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Low Power Dual Synchronous DC/DC Controller With Current Sharing Circuitry
POWER MANAGEMENT Description
The SC1175 is a versatile 2 phase, synchronous, voltage mode PWM controller that may be used in two distinct ways. First, the SC1175 is ideal for applications where point of use output power exceeds any single input power budget. Alternatively, the SC1175 can be used as a dual switcher. The SC1175 features a temperature compensated voltage reference, over current protection with 50% fold-back and internal level-shifted, high-side drive circuitry. In current sharing configuration, the SC1175 can produce a single output voltage from two separate voltage sources (which can be different voltage levels) while maintaining current sharing between the channels. Current sharing is programmable to allow loading each input supply as required by the application. In dual switcher configuration, two feedback paths are provided for independent control of the separate outputs. The device will provide a regulated output from flexibly configured inputs (3.3V, 5V, 12V), provided 5V is present for VCC. The two switchers are 180 out of phase to minimize input and output ripple.
SC1175
Features
u u u u u u u u u u u u u u u u u u
300kHz fixed frequency operation Soft Start and Enable function Power Good output provided Over current protection with 50% fold-back Phase-shifted switchers minimize ripple High efficiency operation, >90% Programmable output(s) as low as 1.25V Industrial temperature range SOIC 20 pin package
Two Phase, Current Sharing Controller Flexible, same or separate VIN Programmable current sharing Combined current limit with fold-back 2 phases operating opposed for ripple reduction Thermal distribution via multi-phase output
Applications
u u u u u u
Graphics cards DDR Memory Peripheral add-in card SSTL Termination Dual-Phase power supply Power supplies requiring two outputs
Two Independent PWM Controllers Flexible, same or separate VIN Independent control for each channel Independent and separate current limit 2 phases operating opposed for ripple reduction (if same VIN used)
Typical Application Circuit
2 Channels with Current Sharing
Revision 7/31/2000
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SC1175
POWER MANAGEMENT Absolute Maximum Rating
Parameter VCC to GND PGND to GND BST to GND Thermal Resi stance Juncti on to C ase Thermal Resi stance Juncti on to Ambi ent Sy mbol VIN Limits -0.3 to 15 1 -0.3 to 26 U nits V V V C /W C /W C C C C
q q
JC
JA
30 90 0 to 85
0 to 125 -65 to +150 300
Operati ng Ambi ent Temperature Range Operati ng Juncti on Temperature Range
Storage Temperature Range Lead Temperature (Solderi ng) 10 sec
TA
TJ TSTG TLEAD
Electrical Characteristics
Unless Specified: VCC = 4.75 to 5.25V, GND = PGND = 0V, FB = VO, 0mV < (CS(+) - CS(-)) < 60mV , TJ = 25C
PAR AMETER Output Voltage Supply Voltage Supply C urrent Reference Load Regulati on Reference Li ne Regulati on Output Li ne Regulati on Gai n (AOL) C urrent Li mi t Voltage Osci llator Frequency Osci llator Max D uty C ycle D H Si nk C urrent D H Si nk C urrent D H Source C urrent D H Source C urrent D H - PGND = 3.5V D H - PGND = 1.75V BSTH - D H = 5.0V BSTH - D H = 2.5V IO = 0.3A to 15A (1) 5V < V C C < 15V 5V < VIN < 15V VOSENSE to VO 60 270 90 1 .5 1 .5 35 70 300 95 80 330 C ON D ITION S IO = 2A(1), VOUT set to 2.75V V CC VCC = 5.0
1.23751
MIN 2.65 4.2
TYP 2.75 10 1.25 1
MAX 2.85 15
1.26251
U N ITS V V mA V %
.5 .5
% % dB mV kHz % A A A A
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SC1175
POWER MANAGEMENT Electrical Characteristics (Cont.)
Unless Specified: VCC = 4.75 to 5.25V, GND = PGND = 0V, FB = VO, 0mV < (CS(+) - CS(-)) < 60mV , TJ = 25C
PAR AMETER D L Si nk C urrent D L Si nk C urrent D L Source C urrent D L Source C urrent D ead Ti me Soft Start C harge C urrent (2) Soft Start Enable Soft Start End Soft Start Transi ti on(2) Power Good Wi ndow(3) Fold Back C urrent Fold Back Voltage Knee Input Bi as C urrent
C ON D ITION S D L - PGND = 3.5V D L - PGND = 1.75V BSTL - D L = 5V BSTL - D L = 2.5V
MIN 1 .5 1 .5 50
TYP
MAX
U N ITS A A A A
100 25 1.4 2.5 3.3 +10 50%
ns A V V V %VOUT I LIM VOUT 1 V A
0% duty cycle 100% duty cycle Synchronous mode VOUT = 0V I = ILIM -IN1, +IN2, -IN2 1.25
NOTES: (1) Specification refers to application circuit. (2) The soft start pin sources 25A to an external capacitor. The converter operates in synchronous mode above the soft start transition threshold and in asynchronous mode below it. (3) Power good is an open collector pulled low when the output voltage is outside the 10% window. (4) This device is ESD sensitive. Use of standard ESD handling precautions is required.
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SC1175
POWER MANAGEMENT Pin Configuration Ordering Information
D EVIC E(1) SC 1175C SW.TR SC 1175EVB-1 PAC K AGE SO-20 C URRENT SHARE VERSION EVALUATION BOARD D UAL C HANNEL VERSION EVALUATION BOARD
SC 1175EVB-2
Notes: (1) Only available in tape and reel packaging. A reel contains 1000 devices
Marking Information Pin Descriptions
EXPANDED PIN DESCRIPTION Pin 1: (VREF) Internal 1.25V reference Connected to the + input of the master channel error amplifier. Pin 2: (+IN) + Input of slave channel error amplifier. Connected to 1.25V reference (Pin 1) for the two independent channel configuration. Pin 3, 18: (-IN2, -IN1) - Inputs of close loop error amplifiers. Works as a feedback inputs (For both modes). Pin 4: (VCC) VCC chip supply voltage. 15V maximum, 10mA typical. Needs a 1F ceramic multilayer decoupling capacitor to GND (Pin 20). Pin 5, 6,15, 16: (CL2-, CL2+, CL1+, CL1-) Pins (-) and (+) of the current limit amplifiers for both channels. Connected to output current sense resistors. Compares that sense voltage to internal 75mV reference. Needs RC filter for noise rejection. Pin 7, 14: (BST2, BST1) BST signal. Supply for high side driver. Can be connected to a high enough voltage source. Usually connected to bootstrap circuit. Pin 8, 13: (DH2, DH1) DH signal (Drive High). Gate drive for top MOSFETs. Requires a small series resistor.
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TOP yyww = Datecode (Example: 9908) xxxx = Semtech Lot # (Example: 90101)
Pin 9, 12: (DL2, DL1) DL signal (Drive Low). Gate drive for bottom MOSFETs. Requires a small series resistor. Pin 10: (PGND) Power GND. Return of gate drive currents. Pin 11: (BSTC) Supply for bottom MOSFETs gate drive. Pin 17: (SS/ENA) Soft start pin. Internal current source connected to external capacitor. Inhibits the chip if pulled down. Pin 19: (PWRGD) Power good signal. Open collector signal . Turns to 0 if output voltage is outside the power good window. Pin 20: (GND) Analog GND. Return of analog signals and bias of chip.
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SC1175
POWER MANAGEMENT Block Diagram
NOTES (1) Block 1 (top) is the Master and Block 2 (bottom) is the Slave in current sharing configuration. (2) For independant operation there is no Master or Slave.
Applications Information
Theory of Operation
Main Loop(s)
The SC1175 is a dual, voltage mode synchronous Buck controller, the two separate channels are identical and share only IC supply pins (Vcc and GND), output driver ground (PGND) and pre-driver supply voltage (BSTC). They also share a common oscillator generating a sawtooth waveform for channel 1 and an inverted sawtooth for channel 2. Each channel has its own current limit comparator. Channel 1 has the positive input of the error amplifier internally connected to Vref. Channel 2 has both inputs of the error amplifier uncommitted and available externally. This allows the SC1175 to operate in two distinct modes. a) Two independent channels with either common or different input voltages and different output voltages. The two channels each have their own voltage feedback path from their own
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output. In this mode, the positive input of error amplifier 2 is connected externally to Vref. If the application uses a common input voltage, the sawtooth phase shift between the channels provides some measure of input ripple current cancellation. b) Two channels operating in current sharing mode with common output voltage and either common input voltage or different input voltages. In this mode, channel 1 operates as a voltage mode Buck controller, as before, but error amp 2 monitors and amplifies the difference in voltage across the output current sense resistors of channel 1 and channel 2 (Master and Slave) and adjusts the Slave duty cycle to match output currents. Because of finite gain and offsets in the loop, the resistor ratio for perfect current matchwww.semtech.com
SC1175
POWER MANAGEMENT
Theory of Operation (Cont.) ing is not 1:1. The Master and Slave channels still have their own current limits, identical to the independent channel case. Power Good The controller provides a power good signal. This is an open collector output, which is pulled low if the output voltage is outside of the power good window. Soft Start/Enable The Soft Start/Enable (SS/ENA) pin serves several functions. If held below the Soft Start Enable threshold, both channels are inhibited. DH1 and DH2 will be low, turning off the top FETs. Between the Soft Start Enable threshold and the Soft Start End threshold, the duty cycle is allowed to increase. At the Soft Start End threshold, maximum duty cycle is reached. In practical applications the error amplifier will be controlling the duty cycle before the Soft Start End threshold is reached. To avoid boost problems during startup in current share mode, both channels start up in asynchronous mode, and the bottom FET body diode is used for recirculating current during the FET off time. When the SS/ENA pin reaches the Soft Start Transition threshold, the channels begin operating in synchronous mode for improved efficiency. The soft start pin sources approximately 25uA and soft start timing can be set by selection of an appropriate soft start capacitor value. The formula is:
V + .1 R (pull - up ) = 100 X 762 X .5 - OUT VSLAVE IN

100W being the value of the resistors connecting the pins 2 and 3 to the two output sense resistors. The estimated voltage drop across the MOSFETs is 0.1V. Positive values go to pin 3, negative to pin 2. If R (pull-up) = +20KW then place a 20WK resistor on pin 3. If R (pull-up) = -20KW then place a 20KW on pin 2. Now that the offset resistor has been fixed, we need to set up the maximum current for each channel. Selection of RSENSE 1 for the master channel: (mohms) RSENSE 1 = 72mV / I max master Selection of RSENSE 2 for the slave channel: (mohms) RSENSE 1 = 48mV / I max master The errors will be minimized if the power components have been sized proportionately to the maximum currents. Independent Channels Calculation of the two current limiting resistors. There is no need for an offset resistor in the independent channels mode, only the two sense resistors are used: Selection of RSENSE 1 for the channel 1: (mohms) RSENSE 1 = 72mV / I max ch 1 Selection of RSENSE 2 for the channel 2: (mohms) RSENSE 1 = 72mV / I max ch 2
SENSE RESISTOR SELECTION
Current Sharing Mode Calculation of the three programming resistors to achieve sharing. Three resistors will determine the current sharing load line. First the offset resistor will ensure that the load line crosses the origin (0 Amp on each channel) for sharing at light current. A pull up resistor from the 5V bias (VCC of the chip) will be used. For low duty cycle on the slave channel (below 50%), the pull up will be on pin 3. For high duty cycle on the slave channel (above 50%), the pull up will be on pin 2.
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SC1175
POWER MANAGEMENT Typical Characteristics - 2 Channels with Current Sharing
Figure 1: VOUT vs IIN(5V) and IIN(12V) with VCC applied and 4A load. Soft start capacitor = 10nF.
PIN Descriptions
Ch1: VOUT Ch2: IIN(5V) (1A/Div) Ch4: IIN(12V) (1A/Div) IOUT: 4.004 Amps
Figure 2: VOUT vs IIN(5V) and IIN(12V) with VCC removed and 4A load. Soft start capacitor = 10nF.
Ch1: VOUT Ch2: IIN(5V) (1A/Div) Ch4: IIN(12V) (1A/Div) IOUT: 4.004 Amps
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SC1175
POWER MANAGEMENT Typical Characteristics - 2 Channels with Current Sharing (Cont.)
Figure 3: VOUT vs IIN(5V) and IIN(12V) with VCC applied and 12A load. Soft start capacitor = 10nF.
Ch1: VOUT Ch2: IIN(5V) (2A/Div) Ch4: IIN(12V) (2A/Div) IOUT: 12 Amps
Figure 4: VOUT vs IIN(5V) and IIN(12V) with VCC removed and 12A load. Soft start capacitor = 10nF.
Ch1: VOUT Ch2: IIN(5V) (2A/Div) Ch4: IIN(12V) (2A/Div) IOUT: 12 Amps
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SC1175
POWER MANAGEMENT Typical Characteristics - 2 Channels with Current Sharing (Cont.)
Figure 5: Efficiency data - current sharing mode.
1.0 0.9 0.8 0.7 Efficiency (%) 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 2 4 6 8 10 12 14 Current (A) VIN(MASTER) = 12V VIN(SLAVE) = 5V VOUT = 2.75V
The Current Sharing Evaluation Board is not intended for a specific application. The power components are not optimized for minimum cost and size. This evaluation board should be used to understand the operation of the SC1175. To design with the SC1175 for specific current sharing applications. Please refer to: Application note AN00-3.
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SC1175
POWER MANAGEMENT Evaluation Board Schematic - 2 Channel with Current Sharing
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SC1175
POWER MANAGEMENT Evaluation Board Bill of Materials - 2 Channels with Current Sharing
Item 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Quantity 2 3 3 1 3 6 2 1 1 2 2 1 7 2 1 1 1 1 C 1,C 7 C 2,C 3,C 4 C 5,C 15,C 16 C8 C 9,C 10,C 14 C 11,C 12,C 13,C 17,C 18,C 19 D 1,D 2 L1 L2 M1,M3 M2,M4 R1 R2,R3,R4,R5,R6,R7,R8 R9,R10 R12 R13 R14 U1 R eference .22uF, 50V 1uF, 50V 10nF, 50V 1nF, 50V 100uF, 6V 150uF, 16V D L4148 7.5uH, 8A 4.7uH, 8A IRF7809 or FD B7030 IRF7811 or FD B7030 Part
124 2.2 100 150
.006
.003
SC 1175
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SC1175
POWER MANAGEMENT Evaluation Board Gerber Plots - 2 Channels with Current Sharing
Top Side Traces
Bottom Side Traces
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SC1175
POWER MANAGEMENT Typical Characteristics - 2 Independent Channels
Figure 6:
Figure 7: Output Current
Input Voltage = 12V @ 5Amps. 2A/DIV.
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SC1175
POWER MANAGEMENT Typical Characteristics - 2 Independent Channels (Cont.)
Figure 8: Peak - Peak Output Ripple @ 5A
Input Voltage = 12V. Output Voltage = 2.0V Figure 9: Phase Node 12V Input @ 5A (without snubber and RC network.
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SC1175
POWER MANAGEMENT Typical Characteristics - 2 Independent Channels (Cont.)
Figure 10: Start-up Power On
Chan. 1 = Output Current. 2A/DIV. Chan. 2 = 5V Bias Voltage Figure 11: Power Off
Chan. 1 = Output Current. 2A/DIV. Chan. 2 = 5V Bias Voltage
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SC1175
POWER MANAGEMENT Typical Characteristics - 2 Independent Channels Efficiency Test
Figure 12:
100 EFFICIENCY 95 90 85 80 75 70 0 1 2 3 4 5 6 OUTPUT CURRENT Vin = 12V Vout = 2.0V Vin = 5V 1.25V Vout =
The Independent Channels Evaluation Board is not intended for a specific application. The power components are not optimized for minimum cost and size. This evaluation board should be used to understand the operation of the SC1175. To design with the SC1175 for specific independent channels applications. Please refer to: Application note AN00-4.
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SC1175
POWER MANAGEMENT Evaluation Board Schematic - 2 Independent Channels
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SC1175
POWER MANAGEMENT Evaluation Board Bill of Materials - 2 Independent Channels
Item 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Quantity 3 3 1 4 9 3 2 1 1 2 2 7 3 1 1 1 2 1 C 1,C 2,C 3 C 4,C 6,C 11 C5 C 7,C 8,C 9,C 10 C 12,C 13,C 14,C 15,C 16,C 17,C 18,C 19,C 20 C 21,C 22,C 23 D 1,D 2 L1 L2 M1,M3 M2,M4 R1,R2,R3,R4,R5,R6,R7 R8,R9,R13 R10 R11 R12 R14,R15 U1 R eference 1uF, 50V .22uF, 50V 1nF, 50V 10nF, 50V 150uF, 6V 100uF, 16V D L4148 7.5uH, 8A 4.7uH, 8A IRF7809 or FD B7030 IRF7811 or FD B7030 2.2 100 .006 220 .003 124 SC 1175 Part
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SC1175
POWER MANAGEMENT Evaluation Board Gerber Plots - 2 Independent Channels
Top Side Traces
Bottom Side Traces
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SC1175
POWER MANAGEMENT PCB Layout
Power and signal traces must be kept separated for noise considerations. Feedback, current sense traces and analog ground should not cross any traces or planes carrying high switching currents, such as the input loop or the phase node. The input loop, consisting of the input capacitors and both MOSFETs must be kept as small as possible. All of the high switching currents occur in this loop. The enclosed loop area must be kept small to minimize inductance and radiated and conducted emissions. Designing for minimum trace length is not always the best approach, often a more optimum layout can be achieved by keeping loop area constraints in mind. It is important to keep gate lengths short, the IC must be close to the power switches. This is more difficult in a dual channel device than a single and requires that the two power paths run on either side of a centrally located controller. Grounding requirements are always conflicting in a buck converter, especially at high power, and the trick is to achieve the best compromise. Power ground (PGND) should be returned to the bottom MOSFET source to provide the best gate current return path. Analog ground (GND) should be returned to the ground side of the output capacitors so that the analog circuitry in the controller has an electrically quiet reference and to provide the greatest feedback accuracy. The problem is that the differential voltage capability of the two IC grounds is limited to about 1V for proper operation and so the physical separation between the two grounds must also be minimized. If the grounds are too far apart, fast current transitions in the connection can generate voltage spikes exceeding the 1V capability, resulting in unstable and erratic behavior. The feedback divider must be close to the IC and be returned to analog ground. Current sense traces must be run parallel and close to each other and to analog ground. The IC must have a ceramic decoupling capacitor across its supply pins, mounted as close to the device as possible. The small ceramic, noise-filtering capacitors on the current sense lines should also be placed as close to the IC as possible.
Outline Drawing - SO-20
Contact Information
Contact Information
Ref. MS-013AC
ECN00-1204
a 2000 Semtech Corp.
Semtech Corporation Power Management Products Division 652 Mitchell Rd., Newbury Park, CA 91320 Phone: (805)498-2111 FAX (805)498-3804
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