SC1175 Low Power Dual Synchronous DC/DC Controller With Current Sharing Circuitry POWER MANAGEMENT Description Features u u u u u u u u u 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. Two Phase, Current Sharing Controller 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. u u u u u Applications 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 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 Two Independent PWM Controllers Graphics cards DDR Memory Peripheral add-in card SSTL Termination Dual-Phase power supply Power supplies requiring two outputs u u u u 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 1 www.semtech.com SC1175 POWER MANAGEMENT Absolute Maximum Rating Parameter Sy mbol Limits U nits VIN -0.3 to 15 V ±1 V -0.3 to 26 V JC 30 °C /W JA 90 °C /W Operati ng Ambi ent Temperature Range TA 0 to 85 °C Operati ng Juncti on Temperature Range TJ 0 to 125 °C Storage Temperature Range TSTG -65 to +150 °C Lead Temperature (Solderi ng) 10 sec TLEAD 300 °C VCC to GND PGND to GND BST to GND Thermal Resi stance Juncti on to C ase q Thermal Resi stance Juncti on to Ambi ent q Electrical Characteristics Unless Specified: VCC = 4.75 to 5.25V, GND = PGND = 0V, FB = VO, 0mV < (CS(+) - CS(-)) < 60mV , TJ = 25°C PAR AMETER C ON D ITION S MIN TYP MAX U N ITS Output Voltage IO = 2A(1), VOUT set to 2.75V 2.65 2.75 2.85 V Supply Voltage V CC 4.2 15 V Supply C urrent VCC = 5.0 1.2375±1 Reference Load Regulati on 10 IO = 0.3A to 15A (1) 1.25 mA 1.2625±1 1 V % Reference Li ne Regulati on 5V < V C C < 15V .5 % Output Li ne Regulati on 5V < VIN < 15V .5 % Gai n (AOL) VOSENSE to VO 35 dB C urrent Li mi t Voltage 60 70 80 mV Osci llator Frequency 270 300 330 kHz Osci llator Max D uty C ycle 90 95 % D H Si nk C urrent D H - PGND = 3.5V 1 A D H Si nk C urrent D H - PGND = 1.75V .5 A D H Source C urrent BSTH - D H = 5.0V 1 A D H Source C urrent BSTH - D H = 2.5V .5 A ã 2000 Semtech Corp. 2 www.semtech.com SC1175 POWER MANAGEMENT Electrical Characteristics (Cont.) Unless Specified: VCC = 4.75 to 5.25V, GND = PGND = 0V, FB = VO, 0mV < (CS(+) - CS(-)) < 60mV , TJ = 25°C PAR AMETER C ON D ITION S MIN D L Si nk C urrent D L - PGND = 3.5V 1 A D L Si nk C urrent D L - PGND = 1.75V .5 A D L Source C urrent BSTL - D L = 5V 1 A D L Source C urrent BSTL - D L = 2.5V .5 A D ead Ti me 50 Soft Start End Soft Start Transi ti on(2) 25 µA 0% duty cycle 1.4 V 100% duty cycle 2.5 V Synchronous mode 3.3 V +10 %VOUT 50% I LIM VOUT = 0V Fold Back Voltage Knee Input Bi as C urrent U N ITS ns Power Good Wi ndow(3) Fold Back C urrent MAX 100 Soft Start C harge C urrent (2) Soft Start Enable TYP I = ILIM -IN1, +IN2, -IN2 1.25 VOUT V 1 µA NOTES: (1) Specification refers to application circuit. (2) The soft start pin sources 25µA 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. ã 2000 Semtech Corp. 3 www.semtech.com SC1175 POWER MANAGEMENT Pin Configuration Ordering Information D EVIC E(1) PAC K AGE SC 1175C SW.TR SO-20 SC 1175EVB-1 C URRENT SHARE VERSION EVALUATION BOARD SC 1175EVB-2 D UAL C HANNEL VERSION EVALUATION BOARD 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 1µF 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. ã 2000 Semtech Corp. 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. 4 www.semtech.com 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) 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. 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. 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 match- 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 ã 2000 Semtech Corp. 5 www.semtech.com SC1175 POWER MANAGEMENT Theory of Operation (Cont.) The formula is: ing is not 1:1. The Master and Slave channels still have their own current limits, identical to the independent channel case. V + .1 R (pull − up ) = 100 Ω X 762 X .5 − OUT VSLAVE IN 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. 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. 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. 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. ã 2000 Semtech Corp. 6 www.semtech.com 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 ã 2000 Semtech Corp. 7 www.semtech.com 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 ã 2000 Semtech Corp. 8 www.semtech.com SC1175 POWER MANAGEMENT Typical Characteristics - 2 Channels with Current Sharing (Cont.) Figure 5: Efficiency data - current sharing mode. 1.0 0.9 0.8 Efficiency (%) 0.7 0.6 0.5 0.4 0.3 VIN(MASTER) = 12V VIN(SLAVE) = 5V VOUT = 2.75V 0.2 0.1 0.0 0 2 4 6 8 10 12 14 Current (A) 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. ã 2000 Semtech Corp. 9 www.semtech.com SC1175 POWER MANAGEMENT Evaluation Board Schematic - 2 Channel with Current Sharing ã 2000 Semtech Corp. 10 www.semtech.com SC1175 POWER MANAGEMENT Evaluation Board Bill of Materials - 2 Channels with Current Sharing Item Quantity 1 2 C 1,C 7 .22uF, 50V 2 3 C 2,C 3,C 4 1uF, 50V 3 3 C 5,C 15,C 16 10nF, 50V 4 1 C8 1nF, 50V 5 3 C 9,C 10,C 14 100uF, 6V 6 6 C 11,C 12,C 13,C 17,C 18,C 19 150uF, 16V 7 2 D 1,D 2 D L4148 8 1 L1 7.5uH, 8A 9 1 L2 4.7uH, 8A 10 2 M1,M3 IRF7809 or FD B7030 11 2 M2,M4 IRF7811 or FD B7030 12 1 R1 124 13 7 R2,R3,R4,R5,R6,R7,R8 2.2 14 2 R9,R10 100 15 1 R12 150 16 1 R13 .006 17 1 R14 .003 18 1 U1 SC 1175 ã 2000 Semtech Corp. R eference 11 Part www.semtech.com SC1175 POWER MANAGEMENT Evaluation Board Gerber Plots - 2 Channels with Current Sharing Top Side Traces Bottom Side Traces ã 2000 Semtech Corp. 12 www.semtech.com SC1175 POWER MANAGEMENT Typical Characteristics - 2 Independent Channels Figure 6: Figure 7: Output Current Input Voltage = 12V @ 5Amps. 2A/DIV. ã 2000 Semtech Corp. 13 www.semtech.com 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. ã 2000 Semtech Corp. 14 www.semtech.com 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 ã 2000 Semtech Corp. 15 www.semtech.com SC1175 POWER MANAGEMENT Typical Characteristics - 2 Independent Channels Efficiency Test Figure 12: EFFICIENCY 100 95 Vin = 12V Vout = 2.0V 90 85 Vin = 5V 1.25V 80 Vout = 75 70 0 1 2 3 4 5 6 OUTPUT CURRENT 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. ã 2000 Semtech Corp. 16 www.semtech.com SC1175 POWER MANAGEMENT Evaluation Board Schematic - 2 Independent Channels ã 2000 Semtech Corp. 17 www.semtech.com SC1175 POWER MANAGEMENT Evaluation Board Bill of Materials - 2 Independent Channels Item Quantity 1 3 C 1,C 2,C 3 1uF, 50V 2 3 C 4,C 6,C 11 .22uF, 50V 3 1 C5 1nF, 50V 4 4 C 7,C 8,C 9,C 10 10nF, 50V 5 9 C 12,C 13,C 14,C 15,C 16,C 17,C 18,C 19,C 20 150uF, 6V 6 3 C 21,C 22,C 23 100uF, 16V 7 2 D 1,D 2 D L4148 8 1 L1 7.5uH, 8A 9 1 L2 4.7uH, 8A 10 2 M1,M3 IRF7809 or FD B7030 11 2 M2,M4 IRF7811 or FD B7030 12 7 R1,R2,R3,R4,R5,R6,R7 2.2 13 3 R8,R9,R13 100 14 1 R10 .006 15 1 R11 220 16 1 R12 .003 17 2 R14,R15 124 18 1 U1 SC 1175 ã 2000 Semtech Corp. R eference 18 Part www.semtech.com SC1175 POWER MANAGEMENT Evaluation Board Gerber Plots - 2 Independent Channels Top Side Traces Bottom Side Traces ã 2000 Semtech Corp. 19 www.semtech.com 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 Ref. MS-013AC Contact Information ECN00-1204 ã 2000 Semtech Corp. Semtech Corporation Power Management Products Division 652 Mitchell Rd., Newbury Park, CA 91320 Phone: (805)498-2111 FAX (805)498-3804 20 www.semtech.com