SC4614 500kHz Voltage Mode PWM Controller POWER MANAGEMENT Description Features The SC4614 is a high-speed, voltage mode PWM controller that provides the control and protection features necessary for a synchronous buck converter. u 500kHz switching frequency u 4V to 25V power rails u 0.5V voltage reference for programmable output The SC4614 is designed to directly drive the top and bottom MOSFETs of the buck converter. It allows the converter to operate at 500kHz switching frequency with 4V to 25V power rail and as low as 0.5V output. It uses an internal 8.2V supply as the gate drive voltage for minimum driver power loss and MOSFET switching loss. u u u u u u u The SC4614 features soft-start, supply power under voltage lockout, and hiccup mode over current protection. The SC4614 monitors the output current by using the Rdson of the bottom MOSFET in the buck converter that eliminates the need for a current sensing resistor. The SC4614 is offered in a MSOP-10 package. voltages Internal LDO for optimum gate drive voltage 1.5A gate drive current Adaptive non-overlapping gate drives provide shoot-through protection for MOSFETs Internal soft start Hiccup mode short circuit protection Power rail under voltage lockout MSOP-10 package, fully RoHS and WEEE compliant Applications u u u u u Embedded, low cost, high efficiency converters Point of load power supplies Set top box power supplies PDP/TFT TVs Consumer electronics Typical Application Circuit 12V IN + 1 2 3 4 5 BST DH OCS PN COMP DL FB VCC GND DRV 10 1.5V OUT 9 1 8 2 7 6 + SC4614 January 16, 2007 1 www.semtech.com SC4614 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 implied. Parameter Symbol Maximum Units Input Supply Voltage VCC 20 V BST to GND VBST 40 V BST to PN VBST_PN 10 V PN to GND VPN -1 to 30 V VPN_PULSE -5 V VDL -1 to +10 V VDL_PULSE -3 V VDH_PN -1 to +10 V VDH_PULSE -3 V VDRV 10 V Operating Ambient Temperature Range TA -40 to 85 °C Operating Junction Temperature TJ -40 to 125 °C Thermal Resistance Junction to Ambient θJA 136 °C/W Thermal Resistance Junction to Case θJC 45 °C/W Lead Temperature (Soldering) 10s TLEAD 300 °C Storage Temperature TSTG -65 to 150 °C PN to GND Negative Pulse (tpulse < 20ns) DL to GND DL to GND Negative Pulse (tpulse < 20ns) DH to PN DH to PN Negative Pulse (tpulse < 20ns) DRV to GND Electrical Characteristics Unless specified: VCC = 5V to 18V; VFB = VO; VBST - VPN = 5V to 8.2V; TA = -40 to 85°C Parameter Symbol Conditions Min Typ Max Units 18 V 7 mA 4 V 10 V 3 mA General VCC Supply Voltage VCC VCC Quiescent Current IQVCC VCC = 12V, VBST -VPN = 8.2V VCC Under Voltage Lockout UVVCC VHYST = 100mV BST to PN Supply Voltage VBST_PN BST Quiescent Current 4 5 4 IQBST VCC = 12V, VBST -VPN = 8.2V LDO Output VDRV 8.6V < VCC < 18V 8.2 V Dropout Voltage VDROP 4V < VCC < 8.6V 0.4 V Internal LDO 2005 Semtech Corp. 2 www.semtech.com SC4614 POWER MANAGEMENT Electrical Characteristics Unless specified: VCC = 5V to 18V; VFB = VO; VBST - VPN = 5V to 8.2V; TA = -40 to 85°C Parameter Symbol Conditions Min Typ Max Units VREF TA = 25°C, VCC = 12V 0.495 0.500 0.505 V Sw itching Regulator Reference Voltage Load Regulation IO = 0.2 to 4A 0.4 % Line Regulation VCC = 10V to 14V 0.4 % 400 500 600 kHz Operating Frequency FS Ramp Amplitude Vm 0.8 V DMAX 97 % TON_MIN 125 ns (2) Maximum Duty Cycle (2) Minimum On-Time (2) DH Rising/Falling Time DL Rising/Falling Time tSRC_DH tSINK_DH tSRC_DL tSINK_DL 6V Swing at CL = 3.3nF VBST-VPN = 8.2V 6V Swing at CL = 3.3nF VDRV = 8.2V DH, DL Nonoverlapping Time 41 27 29 42 ns ns 30 ns 1.5 ms Input Offset Voltage (2) 2 mV Input Offset Current (2) 40 nA Open Loop Gain 80 dB Unity Gain Bandwidth (2) 10 MHz Output Source Current 0.9 mA Output Sink Current 0.9 mA 1.2 V/us TA = 25°C, VCC = 12V Soft Start Time Voltage Error Amplifier (2) Slew Rate (2) For CL=500pF Load Notes: (1) This device is ESD sensitive. Use of standard ESD handling precautions is required. (2) Guaranteed by design, not tested in production. 2005 Semtech Corp. 3 www.semtech.com SC4614 POWER MANAGEMENT Pin Configuration Ordering Information TOP VIEW BST 1 10 DH OCS 2 9 PN COMP 3 8 DL FB 4 7 VCC GND 5 6 DRV Part Numbers P ackag e SC4614MSTRT(1)(2) MSOP-10 S C 4614E V B Note: (1) Only available in tape and reel packaging. A reel contains 2500 devices. (2) Lead free product. This product is fully WEEE and RoHS compliant. (MSOP-10) Pin Descriptions Pin # Pin Name 1 BST Boost input for top gate drive bias. 2 OCS Current limit setting. Connect resistors from this pin to DRV pin and to ground to program the trip point of load current. Refer to Applications Information Section for details. 3 COMP 4 FB 5 GND Chip ground. 6 DRV Internal LDO output. Connect a 1uF ceramic capasitor from this pin to ground for decoupling. This voltage is used for chip bias, including gate drivers. 7 VC C Chip input power supply. 8 DL Gate drive for bottom MOSFET. 9 PN Phase node. Connect this pin to bottom N-MOSFET drain. 10 DH Gate drive for top MOSFET. 2005 Semtech Corp. Pin Function Error amplifier output for compensation. Voltage feed back of sychronous buck converter. 4 www.semtech.com SC4614 POWER MANAGEMENT Block Diagram 8.2V 2005 Semtech Corp. 5 www.semtech.com SC4614 POWER MANAGEMENT Applications Information To program a load trip point for short circuit protection, it is recommended to connect a 3.3k resistor from the OCS pin to the ground, and a resistor Rset from the OCS pin to the DRV pin, as shown in Fig. 1. THEOR Y OF OPERA TION THEORY OPERATION The SC4614 is a high-speed, voltage mode PWM controller that provides the control and protection features necessary for a synchronous buck converter. As shown in the block diagram of the SC4614, the voltage-mode PWM controller consists of an error amplifier, a 500kHz ramp generator, a PWM comparator, a RS latch circuit, and two MOSFET drivers. The buck converter output voltage is fed back to the error amplifier negative input and is regulated to a reference voltage level. The error amplifier output is compared with the ramp to generate a PWM wave, which is amplified and used to drive the MOSFETs in the buck converter. The PWM wave at the phase node with the amplitude of Vin is filtered out to get a DC output. The PWM controller works with softstart and fault monitoring circuitry to meet application requirements. 12V 7 6 DRV Rset 2 OCS SC461 4 3.3k GND 5 UVLO, Start Up and Shut Down To initiate the SC4614, a supply voltage is applied to the Vcc pin. The top gate (DH) and bottom gate (DL) are held low until Vcc voltage exceeds UVLO (Under Voltage Lock Out) threshold, typically 4.0V. Then the internal Soft-Start (SS) capacitor begins to charge, the top gate remains low, and the bottom gate is pulled high to turn on the bottom MOSFET. When the SS voltage at the capacitor reaches 0.4V, the top and bottom gates of PWM controller begin to switch. The switching regulator output is slowly ramping up for a soft turn-on. Fig. 1. Programming load trip point 350 325 Vpn (mV) 300 If the supply voltages at the Vcc pin falls below UVLO threshold during a normal operation, the SS capacitor begins to discharge. When the SS voltage reaches 0.4V, the PWM controller controls the switching regulator output to ramp down slowly for a soft turn-off. 275 250 225 200 175 150 0 100 200 300 400 500 600 Rset (k -ohm) Hiccup Mode Short Circuit Protection The SC4614 uses low-side MOSFET Rdson sensing for over current protection. In every switching cycle, after the bottom MOSFET is on for 150ns, the SC4614 detects the phase node voltage and compares it with an internal setting voltage. If the phase node is lower than the setting voltage, an overcurrent condition occurs. The SC4614 will discharge the internal SS capacitor and shutdown both outputs. After waiting for around 10 milliseconds, the SC4614 begins to charge the SS capacitor again and initiates a fresh startup. The startup and shutdown cycle will repeat until the short circuit is removed. This is called a hiccup mode short circuit protection. 2005 Semtech Corp. V CC Fig. 2. Pull up resistor (Rset) vs. trip voltage Vpn The resistor Rset can be found in Fig. 2 for a given phase node voltage Vpn at the load trip point. This voltage is the product of the inductor peak current at the load trip point and the Rdson of the low-side MOSFET: V pn = I peak ´ Rds _ on The soft start time of the SC4614 is fixed at around 1.5ms. Therefore, the maximum soft start current is de6 www.semtech.com SC4614 POWER MANAGEMENT Applications Information (Cont.) termined by the output inductance and output capacitance. The values of output inductor and output bulk capacitors have to be properly selected so that the soft start peak current does not exceed the load trip point of the short circuit protection. duction losses of the top and bottom MOSFETs are given by: Internal LDO for Gate Drive An internal LDO is designed in the SC4614 to lower the 12V supply voltage for gate drive. A 1uF external ceramic capacitor connected in between DRV pin to the ground is needed to support the LDO. The LDO output is connected to the low gate drive internally, and has to be connected to the high gate drive through an external bootstrap circuit. The LDO output voltage is set at 8.2V. The manufacture data and bench tested results show that, for low Rdson MOSFETs run at applied load current, the optimum gate drive voltage is around 8.2V, where the total power losses of power MOSFETs are minimized. PC _ BOT = I O2 × Rdson × (1 - D ) PC _ TOP = I O2 × Rdson × D If the requirement of total power losses for each MOSFET is given, the above equations can be used to calculate the values of Rdson and gate charge, then the devices can be determined accordingly. The solution should ensure the MOSFET is within its maximum junction temperature at highest ambient temperature. Output Capacitor The output capacitors should be selected to meet both output ripple and transient response criteria. The output capacitor ESR causes output ripple VRIPPLE during the inductor ripple current flowing in. To meet output ripple criteria, the ESR value should be: COMPONENT SELECTION General design guideline of switching power supplies can be applied to the component selection for the SC4614. RESR < Induct or and MOSFET Inductor MOSFETss The selection of inductor and MOSFETs should meet thermal requirements because they are power loss dominant components. Pick an inductor with as high inductance as possible without adding extra cost and size. The higher inductance, the lower ripple current, the smaller core loss and the higher efficiency will be. However, too high inductance slows down output transient response. It is recommended to choose the inductance that creates an inductor ripple current of approximate 20% of maximum load current. So choose inductor value from: L= The output capacitor ESR also causes output voltage transient VT during a transient load current IT flowing in. To meet output transient criteria, the ESR value should be: RESR < VT IT To meet both criteria, the smaller one of above two ESRs is required. The output capacitor value also contributes to load transient response. Based on a worst case where the inductor energy 100% dumps to the output capacitor during the load transient, the capacitance then can be calculated by: V 5 × VO × (1 - O ) I O × f osc VIN The MOSFETs are selected by their Rdson, gate charge, and package specifications. The SC4614 provides 1.5A gate drive current and gives 50nC/1.5A=33ns switching time for driving a 50nC gate charge MOSFET. The switching time ts contributes to the top MOSFET switching loss: I T2 C > L× 2 VT PS = I O ×VIN × t S × f OSC Input Capacitor The input capacitor should be chosen to handle the RMS ripple current of a synchronous buck converter. This value There is no significant switching loss for the bottom MOSFET because of its zero voltage switching. The con 2005 Semtech Corp. L × f OSC × VRIPPLE V VO × (1 - O ) VIN 7 www.semtech.com SC4614 POWER MANAGEMENT Applications Information (Cont.) is given by: SC4614 AND MOSFETS I RMS = (1 - D ) × I 2 IN + D × ( I o - I IN ) 2 REF where Io is the load current, IIN is the input average current, and D is the duty cycle. Choosing low ESR input capacitors will help maximize ripple rating for a given size. + Vc PWM MODULAT OR EA FB - L Vo OUT COMP Zf Bootstrap Circuit The SC4614 uses an external bootstrap circuit to provide a voltage at the BST pin for the top MOSFET drive. This voltage, referring to the Phase Node, is held up by a bootstrap capacitor. Typically, it is recommended to use a 1uF ceramic capacitor with 16V rating and a commonly available diode IN4148 for the bootstrap circuit. Zs Co Resr Fig. 3. Block diagram of the control loop Filters for Supply Power For each pin of DRV and Vcc, it is recommended to use a 1uF/16V ceramic capacitor for decoupling. In addition, place a small resistor (10 ohm) in between the Vcc pin and the supply power for noise reduction. The model is a second order system with a finite DC gain, a complex pole pair at Fo, and an ESR zero at Fz, as shown in Fig. 4. The locations of the poles and zero are determined by: CONTROL LOOP DESIGN FO = The goal of compensation is to shape the frequency response charateristics of the buck converter to achieve a better DC accuracy and a faster transient response for the output voltage, while maintaining the loop stability. FZ = The block diagram in Fig. 3 represents the control loop of a buck converter designed with the SC4614. The control loop consists of a compensator, a PWM modulator, and a LC filter. 1 LC 1 RESR C The compensator in Fig. 3 includes an error amplifier and impedance networks Zf and Zs. It is implemented by the circuit in Fig. 5. The compensator provides an integrator, double poles and double zeros. As shown in Fig. 4, the integrator is used to boost the gain at low frequency. Two zeros are introduced to compensate excessive phase lag at the loop gain crossover due to the integrator (-90deg) and complex pole pair (-180deg). Two high frequency poles are designed to compensate the ESR zero and attenuate high frequency noise. The LC filter and PWM modulator represent the small signal model of the buck converter operating at fixed switching frequency. The transfer function of the model is given by: VO VIN 1 + sRESRC = × VC Vm 1 + sL / R + s 2 LC where VIN is the power rail voltage, Vm is the amplitude of the 500kHz ramp, and R is the equivalent load. 2005 Semtech Corp. 8 www.semtech.com SC4614 POWER MANAGEMENT Applications Information (Cont.) (2). Select the open loop crossover frequency Fc located at 10% to 20% of the switching frequency. At Fc, find the required DC gain. 60 Fp1 (3). Use the first compensator pole Fp1 to cancel the ESR zero Fz. Fp2 COM PENSATOR GAI N 30 GAIN (dB) Fz1 LO Fz2 OP GA IN (4). Have the second compensator pole Fp2 at half the switching frequency to attenuate the switching ripple and high frequency noise. Fo 0 CO Fz NV ER TE RG AI N Fc (5). Place the first compensator zero Fz1 at or below 50% of the power stage resonant frequency Fo. -30 (6). Place the second compensator zero Fz2 at or below the power stage resonant frequency Fo. -60 100 1K 10K 100 K 1M FR EQ UENCY (Hz ) A MathCAD program is available upon request for the calculation of the compensation parameters. Fig. 4. Bode plots for control loop design LA YOUT GUIDELINES LAY C2 C1 R2 C3 The switching regulator is a high di/dt power circuit. Its Printed Circuit Board (PCB) layout is critical. A good layout can achieve an optimum circuit performance while minimizing the component stress, resulting in better system reliability. During PCB layout, the SC4614 controller, MOSFETs, inductor, and power decoupling capacitors have to be considered as a unit. R3 Vo 1 - Vc 2 3 + VREF Rtop Rbot 0.5V The following guidelines are typically recommended for using the SC4614 controller. (1). Place a 4.7uF to 10uF ceramic capacitor close to the drain of top MOSFET for the high frequency and high current decoupling. The loop formed by the capacitor, the top and bottom MOSFETs must be as small as possible. Keep the input bulk capacitors close to the drain of the top MOSFETs. Fig. 5. Compensation network The top resistor Rtop of the voltage divider in Fig. 5 can be chosen from 1k to 5k. Then the bottom resistor Rbot is found from: Rbot = (2). Place the SC4614 over a quiet ground plane to avoid pulsing current noise. Keep the ground return of the gate drive short. 0.5V × Rtop VO - 0.5V where 0.5V is the internal reference voltage of the SC4614. (3). Connect bypass capacitors as close as possible to the decoupling pins (DRV and Vcc) to the ground pin GND. The trace length of the decoupling capacitor on DRV pin should be no more than 0.2” (5mm). The other components of the compensator can be calculated using following design procedure: (4). Locate the components of the bootstrap circuit close to the SC4614. (1). Plot the converter gain, including LC filter and PWM modulator. 2005 Semtech Corp. 9 www.semtech.com SC4614 POWER MANAGEMENT Applications Information (Cont.) TTypical ypical Application Schematics with 12V In put Input 12V Rcc 2R2 C4 10uF Q1 Rli mit R4 3.3k 499k 0 IPD05N03 C15 U1 1 2 3 4 5 0 C8 10nF R13 11.5k BST DH OCS PN COMP DL FB VCC GND DRV SC4614 10 1uF 8 6 1.5V/15A L1 1 1.2uH R11 1R0 Q3 7 1800uF 0 D1 D1N4148 9 + C3 2 R12 14.7k IPD05N03 C18 C9 2.2nF + C5 1800uF C7 + C6 10uF 1800uF C17 1uF 1uF R8 301 C13 2.2nF R15 7.32k 0 C10 680pF 0 Bill of Materials (12V Input) Item Quantity Reference Part Vendor 1 1 C4 10uF/16V Vishay 2 1 C7 10uF/6.3V Vishay 3 1 C3 1800uF/16V Rubycon, MBZ 4 2 C5,C6 1800uF/6.3V Rubycon, MBZ 5 3 C15,C17,C18 1uF Vishay 6 1 C9 2.2nF Vishay 7 1 C13 2.2nF Vishay 8 1 C8 10nF Vishay 9 1 C10 680pF Vishay 10 1 D1 D1N4148 Any 11 1 L1 1.2uH Cooper Electr. Tech 12 2 Q3,Q1 IPD05N03 Infineon 13 1 Rcc 2R2 Vishay 14 1 Rlimit 3.3k Vishay 15 1 R4 499k Vishay 16 1 R8 301 Vishay 17 1 R11 1R0 Vishay 18 1 R12 14.7k Vishay 19 1 R15 7.32k Vishay 20 1 R13 11.5k Vishay 21 1 U1 SC4614 SEMTECH 2005 Semtech Corp. 10 www.semtech.com SC4614 POWER MANAGEMENT Applications Information (Cont.) P er eristics (12V In put) erfformance Charact Characteristics Input) Start up Efficiency (%) vs Load Current 90 85 80 12V Input (5V/DIV) 75 70 65 1.5V Output (1V/DIV) 60 1 3 5 7 9 11 13 15 X=5ms/DIV Load Current (A) Transient Response Load Characteristics (Output vs Load Current) 1.6 1.4 1.5V Output Respo nse (100mV/DIV) 1.2 1.0 0.8 0.6 0.4 0.2 Step Load Current (10A/DIV) 0.0 0 5 10 15 20 X=20us/DIV Load Current(A) Gate Waveforms (Io=15A) Short Circuit Protection Output Short DL (10V/DIV) 1.5V OUT (1V/DIV) DH (10V/DIV) PN (10V/DIV) Output Current (10A/DIV) X=5ms/DIV X=50ns/DIV 2005 Semtech Corp. 11 www.semtech.com SC4614 POWER MANAGEMENT Applications Information (Cont.) TTypical ypical Application Schematics with 25V In put Input Vin=25V Rcc 732 C4 10uF Q1 Rli mit R4 3.3k 499k 0 IRLR7821 C15 U1 1 2 3 4 5 0 C8 4.7nF BST DH OCS PN COMP DL FB VCC GND DRV 10 9 8 1800uF 0 1uF D1 D1N4148 + C3 5V /10A L1 1 2 2.2uH Q3 R11 1R0 R12 22k 7 6 SC4614 C17 1uF C9 2.2nF C7 10uF + C6 1800uF IRLR7821 C18 1uF R8 301 C13 2.2nF R15 2.43k 0 C10 1nF R13 22k D2 0 BZX84B16LT1 0 Note: Zener diode D2 is required when Vin is 18V or higher. Bill of Materials (25V Input) Item Quantity Reference Part Vendor 1 1 C4 10uF/35V Murata 2 1 C7 10uF/6.3V Vishay 3 1 C3 1800uF/35V Rubycon 4 1 C6 1500uF/6.3V Rubycon, MBZ 5 3 C15,C17,C18 1uF Vishay 6 1 C9 2.2nF Vishay 7 1 C13 2.2nF Vishay 8 1 C8 4.7nF Vishay 9 1 C10 1nF Vishay 10 1 D1 D1N4148 Any 11 1 D2 BZX84B16LT1 ON Semi 12 1 L1 2.2uH Cooper Electr. Tech 13 2 Q3,Q1 IRLR7821 IR 14 1 Rcc 732 Vishay 15 1 Rlimit 3.3k Vishay 16 1 R4 499k Vishay 17 1 R8 301 Vishay 18 1 R11 1R0 Vishay 19 1 R12 22k Vishay 20 1 R15 2.43k Vishay 21 1 R13 22k Vishay 22 1 U1 SC4614 SEMTECH 2005 Semtech Corp. 12 www.semtech.com SC4614 POWER MANAGEMENT Applications Information (Cont.) P er eristics (25V In put) erfformance Charact Characteristics Input) Start up Efficiency (%) vs Load Current 92 90 88 25V Input (10V/DIV) 86 84 82 80 5V Output (2V/DIV) 78 76 1 2 3 4 5 6 7 8 9 10 X=5ms/DIV Load Current (A) Gate Waveforms (Io=10A) Transient Response 5V Output Response (200mV/DIV) DL (10V/DIV) DH (10V/DIV) PN (10V/DIV) Step Load Current (10A/DIV) X=100ns/DIV 2005 Semtech Corp. X=20us/DIV 13 www.semtech.com SC4614 POWER MANAGEMENT Outline Drawing - MSOP-10 e A DIM D A A1 A2 b c D E1 E e L L1 N 01 aaa bbb ccc N 2X E/2 ccc C 2X N/2 TIPS E E1 PIN 1 INDICATOR 1 2 B DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX .043 .000 .006 .030 .037 .011 .007 .003 .009 .114 .118 .122 .114 .118 .122 .193 BSC .020 BSC .016 .024 .032 (.037) 10 0° 8° .004 .003 .010 1.10 0.00 0.15 0.75 0.95 0.17 0.27 0.08 0.23 2.90 3.00 3.10 2.90 3.00 3.10 4.90 BSC 0.50 BSC 0.40 0.60 0.80 (.95) 10 0° 8° 0.10 0.08 0.25 D aaa C A2 SEATING PLANE H A bxN bbb c GAGE PLANE A1 C C A-B D 0.25 L (L1) DETAIL SEE DETAIL SIDE VIEW 01 A A NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. DATUMS -A- AND -B- TO BE DETERMINED AT DATUM PLANE -H- 3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. 4. REFERENCE JEDEC STD MO-187, VARIATION BA. Land Pattern - MSOP-10 X DIM (C) G C G P X Y Z Z Y DIMENSIONS INCHES MILLIMETERS (.161) .098 .020 .011 .063 .224 (4.10) 2.50 0.50 0.30 1.60 5.70 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 2005 Semtech Corp. 14 www.semtech.com