UCC28C43AN

Application ReportSLUU143 - February 2003UCC38C42 30-W Synchronous Buck Converter ReferenceDesign (PR112B)Lisa DinwoodieABSTRACTThis reference design presents a synchronous buck converter using the UCC38C43BiCMOS low-power current-mode PWM controller, the TPS2838 synchronous buckMOSFET driver with drive regulator, and the INA138 high-side measurement current shuntmonitor. The input voltage for this converter is from a 3.3-Vdc rail which draws the biasvoltage for the devices from an available 12-Vdc bus. The converter is designed to operateat a switching frequency of 400 kHz and supports a non-isolated 1.8-Vdc, 17-A output. Thecomplete schematic, board layout, circuit description, list of materials, and circuitperformance curves are included.System PowerContents12345678Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Reference Design Layout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Circuit Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Performance Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8List of Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Predictive Gate Drivet, PowerPADt and TrueDrivet are trademarks of Texas Instruments Incorporated1SLUU1431IntroductionSynchronous buck converters have significantly better efficiency than conventional buckconverters because the commutation diode is replaced by a power MOSFET, reducing thetypical diode forward voltage drop to less than 0.1 V thanks to the low RDS(on) of the FET. Buckconverters are relatively simple to design due to the absence of a transformer. The controllerused is the UCC38C43. Its low start-up and operating currents, high-frequency operation, andindustry standard familiarity make this general-purpose controller easy to use. The TPS2838driver enhances the converter’s performance because of its high sink and source peak currentsand shoot through protection with its adaptive/adjustable dead-time control. The INA138 currentshunt monitor, combined with an LM311 comparator, provides over current protection for thisvoltage-mode converter. A single resistor provides the gain to the differential voltage across acurrent sense resistor, which is then compared to a threshold voltage corresponding to anovercurrent load condition. The UCC38C43 controller is then forced into a hiccup mode until thefault is removed.2Features•••••••••Fixed input range: 3.3 VDC1.8 VDC output voltage17-A maximum output load, 30-W maximum continuous output powerHigh-efficiency 400-kHz switching frequencySynchronous buck topologyVoltage mode controlOvercurrent hiccup modeSoft startSynchronization input3SchematicFigure 1 shows the schematic of the design.2UCC38C42 30-W Synchronous Buck Converter Reference Design (PR112B)SLUU143+1234T+––UNNCOIICTIVMETBUT+OS/CLLLCOAAVCBB8765Figure 1.Synchronous Buck Converter Featuring the UCC38C43 UCC38C42 30-W Synchronous Buck Converter Reference Design (PR112B)3SLUU1434Reference Design LayoutFigure 2.Top Layer AssemblyFigure 3.Top Layer Route4UCC38C42 30-W Synchronous Buck Converter Reference Design (PR112B)SLUU143Figure 4.Bottom Layer AssemblyFigure 5.Bottom Layer Route UCC38C42 30-W Synchronous Buck Converter Reference Design (PR112B)5SLUU1435Circuit DescriptionA brief description of the circuit elements follows:•Input capacitors C1 through C7, MOSFETs Q2 and Q4, inductor L1, and output capacitorsC18, C20 through C24, and C26 form the power stage of the converter. Transistor Q2 is thepower switch while Q4 is the synchronous switch. Diode D1, in parallel with Q4, allowsinductor current to flow during the dead time when Q2 turns off and Q4 has not yet turnedon, improving converter efficiency by decreasing the body diode conduction time of thesynchronous switch.Resistor R21 provides biasing to the PWM controller, U2, from the 12V bus. Zener diode D2protects the IC by clamping the bias voltage.PNP transistor Q1, resistor R6, and capacitor C8 provide soft start.NPN transistor Q3, resistors R13 and R15 convert the current mode controller into voltagemode operation by dividing down the oscillator timing capacitor waveform and feeding thissignal into the current sense pin.Resistor R7 and capacitor C12 provide a charge and discharge path for the internaloscillator, setting the switching frequency of the controller. Resistor R8 provides a means ofinserting an external synchronization pulse into the circuit.The voltage sense feedback loop uses the internal error amplifier in the UCC38C43.Resistors R24 and R10 bias up the 1.8-V output to the 2.5-V feedback threshold level.Resistors R12 and R23, capacitors C13, C14, and C25, along with R24, set up a Type-IIIcompensator and provide the necessary gain, poles, and zeros to stabilize the control loop.Resistor R22 provides an impedance port for loop measurement with a gain phase analyzer.Decoupling to the PWM is performed by capacitors C17 and C19 while C9 and C11decouple the MOSFET driver, U1.Resistors R1, R2, and R3 provide pull-up to the 12-V rail to the digital control signalsENABLE, PWRRDY, and SYNC of the driver device.Capacitor C10 is needed to configure the floating bootstrap voltage for the high-sideMOSFET.Resistors R9 and R5 adjust the driver regulator output to 2-V below VCC for optimumefficiency.The overcurrent detect circuit consists of the power resistor R18, which senses the inductorcurrent while current shunt monitor U4 converts this current into a differential voltage. Thisvoltage is then transformed into an output current, which is converted into a proportionallygained output voltage with the help of resistor R16 and small filter capacitor C16. This outputvoltage is used as the input to the inverting terminal of an LM311 comparitor. Thenon-inverting terminal input is a threshold voltage, which is derived from the R19/R20 dividerfrom the 12-Vdc rail. Resistor R17 provides hysteretic feedback to the comparator. Theoutput of the comparator is used to drive the base of the PNP transistor Q5. The emitter ofthis bipolar transistor is used to pull down the COMP pin of the controller, initiating a hiccupmode in the event of an inductor over current. A small impedance, R11, between thecollector and ground, ensures the COMP pin does not get pulled below ground.••••••••••6UCC38C42 30-W Synchronous Buck Converter Reference Design (PR112B)SLUU1436Performance DataThe following figures show the performance of a circuit built as described, using inductors fromthree different vendors; TDK, Panasonic, and Pulse Engineering.Efficiencies greater than 89.4% are achieved with this reference design (see Figure 6). Loadregulation is measured to be better than 0.2% (see Figure 7).EFFICIENCYvsOUTPUT POWERLOAD REGULATIONvsOUTPUT LOAD CURRENT900TDKRegulation – %85Efficiency – %PanasonicPulse–0.1TDKPulse–0.2Panasonic–0.380–0.47536915182124POUT – Output Power – W122730–0.5024681012POUT – Output Power – W141618Figure 6.Figure 7.VRIPPLE (10 mV/div)VRIPPLE (20 mV/div)t – Time – 1 µs/divt – Time – 1 µs/divFigure 8.Output Ripple and Noise at Minimum LoadFigure 9.Output Ripple and Noise at Maximum Load UCC38C42 30-W Synchronous Buck Converter Reference Design (PR112B)7SLUU143Q4 GATE TURN-OFF (2V/div)Q2 GATE TURN-OFF (2V/div)Q4 GATE TURN-ON (2V/div)Q2 GATE TURN-ON (2V/div)t – Time – 25 ns/divt – Time – 25 ns/divFigure 10.Figure 11.Figure 12 shows a Bode plot with a crossover frequency of 40 kHz and a phase margin ofapproximately 60 degrees.GAIN AND PHASEvsFREQUENCY3018020PHASE1200GAIN0–10–60–20–120–301 k10 kf – Frequency – Hz–180100 kFigure 12.7References1.BiCMOS Low-Power Current-Mode PWM Controller, Texas Instruments Literature No.SLUS458.2.Andreycak, Bill, The UCC38C42 Family of High-Speed, BiCMOS Current-Mode PWMControllers, Texas Instruments Literature No. SLUA257.8UCC38C42 30-W Synchronous Buck Converter Reference Design (PR112B)Phase – Degrees10Gain – dB60SLUU1438List of MaterialsTable 1.UCC38C43EVM List of Materials CapacitorQTY1111321112111Diode11InductorTransistor1221Resistor61111111211121111REFERENCEDESIGNATORC1C2, C3, C4,C5, C6, C7,C18, C20,C21, C22, C23C8C9, C11, C17C10, C19C12C13C14C15, C26C16C24C25D1D2L1Q1, Q5Q2, Q4Q3R1, R2, R3,R6, R14, R20R10R12R13R15R16R17R18R19, R24R21R22R23R4, R11R5R7R8R9DESCRIPTIONCeramic, 0.01 µF, 50 V, X7R, ±10%Ceramic, 47 µF, 6.3 V, X5R, ±20%Ceramic, 2.2 µF, 6.3 V, X5R, ±20%Ceramic, 10 µF, 16 V, X5R, ±20%Ceramic, 1 µF, 25 V, X7R, ±10%Ceramic, 470-pF, 50 V, C0G, ±10%Ceramic, 18 pF, 50 V, C0G, ±10%Ceramic, 6800 pF, 50 V, X7R, ±10%Ceramic, 0.01 µF, 50 V, X7R, ±10%Ceramic, 8 pF, 50 V, C0G, ±10%Tantalum, 470 µF, 6.3 V, ±10%Ceramic, 15000 pF, 50 V, X7R, ±10%Schottky, 12 A, 30 V,Zener, 12 V, 200 mWSMT, 1.0 µH, 22 A, 2.28 mΩBipolar, PNP, 60 V, 150 mA, 350 mΩMOSFET, N-channel, 30 V, 45 A, 2.9 mΩBipolar, NPN, 40 V, 500 mA, 350 mΩChip, 10 kΩ, 1/16W, ±1%Chip, 3.92 kΩ, 1/16W, ±1%Chip, 1.96 kΩ, 1/16W, ±1%Chip, 1.47 kΩ, 1/16W, ±1%Chip, 1.1 kΩ, 1/16W, ±0.1%Chip, 324 kΩ, 1/16W, ±1%Chip,100 kΩ, 1/16W, ±5%Metal strip, 1 mΩ, 1W, ±1%Chip, 49.9 Ω, 1/16W, ±1%Chip, 249 Ω, 1/16W, ±1%Chip, 49.9 Ω, 1/10W, ±1%Chip, 5.11 Ω, 1/16W, ±1%Chip, 10 Ω, 1/16W, ±5%Chip, 30.1 kΩ, 1/16W, ±5%Chip, 7.15 kΩ, 1/16W, ±1%Chip, 24 Ω, 1/16W, ±5%Chip, 215 kΩ, 1/16W, ±1%SIZE0603121006031206080506030603060306030603E size0603TO–252AASOT–3230.524 ×0.492SOT–23LFPAKSOT–2306030603060306030603060306032512060306030805060306030603060306030603TDKTDKTDKTDKTDKTDKTDKTDKTDKTDKVishay SpragueTDKInternational RectifierDiodes, Inc.TDKDiodes, Inc.HITACHIDiodes, Inc.Panasonic–ECGPanasonic–ECGPanasonic–ECGPanasonic–ECGPanasonic–ECGPanasonic–ECGPanasonic–ECGVishay DalePanasonic–ECGPanasonic–ECGPanasonic–ECGYageo AmericaPanasonic–ECGPanasonic–ECGPanasonic–ECGPanasonic–ECGPanasonic–ECGMANUFPART NUMBERC1608X7R1H103KC3225X5R0J476MC1608X5R0J225MC3216X5R1C106MC2012X7R1E105KC1608C0G1H471JC1608C0G1H180JC1608X7R1H682KC1608X7R1H103KC1608C0G1H080D293D477X96R3E2TC1608X7R1H153K12CWQ03FNBZX84C12W–7SPM12550T1R0M220MMBT2907A–7HAT2166HMMBT2222A–7ERJ–3EKF1002VERJ–3EKF3921VERJ–3EKF1961VERJ–3EKF1471VERA–3YEB112VERJ–3EKF3243VERJ–3GEYJ104VWSL–2512 0.001 ±1%ERJ–3EKF49R9VERJ–3EKF2490VERJ–6ENF49R9V9C06031A5R11FKHFTERJ–3GEYJ100VERJ–3EKF3012VERJ–3EKF7151VERJ–3GEYJ240VERJ–3EKF2153V UCC38C42 30-W Synchronous Buck Converter Reference Design (PR112B)9SLUU143 IntegratedgCiiCircuitREFERENCEDESIGNATORU1U2U3U4J1, J2J3, J4JP1JP1 MateTP1, TP2, TP3SH1QTY1111DESCRIPTIONSynchronous-buck MOSFET driverBiCMOS, low-power current-mode PWMcontrollerDifferential comparators with strobesHigh–sdide measurement current shuntmonitorTerminal block, 2–pin, 15 A, 5.1mmTerminal block, 4–pin, 15 A, 5.1mmHeader, 3–pin, 100-mil spacing,(36–pin strip)Shorting jumper, single, 2 positionTest Point, O.050 Hole”SIZEPWP–16MSOP–8PS–8SOT23–50.40×0.350.80×0.350.100 × 3”MANUFPART NUMBERTPS2838PWPTexasInstrumentsUCC38C43DGKLM311PWINA138NAConnector2211On ShoreTechnology Inc.SullinsElectronicsCorp.Mill MaxED 120/2DSED2227PTC36SAANSTC02SYAN3156-2-00-01-00-00-08-0Terminal31This part is designed to be used for keeping GNDs separate when laying out PCB’s.10UCC38C42 30-W Synchronous Buck Converter Reference Design (PR112B)IMPORTANT NOTICE

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