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Dual DC-DC Buck Converter with High Current Capability
POWER MANAGEMENT Description
The SC198 is a high-efficiency, dual synchronous buck converter designed for use in low power applications. Each converter is capable of supplying up to 600mA of average current at one of eight programmable output voltages commonly used as microprocessor supply voltages. The SC198 uses a single control logic block for the two converters, but their controls can be set independently. The SC198 employs a flexible clocking scheme that allows the user to use the fixed internal 1MHz oscillator or synchronize the clock to an external source. The device also has an optional power-save mode that optimizes efficiency for light loads. The SC198 provides output voltage flexibility while conserving board space by using control pins to select the output voltage instead of external resistor voltage divider circuits. This eliminates at least four parts from the traditional dual buck converter bill of materials.
SC198
Features
Dual Buck Converters with Independent Control High Efficiency - Over 90% Peak 8 Programmable Output Voltages Input Voltage Range Between 2.7V to 5.5V Fixed-Frequency Operation - 1MHz Optional SYNC Clock Input Optional Power-Save Under Light-Load Conditions Shutdown Current <1A Soft-Start to Limit In-Rush Current 4mm x 4mm MLPQ Package
Applications
Mobile Phones Cordless Phones MP3 Players Digital Cameras PDAs Single Li-ion Cell or (3) NiMH/NiCd Cell Devices Digital Multimedia Broadcast (DMB) Devices Portable Gaming Systems
Typical Application Circuit
SC198
LXA VOUTA GNDA
1 18 15
10F
VINA VINR VINB ENA CTLA0 CTLA1 CTLA2 ENB CTLB0 CTLB1 CTLB2 SYNC
2 4 3
4.7H
VOUTA
10F
20 5 6 7 16 11 10 9 17
LXB VOUTB GNDB GNDR
14 12 13 8
4.7H
VOUTB
10F
October 19, 2006
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SC198
POWER MANAGEMENT Absolute Maximum Ratings
Exceeding the specifications below may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended.
Parameter VIN EN, SYNC Input LXA and LXB Voltages VOUT CTLA0, CTLA1, CTLA2, CTLB0, CTLB1 & CTLB2 Voltages Output Short Circuit to GND LXA and LXB Currents Thermal Impedance (Junction to Ambient) Operating Ambient Temperature Maximum Junction Temperature Storage Temperature Reflow Solder Temperature ESD Protection Level(2)
(1)
Symbol VIN VEN ,VSYNC VLXA, VLXB VOUT tSC ILXA and ILXB JA TA TJ TS TPKG VESD
Maximum -0.3 to 7 -0.3 to VIN +0.3 -1 to VIN +1 -0.3 to VIN +0.3 -0.3 to VIN +0.3 Indefinite 1.2 40 -40 to +85 150 -65 to +150 260 2
Units V V V V V s A C/W C C C C kV
Note: (1) Calculated from package in still air, mounted to 3" x 4.5", 4 layer FR4 PCB with thermal vias under the exposed pad as per JESD51 standards. (2) Tested according to JEDEC standard JESD22-A114-B.
Electrical Characteristics
Unless otherwise noted: VIN = 3.6V, SYNC = VIN, L = 4.7H, C = 10F, TA = -40C to +85C. Typical values are at TA = 25C.
Parameter Input Voltage Range VOUT Accuracy(1) Line Regulation Load Regulation Minimum Duty Cycle LXA, LXB Current Limit Quiescent Current Shutdown Current LXA and LXB Leakage Current P-Channel On Resistance N-Channel On Resistance
(c) 2006 Semtech Corp.
Symbol VIN VOUT VLINEREG VLOADREG DCMIN ILIM IQ ISD ILXA, ILXB RDSP RDSN
Conditions
Min 2.7
Typ
Max 5.5 3 0.2 0.0008 18
Units V % %/V %/mA % A
IOUT = 10mA to 600mA VIN = 2.7 to 5.5V, VOUT = 1.8V, IOUT = 300mA IOUT = 10mA to 600mA
-3
0.9 IOUT = 0mA, SYNC = 0, ENA, ENB = VIN ENA, ENB = 0, LXA, LXB = Open VIN = 5.5V, LX = 0V, PMOS VIN = 5.5V, LX = VIN, NMOS ILXA, ILXB = 100mA ILXA, ILXB = 100mA
2
50 0.1 0.1 -2 0.1 0.35 0.25 1 1
A A A A
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SC198
POWER MANAGEMENT Electrical Characteristics (Cont.)
Parameter Start-Up Time Oscillator Frequency SYNC Frequency (upper) SYNC Frequency (lower) Thermal Shutdown Thermal Shutdown Hysteresis UVLO Threshold UVLO Hysteresis Logic Input High(2) Logic Input Low(2) Logic Input High(2) Logic Input Low(2) Symbol tSTART fOSC fSYNCU fSYNCL THI THYSR VUVL VUVLHYS VIH VIL IIH IIL -2 -2 0.1 0.1 1.6 0.6 2 2 Decreasing VIN 2.4 145 10 2.5 50 2.6 0.85 1.25 0.75 1.1 Conditions Min Typ Max 5 1.25 Units ms MHz MHz MHz C C V mV V V A A
Notes: (1) See Output Voltage Options table in the Application Information section. (2) For ENA, ENB, SYNC, CTLA0, CTLA1, CTLA2, CTLB0, CTLB1, CTLB2.
(c) 2006 Semtech Corp. 3
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SC198
POWER MANAGEMENT Electricalguration Pin Confi Characteristics (Cont.)
VINR ENA SYNC ENB
Ordering Information
DEVICE PACKAGE(
2)(1) 1)
NC
SC198MLTRT((1)(2) SC198EVB
15
MLPQ-20 Evaluation Board
20
19
18
17
16
VINA LXA GNDA VOUTA CTLA0
1 2 3 4 5 6 7 T 8 9 10
VINB LXB GNDB VOUTB CTLB0
TOP VIEW
14 13 12 11
Notes: (1) Available in tape and reel only. A reel contains 3,000 devices. (2) Available in lead-free package only. Device is WEEE and RoHS compliant.
CTLA1
CTLA2
MLPQ20: 4X4 20 LEAD
Marking Information
SC198 yyww xxxxx xxxxx
yy = two digit year of manufacture ww = two digit week of manufacture xxxxx = lot number
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CTLB2
CTLB1
GNDR
SC198
POWER MANAGEMENT Pin Descriptions
Pin # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 T Pin Name VINA LXA GNDA VOUTA CTLA0 CTLA1 CTLA2 GNDR CTLB2 CTLB1 CTLB0 VOUTB GNDB LXB VINB ENB SYNC VINR NC ENA THERMAL PAD Pin Function Input power supply voltage converter A Connection point for the inductor on converter A Ground reference to converter A Feedback control input for converter A Voltage select control pin 0 for output A (see Table A -- Output Voltage Options).(1) Voltage select control pin 1 for output A (see Table A -- Output Voltage Options). (1) Voltage select control pin 2 for output A (see Table A -- Output Voltage Options). (1) Ground for reference supply Voltage select control pin 2 for output B (see Table A -- Output Voltage Options). (1) Voltage select control pin 1 for output B (see Table A -- Output Voltage Options). (1) Voltage select control pin 0 for output B (see Table A -- Output Voltage Options). (1) Feedback control input for converter B Ground reference for converter B Connection point for the inductor on converter B Input power supply voltage for converter B Enable pin for output B Oscillator synchronization input - connect to VIN for forced PWM mode, ground for power-save mode or connect to an external clock for frequency synchronization. Reference supply input Not connected Enable pin for output A Thermal pad for heatsinking purposes -- connect to ground plane using multiple vias -- not connected internally
Note: (1) All CTL pins must be pulled high or low for proper operation.
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SC198
POWER MANAGEMENT Block Diagram
SYNC
VINA
Oscillator and Slope Generator Error Amp A Current Amp P Limit Amp VREF1 PWM Comp PWM Comp Error Amp B VREF1 P Limit Amp Current Amp
VINB
N Limit Amp LXA VREF2
N Limit Amp
Switching Control Logic
LXB VREF2
GNDA
GNDB
ENA
ENB
VOUTA CTLA0 Voltage Select CTLA1 CTLA2 VREF1 VINR Voltage References VREF2 VREF3 Error Amp A Error Amp B
VOUTB CTLB0 Voltage Select CTLB1 CTLB2
VREF3
VREF3
GNDR
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SC198
POWER MANAGEMENT Applications Information
SC198 Detailed Description The SC198 is a dual step-down, pulse-width modulated DC-DC converter with programmable output voltage and power-save mode. This device has a 1MHz internal oscillator that can be used during forced PWM mode or power-save mode (PSAVE). Operation During normal operation, the PMOS switch is activated on each rising edge of the internal oscillator. Current feedback for the switching regulator is through the PMOS current path, and it is amplified and summed with the internal slope compensation network. The voltage feedback loop uses an internal feedback divider. The ontime is determined by comparing the summed current feedback and the output of the error amplifier. The period is set by the onboard oscillator or by an external clock attached to the SYNC/PWM pin. When synchronizing to an external frequency, the SC198 operates in PWM mode only. The device has an internal synchronous NMOS rectifier and does not require a Schottky diode on the LX pin. The device is designed to operate as a buck converter in PWM mode with a fixed frequency of 1MHz. At light loads the part can enter PSAVE mode to maximize efficiency. Output Filter The SC198 converter also has internal loop compensation. The internal compensation is designed to work with a certain output filter corner frequency defined by the equation: 1 fC = ---------- ------ 2 LxC When selecting output filter components, the LC product should not vary over a wide range. Selection of smaller inductor and capacitor values will move the corner frequency, potentially impacting system stability. Due to this issue the practical lower limit for the inductor value is 4.7H. The internal compensation is designed to operate with a single-pole output filter, L = 4.7H and COUT = 10F. Programmable Output Voltage The SC198 has three control pins per output to allow the user to select a voltage level from eight predetermined output voltage levels (see Table A, Output Voltage Options below). The output voltage can be selected at any time, including while the output is enabled and loaded. Table A -- Output Voltage Options
Programmable Output Voltage CTLA2/B2 0 0 0 0 1 1 1 1 CTLA1/B1 0 0 1 1 0 0 1 1 CTLA0/B0 0 1 0 1 0 1 0 1 Nominal Output Voltage (V) 1 1.2 1.375 1.4 1.5 1.575 1.8 1.875
Continuous Conduction and Oscillator Synchronization The SC198 is designed to operate in a continuous conduction mode, where it maintains a fixed frequency. When the SYNC pin is tied high, the part runs under control of the internal oscillator in PWM mode. The part can be synchronized to an external clock by connecting a clock signal to the SYNC pin. The part synchronizes to the rising edge of the clock. Power-Save Mode Operation The SC198 has a power-save mode (PSAVE) which may be selected by tying the SYNC pin to GND. Selecting PSAVE mode enables automatic activation of PSAVE mode at light loads, thus maximizing efficiency across the full load range. The SC198 will automatically detect the load current at which it should enter PSAVE mode. The SC198 is optimized to track maximum efficiency with respect to VIN.
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SC198
POWER MANAGEMENT Applications Information (Cont.)
Upper Threshold Limit Lower Threshold Limit
VOUT
BURST OFF Higher Load Applied
Protection Features The SC198 provides the following protection features: Thermal Shutdown Current Limit Over-Voltage Protection Soft-Start Thermal Shutdown The device has a thermal shutdown feature to protect the SC198 if the junction temperature exceeds 145C. In thermal shutdown the on-chip power devices are disabled, tri-stating the LX output. Switching will resume when the temperature drops by 10C. During this time if the output voltage decreases by more than 60% of its programmed value, a soft-start will be invoked. Current Limit The PMOS and NMOS power devices of the buck switcher stage are protected by current limit functions. In the case of a short to ground on the output, the part enters frequency foldback mode, that causes the switching frequency to divide by a factor determined by the output voltage. This prevents the inductor current from "staircasing". Over-Voltage Protection Over-voltage protection is provided on the SC198. Hysteresis is provided to prevent toggling in and out of operation. Soft-Start The soft-start mode operates by limiting the current through the inductor and controlling the switching frequency. The current limit is increased over several milliseconds the in discrete steps. This has the desired effect of limiting inrush current from the input supply. The soft-start mode is cancelled once the output reaches regulation. Soft-start is only re-enabled by power cycling, toggling enable, UVLO event, or shutdown.
PSAVE Mode at Light Load
PWM Mode at Medium/ High Load
PSAVE Exit Threshold
Inductor Current
0A
Time
PSAVE Operation In PSAVE mode, VOUT regulation is controlled by bursts in switching. While the output voltage is between the low and high voltage thresholds, switching is stopped to reduce loss in the regulator due to switching. When the voltage drops to the lower threshold, the switch bursts at a fixed on time until the upper threshold is met. At this point the output voltage is allowed to ramp down to the lower threshold again without switching to conserve power. In the PSAVE mode, less switching is used. Less switching results in less power loss which can be seen in the difference of efficiency between PWM mode and PSAVE mode at light loads. The PSAVE switching burst frequency is controlled such that the inductor current ripple during the burst is similar to that in PWM mode. The minimum switching frequency during this period is limited to 650kHz. The SC198 automatically detects when to exit PSAVE mode. The device sets a maximum peak current in PSAVE mode, and then detects when VOUT falls due to the load increasing above the level that the peak current will support. For the SC198 to exit PSAVE mode, the load must be increased, which causes VOUT to decrease until the PSAVE exit threshold is reached. PSAVE levels are set high to minimize the undershoot when the SC198 exits PSAVE.
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SC198
POWER MANAGEMENT Applications Information (Cont.)
Inductor Selection The SC198 was designed for optimum performance when using a 4.7H inductor. The magnitude of the inductor current ripple is dependent on the inductor value and can be determined by the following equation: VOUT VOUT IL = ------ 1 + ------ L x fOSC VIN Capacitors with X7R or X5R ceramic dielectric are strongly recommended for their low ESR and superior temperature and voltage characteristics. Y5V capacitors should not be used as their temperature coefficients make them unsuitable for this application. Table C lists some manufacturers of recommended capacitor options.
(
)
This equation demonstrates the relationship between input voltage, output voltage, and inductor ripple current. The inductor should also have a low DCR to minimize the conduction losses and maximize efficiency. The minimum DC current rating of the inductor should be equal to the maximum load current plus half of the inductor current ripple as shown by the following equation: IL IL(PK) = IOUT(MAX) + ---- 2 Final inductor selection will depend on various design considerations such as efficiency, EMI, size and cost. Table B lists some manufactuers of practical inductor options. Capacitor Selection Input Capacitor The source input current to a buck converter is non-ontinuous. To prevent large input voltage ripple a low ESR ceramic capacitor is required. A minimum value of 4.7F should be used for sufficient input voltage filtering and a 22F MLCC may be used for optimum input voltage filtering. Output Capacitor A 4.7F ceramic capacitor is the minimum recommended for the output filter capacitor. A capacitor value of at least 10F is recommended if the part will be used in powersave mode. Output voltage ripple is dominated by the filter capacitance ESR as shown in the following equation: VOUT(P-P) = IL x ESRCOUT
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SC198
POWER MANAGEMENT Applications Information (Cont.)
Table B -- Recommended Inductors
Manufacturer Part Number LPO6610-472ML(1) 1812PS-472KLB VLCF4018T- 4R7N1R0-2 HM66304R7 Value H 4.7 4.7 4.7 4.7 DCR 0.20 0.11 0.101 0.072 Rated Current A 1.1 2.10 1.07 1.32 Tolerance % 20 20 30 20 Dimensions (LxWxH) mm 6.60 x 5.50 x 1.10 5.87 x 4.89 x 3.81 4.3 x 4.0 x 1.8 4.7 x 4.7 x 3.0
Coilcraft TDK BI Technologies
Note: (1) This inductor was used while obtaining data for the Typical Characteristics Section.
Table C -- Recommended Capacitors
Manufacturer Part Number GRM 21BR60J226ME39L Murata GRM188R60J106MKE19 GRM188R60G106ME47 TDK C2012X5R0J106K Value F 22 10 4.7 10 Rated Voltage VDC 6.3 6.3 6.3 6.3 Type % X5R X5R X5R X5R Case Size 0805 0603 0603 0603
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SC198
POWER MANAGEMENT Applications Information (Cont.)
PCB Layout Considerations Poor layout can degrade the performance of the DC-DC converter and can be a contributory factor in EMI problems, ground bounce, thermal issues, and resistive voltage losses. Poor regulation and instability can result. The following design rules are recommended: 1. Place the inductor and filter capacitors as close to the device as possible and use short wide traces between the power components. 2. Route the output voltage feedback path away from inductor and LX node to minimize noise and magnetic interference. 3. Use a ground plane to further reduce noise interference on sensitive circuit nodes.
VIN
CinR CinA CinB
LXA SC198 GND
LXB
GND
VOUTA
VOUTB
Note: Control pin routings are not shown.
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SC198
POWER MANAGEMENT Typical Characteristics
Efficiency vs. Load @ 2.7VIN
100 90 80 70
Efficiency vs. Load @ 3.6VIN
100 90 80 70
Efficiency (%)
Efficiency (%)
60 50 40 30 20 10
VOUT = 1V PSAVE
VOUT = 1.8V PSAVE
60 50 40 30 20 10
VOUT = 1V PWM VOUT = 1V PSAVE
VOUT = 1.8V PSAVE
VOUT = 1.8V PWM
VOUT = 1.8V PWM
VOUT = 1V PWM
ENA=ENB=VIN
0 0.001
ENA=ENB=VIN
0.01
0.1
1
0 0.001
0.01
0.1
1
Load (A)
Load (A)
Efficiency vs. Load @ 5.5VIN
100 90
Normalized Frequency (kHz)
Oscillator Frequency vs. Input Voltage
4.0
80 70
2.0
85C
0.0
Efficiency (%)
60
VOUT = 1.8V PSAVE
50 40 30 20 10 0 0.001
VOUT = 1V PWM
ENA=ENB=VIN
-2.0
25C -40C
VOUT = 1V PSAVE VOUT = 1.8V PWM
-4.0
-6.0
0.01
0.1
1
-8.0 2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Load (A)
Input (V)
Line Regulation at 300mAOUT
0.2 0.15 0.1
VOUT = 1V
Load Regulation at 3.6VIN (PWM)
1 0.8 0.6 0.4
VOUT = 1.8V
Output (%)
Output (%)
0.05 0
VOUT = 1.8V
0.2 0 -0.2 -0.4
VOUT = 1V
-0.05 -0.1 -0.15 -0.2 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
-0.6 -0.8 -1 0.001
0.01
0.1
1
Input (V)
Load (A)
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SC198
POWER MANAGEMENT Typical Characteristics (Cont.)
Efficiency vs. Line @ VOUT = 1.8V
100
30mA PSAVE
Efficiency vs. Line @ VOUT = 1V
100
30mA PSAVE
90
90
Efficiency (%)
Efficiency (%)
80
300mA Load
80
70
30mA PWM
70
30mA PWM
60
60
300mA Load
50 2.7 3.7 4.7
50 2.7 3.7 4.7
Input (V)
Input (V)
Output Ripple for VIN = 3.6V and VOUT = 1.8V @ IOUT = 50mA
20mV/div
VIN
Output Ripple for VIN = 3.6V and VOUT = 1.8V @ IOUT = 10mA
20mV/div
VIN
10mV/div
VOUT
10mV/div
VOUT
2.0V/div
2.0V/div
LX LX
400ns/div
400ns/div
PSAVE @ 50mA Load
50mV/div
VIN
PSAVE @ 30mA Load
50mV/div
VIN
50mV/div
VOUT
50mV/div
VOUT
2V/div
LX
2V/div
LX
2ms/div
(c) 2006 Semtech Corp. 13
2ms/div
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SC198
POWER MANAGEMENT Typical Characteristics (Cont.)
PSAVE @ 50mA Load with COUT = 22F
50mV/div
VIN
Transient Step in PSAVE
200mV/div
VIN
50mV/div
VOUT
200mV/div
VOUT
5.0V/div 2.0V/div
LX LX
200mA/div
IOUT
4.0s/div
400s/div
Input Transient Step from 3.1V to 3.6V
Input Transient Step from 3.6V to 3.1V
500mV/div 500mV/div
VIN
VIN
20mV/div
VOUT
20mV/div
VOUT
1ms/div Output Transient @ PWM
50mV/div
VOUTA
1ms/div
Output Transient in PSAVE
50mV/div
VOUTA
C
2V/div
CTL0 1 and 2
2V/div
CTL0 1 and 2
2ms/div
(c) 2006 Semtech Corp. 14
2ms/div
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SC198
POWER MANAGEMENT Outline Drawing - MLPQ-20 4 x 4
A
D
B
DIM
A A1 A2 b D D1 E E1 e L N aaa bbb
SEATING PLANE A1 C D1 LxN E/2 E1 2 1
PIN 1 INDICATOR (LASER MARK)
E
A2 A aaa C
.031 .035 .040 .000 .001 .002 - (.008) .007 .010 .012 .153 .157 .161 .100 .106 .110 .153 .157 .161 .100 .106 .110 .020 BSC .011 .016 .020 20 .004 .004
DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX
0.80 0.90 1.00 0.00 0.02 0.05 - (0.20) 0.18 0.25 0.30 3.90 4.00 4.10 2.55 2.70 2.80 3.90 4.00 4.10 2.55 2.70 2.80 0.50 BSC 0.30 0.40 0.50 20 0.10 0.10
N bxN bbb e D/2 CAB
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
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SC198
POWER MANAGEMENT Land Pattern - MLPQ-20 4 x 4
K
DIMENSIONS DIM C G H K P X Y Z INCHES (.155) .122 .106 .106 .021 .010 .033 .189 MILLIMETERS (3.95) 3.10 2.70 2.70 0.50 0.25 0.85 4.80
(C)
H
G
Z
Y X P
NOTES:
1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET.
Contact Information
Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805) 498-2111 Fax: (805) 498-3804
www.semtech.com
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