC switches normally. Use a low-ESR ceramic capacitor placed as close as possible to the IC at soft-start. The value is determined as follows: CSS = tSS 530
MAX8541EEE
13 SKTON 12 CS 11 SKTOFF 10 ILIM 9 OPTO
where tSS is the desired soft-start period in ms and CSS is in F.
QSOP
Applications Information
Refer to the following application notes for the MAX8540/MAX8541 application circuits and applications information: 50W Current-Mode Forward Converter Design with the MAX8540 and 50W VoltageMode Forward Converter Design with the MAX8541.
Chip Information
TRANSISTOR COUNT: 2704 PROCESS: BiCMOS
12
______________________________________________________________________________________
Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies
Functional Diagrams
MAX8540/MAX8541
VCC
VCC UVLO COMPARATOR
TEMP OK
VCC
MAX8540
8V
SS_RESET OVERVOLTAGE COMPARATOR
5V REFERENCE
REF
3.02V
ENABLE OV UV UNDERVOLTAGE COMPARATOR CLK ENABLE DRIVE LOGIC DRV
1.25V
EN CS 1.2V SKTOFF SKTON SHORT-CIRCUIT TIMER PWM COMPARATOR SLOPE COMPENSATION SCOMP
OPTO CS 80% MAX
ILIM SS MAX TON 5A SS_RESET REF OSC DECODER RAMP CLK
GND
MAXDTY
FREQ/SYNC
Figure 2. MAX8540 Functional Diagram ______________________________________________________________________________________ 13
Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies MAX8540/MAX8541
Functional Diagrams (continued)
VCC
VCC UVLO COMPARATOR
TEMP OK
VCC
MAX8541
8V
SS_RESET OVERVOLTAGE COMPARATOR
5V REFERENCE
REF
3.02V
ENABLE OV UV UNDERVOLTAGE COMPARATOR CLK ENABLE DRIVE LOGIC DRV
1.25V
EN CLK 1.2V SKTOFF SKTON SHORT-CIRCUIT TIMER PWM COMPARATOR VOLTAGE RAMP PRAMP
OPTO CS 80% MAX
ILIM SS MAX TON 5A SS_RESET REF OSC DECODER RAMP CLK
GND
MAXDTY
FREQ/SYNC
Figure 3. MAX8541 Functional Diagram 14 ______________________________________________________________________________________
Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies
Typical Operating Circuit
INPUT 36V TO 76V OUTPUT 2.5V AT 20A
MAX8540/MAX8541
IN
OUT FB VIN
MAX8515
PGND GND
OPTIONAL SYNCHRONIZATION
OV UV FREQ/SYNC SS REF U1 DRV VCC
MAX8540
CS MAXDTY SKTOFF SKTON
ILIM SCOMP
OFF ON
OPTO
EN
GND
______________________________________________________________________________________
15
Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies MAX8540/MAX8541
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
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.
16 ____________________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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