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SC198A
Dual DC-DC Buck Converter with High Current Capability
POWER MANAGEMENT Features
Input voltage range -- 2.7V to 5.5V Dual buck converters with independent control High efficiency -- over 90% peak Up to 800mA Load Programmable output voltages per channel -- 8 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
Description
The SC198A is a dual high-efficiency synchronous buck converter designed for use in low power applications. Each converter is capable of supplying up to 800mA of average current at one of eight programmable output voltages used as microprocessor supply voltages. The SC198A uses a single control logic block for the two converters, but their controls can be set independently. The device uses a flexible clocking method allowing the user to set 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 SC198A provides output voltage flexibility while conserving board space by using control pins to select the output voltage. This eliminates at least four parts from the traditional dual buck converter bill of materials.
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
VINA VBATT VINR VINB CIN ENA CTLA0 CTLA1 CTLA2 ENB CTLB0 CTLB1 CTLB2
SC198A
LXA VOUTA GNDA
4.7H
VOUTA
COUTA
4.7H LXB VOUTB GNDB GNDR COUTB
VOUTB
MODE/SYNC
September 26, 2007
1
SC198A
Pin Configuration
MODE/SYNC
Ordering Information
Device
SC198AMLTRT(1) (2)
ENB
Package
MLPQ-20 Evaluation Board
ENA
VINR
NC
SC198AEVB
20
19
18
17
16 15
VINA LXA GNDA VOUTA CTLA0
1 2 3 4 5 6 7 T 8 9 10
VINB LXB GNDB VOUTB CTLB0
Notes: (1) Available in tape and reel only. A reel contains 3,000 devices. (2) Lead-free package only. Device is WEEE and RoHS compliant.
TOP VIEW
14 13 12 11
CTLA1
MLPQ: 4mm x 4mm 20 Lead JA = 31C/W
Marking Information
yyww = Date Code xxxx = Semtech Lot No. xxxx = Semtech Lot No.
CTLA2
198A yyww xxxxx xxxxx
CTLB2
GNDR
CTLB1
2
SC198A
Absolute Maximum Ratings
VIN (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +7 VOUT (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to VIN +0.3 VLXA and VLXB (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-1 to VIN +1 Pin Voltage - All Other Pins (V) . . . . . . . . . -0.3 to (VIN + 0.3) Output Short Circuit to GND Duration . . . . . Continuous LXA and LXB Currents (A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 ESD Protection Level(1) (kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Recommended Operating Conditions
Ambient Temperature Range (C) . . . . . . . . . . . . . -40 to +85 VIN (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 to 5.5
Thermal Information
Thermal Resistance, Junction to Ambient(2) (C/W) . . . . 31 Maximum Junction Temperature (C) . . . . . . . . . . . . . . +150 Storage Temperature Range (C) . . . . . . . . . . . . -65 to +150 Peak IR Reflow Temperature (10s to 30s) (C) . . . . . . . +260
Exceeding the above specifications 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. NOTES: (1) Tested according to JEDEC standard JESD22-A114-B. (2) Calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards.
Electrical Characteristics
Unless otherwise noted: VIN = 3.6V, VMODE/SYNC = VIN, L = 4.7H, C = 10F, TA = -40C to +85C. Typical values are at TA = 25C for both outputs.
Parameter
Input Voltage Range
Symbol
VIN
Conditions
VOUT = 1.8V VOUT = 3.3V
Min
2.7 4.0 -3
Typ
Max
5.5
Units
V
5.5 3 %
VOUT Accuracy(1)
VOUT
IOUT = 10mA to 800mA , VIN 0.7V + VOUT VIN = 2.7 to 5.5V, VOUT = 1.8V, IOUT = 300mA
Line Regulation
VLINE VIN = 4.0 to 5.5V, VOUT = 3.3V, IOUT = 300mA IOUT = 10mA to 800mA, VOUT = 1.8V
0.2
%/V
Load Regulation
VLOAD IOUT = 10mA to 800mA, VOUT = 3.3V, VIN = 5.0V
0.0008
%/mA
Minimum Duty Cycle LXA, LXB Current Limit Quiescent Current Shutdown Current LXA and LXB Leakage Current P-Channel On Resistance
DCMIN ILIM IQ ISD IOUT = 0mA, SYNC = 0, ENA, ENB = VIN VENA, VENB = 0, LXA, LXB = Open VIN = 5.5V, VLX = 0V, PMOS ILXA, ILXB VIN = 5.5V, VLX = VIN, NMOS RDSP ILXA, ILXB = 100mA -2 0.1 0.35 0.1 0.1 1.2
18
% A
50 1 1
A A A A
3
SC198A
Electrical Characteristics (continued)
Parameter
N-Channel On Resistance 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)
Symbol
RDSN tSTART fOSC fSYNCU
Conditions
ILXA, ILXB = 100mA
Min
Typ
0.25
Max
Units
5 0.95 1.25 1.1 1.25
ms MHz MHz
fSYNCL
THI THYSR VUVL VUVLHYS VIH VIL 1.6 Decreasing VIN 2.4 145 10 2.5 50
0.75
MHz C C
2.6
V mV V
0.6
V
Notes: (1) See Output Voltage Options table in the Application Information section. (2) For ENA, ENB, SYNC, CTLA0, CTLA1, CTLA2, CTLB0, CTLB1, CTLB2.
4
SC198A
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 MODE/SYNC VINR NC ENA THERMAL PAD
Pin Function
Input power supply voltage for 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 1 -- Output Voltage Options). Voltage select control pin 1 for output A (see Table 1 -- Output Voltage Options). Voltage select control pin 2 for output A (see Table 1 -- Output Voltage Options). Ground for reference supply Voltage select control pin 2 for output B (see Table 1 -- Output Voltage Options). Voltage select control pin 1 for output B (see Table 1 -- Output Voltage Options). Voltage select control pin 0 for output B (see Table 1 -- Output Voltage Options). 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 -- Leave open Enable pin for output A Thermal pad for heatsinking purposes -- connect to ground plane using multiple vias -- not connected internally
5
SC198A
Block Diagram
SYNC 17 Oscillator and Slope Generator Error Amp A Error Amp B
VINA 1
15 VINB
VREF1 Current Amp P Limit Amp PWM Comp
VREF1 PWM Comp P Limit Amp
Current Amp
N Limit Amp LXA 2 VREF2
N Limit Amp
Switching Control Logic
VREF2
14 LXB
GNDA 3 20 4 5 Voltage Select VREF3 Error Amp A Error Amp B Voltage Select
13 GNDB 16 ENB 12 VOUTB 11 CTLB0 10 CTLB1 9 CTLB2 Voltage References VREF1 VREF2 VREF3 8 GNDR
ENA VOUTA CTLA0
CTLA1 6 CTLA2 7 VINR 18
VREF3
6
SC198A
Typical Characteristics
Efficiency vs. Load at VIN = 2.7V, Side A
100 90 80 70
Efficiency vs. Load at VIN = 3.6V, Side A
100 90
VOUT = 1.8V PSAVE VOUT = 1.0V PSAVE
VOUT = 1.8V PSAVE VOUT = 1.0V PSAVE
80 70
Efficiency (%)
Efficiency (%)
60 50 40 30 20 10 0
VOUT = 1.8V PWM VOUT = 1.0 PWM
60 50 40
VOUT = 1.8V PWM
VOUT = 1.0 PWM
30 20 10 0
0.001
0.01
0.1
1
Load (A)
0.001
0.01
0.1
1
Load (A)
Efficiency vs. Load at VIN = 5.5V, Side A
100 90 80 70
100
Efficiency vs. Load at VIN = 5.5V, Side B
VOUT = 1.8V PSAVE
90 80
VOUT = 1.8V PSAVE
VOUT = 1.0V PSAVE
VOUT = 1.0V PSAVE
Efficiecny (%)
70 60
Efficiecny (%)
60 50 40 30 20 10 0 0.001 0.01 0.1 1
VOUT = 1.8V PWM
50 40 30 20 10 0 0.001 0.01 0.1 1
VOUT = 1.8V PWM
VOUT = 1.0 PWM
VOUT = 1.0 PWM
Load (A)
Load (A)
Line Regulation at 300mA
0.2
3
Load Regulation -- VOUT=1.8V, VIN=3.6V
0.15
VOUT = 1.8V PWM
2
0.1
VOUT = 1.0 PWM
Output (%)
Output (%)
0.05
1
VOUT = 1.8V PWM
0
0
-0.05
-1
VOUT = 1.0 PWM
-0.1
-2
-0.15
-3
-0.2 2.7 3.26 3.82 4.38 4.94 5.5
0.001
0.01
0.1
1
Input (V)
Load (A)
7
SC198A
Typical Characteristics (continued)
VOUT vs. VIN -- 3.3V Programmed Output
4
VOUT vs. VIN -- 1.8V Programmed Output
1.854
IOUT = 300mA
3.5
IOUT = 300mA
3
1.836
IOUT = 800mA
IOUT = 800mA
1.818
Output (V)
Output (V)
2.5
2 1.5
1.8
1.782
1
0.5
1.764
0 2.7 3.26 3.82 4.38 4.94 5.5
1.746 2.7 3.26 3.82 4.38 4.94 5.5
Input (V)
Input (V)
Crosstalk in PSAVE
VIN=5.0V, 25C, VOUTA = 1.0V, IOUTA = 80mA, VOUTB = 3.3V, IOUTB = 0mA
Crosstalk -- Side A (PSAVE) Side B (PWM)
VIN=5.0V, 25C, VOUTA = 1.0V, IOUTA = 80mA, VOUTB = 3.3V, IOUTB = 800mA
(50mV/div)
(100mV/div)
(50mV/div)
(100mV/div)
10s
10s
Transient in PSAVE
VOUT=1.0V, VIN=3.6V, 25C, IOUT = 0 to 800mA (5.0mV/div) (5.0mV/div)
Transient in PWM
VOUT=1.8V, VIN=3.6V, 25C, IOUT = 0 to 800mA
(200mV/div)
(200mV/div)
(200mV/div)
(200mV/div)
(1.0A/div) Time (200s/div)
(1.0A/div) Time (200s/div)
8
SC198A
Typical Characteristics (continued)
Switching Waveforms -- VOUT=1.0V
VIN=3.6V, 25C, IOUT = 800mA
Switching Waveforms -- VOUT=1.8V
VIN=3.6V, 25C, IOUT = 800mA
(2V/div)
(2V/div)
(10mV/div)
(10mV/div)
Time (400ns/div)
Time (400ns/div)
Switching Waveforms in PSAVE
VOUT=1.0V, VIN=3.6V, 25C, IOUT = 0mA
Switching Waveforms in PSAVE
VOUT=1.8V, VIN=3.6V, 25C, IOUT = 80mA
(2V/div) (2V/div)
(10mV/div) (50mV/div)
Time (20ms/div)
Time (4s/div)
9
SC198A
Application Information
Detailed Description
The SC198A 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 on-time 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 MODE/SYNC pin. When synchronizing to an external frequency, the SC198A 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 SC198A converter also has internal loop compensation. The internal compensation is designed to work with a specific output filter corner frequency defined by the equation
fC 1 L COUT
Programmable Output Voltage The SC198A has three control pins per output to allow selection of a voltage level from eight predetermined output voltage levels (see Table 1). CAUTION -- All CTL pins much be pulled high or low for proper operation (they must not be allowed to float). The output voltage can be selected while the device is enabled and loaded. Table 1 -- Output Voltage Options
Programmable Output Voltage Nominal Output Voltage (V) A
0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.8
CTLA2/B2
CTLA1/B1
CTLA0/B0
B
1.8 2.2 2.5 2.7 2.8 2.9 3.0 3.3
2
Continuous Conduction and Oscillator Synchronization The SC198A is designed to operate in continuous conduction mode, where it maintains a fixed frequency. When the MODE/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 MODE/SYNC pin. The part synchronizes to the rising edge of the clock. Power-Save Mode Operation The SC198A has a power-save mode (PSAVE) which may be selected by tying the MODE/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 device will automatically detect the load current at which it should enter PSAVE mode. The device is optimized to track maximum efficiency.
When selecting output filter components, the LC product should not vary over a wide range. The practical lower limit for the inductor value is 4.7H to ensure system stability. Selection of smaller inductor and capacitor values will shift the corner frequency higher, which may impact stability. The internal compensation is designed to operate with a single-pole output filter where L = 4.7H and COUT = 10F.
10
SC198A
Application Information (continued)
In PSAVE mode (see Figure 1), 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. When the voltage drops to the lower threshold, the switch bursts at a fixed on time until the upper threshold is reached. At this point the output voltage is allowed to ramp down to the lower threshold without switching to conserve power. In the PSAVE mode, less switching is used. This results in less power loss which can be seen in the difference of efficiency between PWM mode and PSAVE mode at light loads.
PSAVE Mode at Moderate Load Upper Thershold Limit Lower Thershold Limit VOUT
BURST OFF BURST Higher Load Applied OFF
Protection Features
The SC198A provides the following protection features:
* * * *
Thermal Shutdown Current Limit Over-Voltage Protection Soft-Start
PSAVE Mode at High Load
BURST
PWM Mode at High Load
PWM Mode
Thermal Shutdown The SC198A has a thermal shutdown feature to protect against damage if the junction temperature exceeds 145C. In thermal shutdown, the on-chip power devices are disabled, tri-stating the LX output. Switching resumes 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 internal power devices are protected by current limit functions. In the event of a short to ground on the output, the part enters frequency foldback mode. This causes the switching frequency to decrease by a factor determined by the output voltage. This prevents the inductor current from staircasing. Over-Voltage Protection In the event of an over-voltage on the output in switcher mode, the PWM drive is disabled, effectively tri-stating the LX output. The part will not resume switching until the output voltage has fallen below 2% of the regulation voltage. 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 in discrete steps. This has the desired effect of limiting in-rush current from the input supply. The softstart mode is cancelled once the output reaches regulation. Soft-start is only re-enabled by power cycling, toggling enable, a UVLO event, or shutdown.
PSAVE Exit Threshold
Inductor Current
0A
Time
Figure 1 -- PSAVE Operation The PSAVE switching burst frequency is controlled such that the inductor current ripple during the burst is similar to that in PWM mode. The SC198A 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 SC198A 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 SC198A exits PSAVE. Figure 1 shows PSAVE mode operation and exiting into PWM mode at increased load.
11
SC198A
Applications Information (continued)
Inductor Selection
The SC198A 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 is determined by the equation
VOUT VOUT 1 L fosc VIN
Output Capacitor Output voltage ripple is dominated by the filter capacitance ESR as shown in the equation VOUT(P-P) = DIL x ESRCOUT 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. When selecting capacitors for the application, first consider the DC voltage characteristic of the capacitor. The capacitance value at the DC operating voltage may be considerably lower than the rated value. Table 3 lists recommended capacitor values which have been chosen to minimize the impact of this limitation. A 10F ceramic capacitor is the minimum recommended value for the output filter capacitor. It is important to ensure the capacitor value does not drop below 9F for the operating range of the application. Table 3 -- Recommended Capacitors
Manufacturer/ Part No. Murata GRM21BR60J226ME39 Murata GRM188R60J106ME47 Murata GRM21BR60J106KE01B Value (F) Rated Voltage (VDC) 6.3 6.3 6.3 Type Case Size
IL
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 equation
IL 2
IL(Peak)
IOUT(MAX)
Final inductor selection will depend on various design considerations such as efficiency, EMI, size, and cost. Table 2 lists some manufacturers of practical inductor options. Table 2 -- Recommended Inductors
Manufacturer/ Part No. Coilcraft LPO6610-472ML Coilcraft 1812PS-472KLB Coilcraft D01608C-472ML Value (H) 4.7 DCR () 0.20 Rate Tolerance Current (+%) (A) 1.1 20 Dimensions LxWxH (mm) 6.60 x 5.50 x 1.10
22 10 10
X5R X5R X5R
0805 0603 0805
4.7
0.11
2.10
20
5.87 x 4.89 x 3.81
4.7
0.09
1.5
20
6.60 x 4.50 x 13.0
Capacitor Selection
Input Capacitor The source input current to a buck converter is non-continuous. 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.
12
SC198A
Applications Information (continued)
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
GND CinA LXA
CinR
GND CinB
LA
VOUTA
CoutA
GND
GND
CoutB
VOUTB
LB
LXB
13
SC198A
Outline Drawing - MLPQ-20 4x4
A D B DIM DIMENSIONS MILLIMETERS INCHES MIN NOM MAX MIN NOM MAX . 031 . 035 . 040 0. 80 0. 90 1. 00 A A 1 . 000 . 001 . 002 0. 00 0. 02 0. 05 - (0.20) - (.008) A2 b . 007 . 010 . 012 0. 18 0. 25 0. 30 D . 153 . 157 . 161 3. 90 4. 00 4. 10 D 1 . 100 . 106 . 110 2. 55 2. 70 2. 80 E . 153 . 157 . 161 3. 90 4. 00 4. 10 E1 . 100 . 106 . 110 2. 55 2. 70 2. 80 e . 020 BSC 0. 50 BSC L . 011 . 016 . 020 0. 30 0. 40 0. 50 20 20 N aaa . 004 0. 10 bbb . 004 0. 10
PIN 1 INDICATOR (LASER MARK )
E
A2 A aaa C A1 D1 LxN E/2 E1 2 1 C
SEATING PLANE
N bxN bbb e D/2 NOTES : 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS CAB
14
SC198A
Land Pattern - MLPQ-20 4x4
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 Mangement Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805) 498-2111 Fax: (805) 498-3804 www.semtech.com
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