CS51031 Fast PFET Buck Controller The CS51031 is a switching controller for use in DC–DC converters. It can be used in the buck topology with a minimum number of external components. The CS51031 consists of a VCC monitor for controlling the state of the device, 1.0 A power driver for controlling the gate of a discrete P–channel transistor, fixed frequency oscillator, short circuit protection timer, programmable soft start, precision reference, fast output voltage monitoring comparator, and output stage driver logic with latch. The high frequency oscillator allows the use of small inductors and output capacitors, minimizing PC board area and systems cost. The programmable soft start reduces current surges at start up. The short circuit protection timer significantly reduces the duty cycle to approximately 1/30 of its cycle during short circuit conditions. The CS51031 is available in an 8 Lead SO plastic package. Features • 1.0 A Totem Pole Output Driver • High Speed Oscillator (700 kHz max) • No Stability Compensation Required • Lossless Short Circuit Protection • VCC Monitor • 2.0% Precision Reference • Programmable Soft Start • Wide Ambient Temperature Range: – Industrial Grade: –40°C to 85°C – Commercial Grade: 0°C to 70°C 5.0 V–12 V http://onsemi.com MARKING DIAGRAM 8 SO–8 D SUFFIX CASE 751 8 1 51031 ALYW 1 A WL, L YY, Y WW, W = Assembly Location = Wafer Lot = Year = Work Week PIN CONNECTIONS VGATE 1 VC PGND CS COSC GND VCC VFB ORDERING INFORMATION* CIN 47 µF Device MP1 IRF7416 VGATE MBRS360 VC VGATE Package Shipping CS51031YD8 SO–8 95 Units/Rail CS51031YDR8 SO–8 2500 Tape & Reel CS51031GD8 SO–8 95 Units/Rail CS51031GDR8 SO–8 2500 Tape & Reel PGND COSC 470 pF COSC CS51031 D1 GND CS RVCC 100 Ω VCC CS 0.1 µF CVCC 0.1 µF *Additional ordering information can be found on page 9 of this data sheet. L 4.7 µH RB VFB 2.5 kΩ RA 1.5 kΩ CRR 0.1 µF VO 3.3 V @ 3 A CO 100 µF × 2 Figure 1. Typical Application Diagram Semiconductor Components Industries, LLC, 2002 December, 2001 – Rev. 9 1 Publication Order Number: CS51031/D CS51031 MAXIMUM RATINGS* Rating Value Unit Power Supply Voltage, VCC 20 V Driver Supply Voltage, VC 20 V Driver Output Voltage, VGATE 20 V COSC, CS, VFB (Logic Pins) 6.0 V Peak Output Current 1.0 A Steady State Output Current 200 mA Operating Junction Temperature, TJ 150 °C Operating Temperature Range, TA –40 to 85 °C Storage Temperature Range, TS –65 to 150 °C 2.0 kV 260 peak 230 peak °C °C ESD (Human Body Model) Lead Temperature Soldering: Wave Solder: (through hole styles only) (Note 1) Reflow (SMD styles only) (Note 2) 1. 10 sec. maximum. 2. 60 sec. max above 183°C. *The maximum package power dissipation must be observed. ELECTRICAL CHARACTERISTICS (Specifications apply for 4.5 ≤ VCC ≤ 16 V, 3.0 V ≤ VC ≤ 16 V; Industrial Grade: –40°C < TA < 85°C; –40°C < TJ < 125°C: Commercial Grade: 0°C < TA < 70°C; 0°C < TJ < 125°C, unless otherwise specified.) Test Conditions Characteristic Oscillator Min Typ Max Unit 160 200 240 kHz VFB = 1.2 V Frequency COSC = 470 pF Charge Current 1.4 V < VCOSC < 2.0 V – 110 – µA Discharge Current 2.7 V > VCOSC > 2.0 V – 660 – µA Maximum Duty Cycle 1 – (tOFF/tON) 80.0 83.3 – % Short Circuit Timer VFB = 1.0 V; CS = 0.1 F; VCOSC = 2.0 V Charge Current 1.0 V < VCS < 2.0 V 175 264 325 µA Fast Discharge Current 2.55 V > VCS > 2.4 V 40 66 80 µA Slow Discharge Current 2.4 V > VCS > 1.5 V 4.0 6.0 10 µA Start Fault Inhibit Time 0 V < VCS < 2.5 V 0.70 0.85 1.40 ms Valid Fault Time 2.6 V > VCS > 2.4 V 0.2 0.3 0.45 ms GATE Inhibit Time 2.4 V > VCS > 1.5 V 9.0 15 23 ms – 2.5 3.1 4.6 % – – 2.5 – V Fault Duty Cycle CS Comparator VFB = 1.0 V Fault Enable CS Voltage Max. CS Voltage VFB = 1.5 V – 2.6 – V Fault Detect Voltage VCS when GATE goes high – 2.4 – V Fault Inhibit Voltage Minimum VCS – 1.5 – V Hold Off Release Voltage VFB = 0 V 0.4 0.7 1.0 V Regulator Threshold Voltage Clamp VCS = 1.5 V 0.725 0.866 1.035 V http://onsemi.com 2 CS51031 ELECTRICAL CHARACTERISTICS (continued) (Specifications apply for 4.5 ≤ VCC ≤ 16 V, 3.0 V ≤ VC ≤ 16 V; Industrial Grade: –40°C < TA < 85°C; –40°C < TJ < 125°C: Commercial Grade: 0°C < TA < 70°C; 0°C < TJ < 125°C, unless otherwise specified.) Characteristic Test Conditions VFB Comparators Min Typ Max Unit VCOSC = VCS = 2.0 V Regulator Threshold Voltage TJ = 25°C (Note 3) TJ = –40 to 125°C 1.225 1.210 1.250 1.250 1.275 1.290 V V Fault Threshold Voltage TJ = 25°C (Note 3) TJ = –40 to 125°C 1.12 1.10 1.15 1.15 1.17 1.19 V V Threshold Line Regulation 4.5 V ≤ VCC ≤ 16 V – 6.0 15 mV Input Bias Current VFB = 0 V – 1.0 4.0 µA Voltage Tracking (Regulator Threshold – Fault Threshold Voltage) 70 100 120 mV – – 4.0 20 mV Input Hysteresis Voltage Power Stage VCC = VC = 10 V; VFB = 1.2 V GATE DC Low Saturation Voltage VCOSC = 1.0 V; 200 mA Sink – 1.2 1.5 V GATE DC High Saturation Voltage VCOSC = 2.7 V; 200 mA Source; VC = VGATE – 1.5 2.1 V Rise Time CGATE = 1.0 nF; 1.5 V < VGATE < 9.0 V – 25 60 ns Fall Time CGATE = 1.0 nF; 9.0 V > VGATE > 1.5 V – 25 60 ns VCC Monitor Turn On Threshold – 4.200 4.400 4.600 V Turn Off Threshold – 4.085 4.300 4.515 V Hysteresis – 65 130 200 mV Current Drain ICC 4.5 V < VCC < 16 V, Gate switching – 4.5 6.0 mA IC 3.0 V < VC < 16 V, Gate non–switching – 2.7 4.0 mA Shutdown ICC VCC = 4.0 – 500 900 µA 3. Guaranteed by design, not 100% tested in production. PACKAGE LEAD DESCRIPTION PACKAGE PIN NUMBER SO–8 PIN SYMBOL 1 VGATE Driver pin to gate of external PFET. 2 PGND Output power stage ground connection. 3 COSC Oscillator frequency programming capacitor. 4 GND Logic ground. 5 VFB Feedback voltage input. 6 VCC Logic supply voltage. 7 CS Soft start and fault timing capacitor. 8 VC Driver supply voltage. FUNCTION http://onsemi.com 3 CS51031 VC VREF RG IC Oscillator COSC 7IC VGATE VGATE Flip–Flop + Comparator A1 G1 – Q R F2 2.5 V – VFB Comparator – + A6 + 0.7 V – + + – VREF 3.3 V – Fault Comp + G4 VREF = 3.3 V 1.15 V CS Charge Sense Comparator – + VCCOK VCC – + Hold Off Comp VCC G3 + A4 IT CS Comparator + A2 – F1 2.5 V 2.4 V 2.3 V Q R G5 – + 1.5 V – + IT 5 – + IT 55 – + CS VFB 1.25 V – – + 1.5 V PGND Q S G2 – A3 + Slow Discharge Comparator S Q Slow Discharge Flip–Flop GND Figure 2. Block Diagram CIRCUIT DESCRIPTION THEORY OF OPERATION of the charge time. The PFET gets turned off and remains off during the oscillator’s discharge time with the maximum duty cycle to 80%. It requires 7.0 mV typical, and 20 mV maximum ripple on the VFB pin is required to operate. This method of control does not require any loop stability compensation. Control Scheme The CS51031 monitors the output voltage to determine when to turn on the PFET. If VFB falls below the internal reference voltage of 1.25 V during the oscillator’s charge cycle, the PFET is turned on and remains on for the duration http://onsemi.com 4 CS51031 Startup Lossless Short Circuit Protection The CS51031 has an externally programmable soft start feature that allows the output voltage to come up slowly, preventing voltage overshoot on the output. At startup, the voltage on all pins is zero. As VCC rises, the VC voltage along with the internal resistor RG keeps the PFET off. As VCC and VC continue to rise, the oscillator capacitor (COSC ) and the Soft Start/Fault Timing capacitor (CS) charges via internal current sources. COSC gets charged by the current source IC and CS gets charged by the IT source combination described by: The CS51031 has “lossless” short circuit protection since there is no current sense resistor required. When the voltage at the CS pin (the fault timing capacitor voltage ) reaches 2.5 V during startup, the fault timing circuitry is enabled by A2. During normal operation the CS voltage is 2.6 V. During a short circuit or a transient condition, the output voltage moves lower and the voltage at VFB drops. If VFB drops below 1.15 V, the output of the fault comparator goes high and the CS51031 goes into a fast discharge mode. The fault timing capacitor, CS, discharges to 2.4 V. If the VFB voltage is still below 1.15 V when the CS pin reaches 2.4 V, a valid fault condition has been detected. The slow discharge comparator output goes high and enables gate G5 which sets the slow discharge flip flop. The VGATE flip flop resets and the output switch is turned off. The fault timing capacitor is slowly discharged to 1.5 V. The CS51031 then enters a normal startup routine. If the fault is still present when the fault timing capacitor voltage reaches 2.5 V, the fast and slow discharge cycles repeat as shown in figure 3. If the VFB voltage is above 1.15 V when CS reaches 2.4 V a fault condition is not detected, normal operation resumes and CS charges back to 2.6 V. This reduces the chance of erroneously detecting a load transient as a fault condition. I I ICS IT T T 55 5 The internal Holdoff Comparator ensures that the external PFET is off until VCS > 0.7 V, preventing the GATE flip–flop (F2) from being set. This allows the oscillator to reach its operating frequency before enabling the drive output. Soft start is obtained by clamping the VFB comparator’s (A6) reference input to approximately 1/2 of the voltage at the CS pin during startup, permitting the control loop and the output voltage to slowly increase. Once the CS pin charges above the Holdoff Comparator trip point of 0.7 V, the low feedback to the VFB Comparator sets the GATE flip–flop during COSC’s charge cycle. Once the GATE flip–flop is set, VGATE goes low and turns on the PFET. When VCS exceeds 2.3 V, the CS charge sense comparator (A4) sets the VFB comparator reference to 1.25 V completing the startup cycle. 2.6 V VCS S2 2.4 V S2 S1 S3 S3 S1 S3 S1 S3 1.5 V 0.7 V 2.5 V S2 0V 0V TSTART START td1 NORMAL OPERATION tFAULT tRESTART td2 FAULT VGATE 1.25 V 1.15 V VFB Figure 3. Voltage on Start Capacitor (VGS), the Gate (VGATE), and in the Feedback Loop (VFB), During Startup, Normal and Fault Conditions. http://onsemi.com 5 tFAULT CS51031 Buck Regulator Operation and R2 and the reference voltage VREF, the power transistor Q1 switches on and current flows through the inductor to the output. The inductor current rises at a rate determined by (VIN – VOUT)/L. The duty cycle (or “on” time) for the CS51031 is limited to 80%. If output voltage remains higher than nominal during the entire COSC change time, the Q1 does not turn on, skipping the pulse. A block diagram of a typical buck regulator is shown in Figure 4. If we assume that the output transistor is initially off, and the system is in discontinuous operation, the inductor current IL is zero and the output voltage is at its nominal value. The current drawn by the load is supplied by the output capacitor CO. When the voltage across CO drops below the threshold established by the feedback resistors R1 L Q1 VIN R1 CIN CO D1 RLOAD R2 Control Feedback Figure 4. Buck Regulator Block Diagram. APPLICATIONS INFORMATION CS51031 DESIGN EXAMPLE If VF = 0.60 V and VSAT = 0.60 V then the above equation becomes: Specifications 12 V to 5.0 V, 3.0 A Buck Controller DMAX 5.6 0.62 9.0 • VIN = 12 V ±20% (i.e. 14.4 V max., 12 V nom., 9.6 V • • • • • min.) VOUT = 5.0 V ±2% IOUT = 0.3 A to 3.0 A Output ripple voltage < 50 mV max. Efficiency > 80% fSW = 200 kHz DMIN 5.6 0.40 13.8 2) Switching Frequency and On and Off Time Calculations Given that fSW = 200 kHz and DMAX = 0.80 T 1.0 5.0 s fSW 1) Duty Cycle Estimates Since the maximum duty cycle D, of the CS51031 is limited to 80% min., it is necessary to estimate the duty cycle for the various input conditions over the complete operating range. The duty cycle for a buck regulator operating in a continuous conduction mode is given by: D TON(max) T DMAX 5.0 s 0.62 3.0 s TON(min) T DMIN 5.0 s 0.40 2.0 s TOFF(max) TON(min) 5.0 s 2.0 s 3.0 s VOUT VF VIN VSAT 3) Oscillator Capacitor Selection The switching frequency is set by COSC, whose value is given by: where: VSAT = Rds(on) × IOUT max. and Rds(on) is the value at TJ 100°C. COSC in pF http://onsemi.com 6 95 106 SW 3010 FSW 1 310 6 F F SW 3 2 CS51031 4) Inductor Selection 6) VFB Divider The inductor value is chosen for continuous mode operation down to 0.3 Amps. The ripple current ∆I = 2 × IOUTmin = 2 × 0.3 A = 0.6 A. L min (VOUT VD) TOFF(max) I I LMIN 5.0 V R1 R2 5.0 K 1.0 mA 5.6 V 2.0 s 0.4 A 28 H Let R2 = 1.0 K Rearranging the divider equation gives: The core must not saturate with the maximum expected current, here given by: OUT 1.0 1.0 k5.0 V 1.0 3.0 k V1.25 1.25 R1 R2 IMAX IOUT I2 3.0 A 0.4 A2 3.2 A 7) Divider Bypass Capacitor CRR 5) Output Capacitor Since the feedback resistors divide the output voltage by a factor of 4.0, i.e. 5.0 V/1.25 V= 4.0, it follows that the output ripple is also divided by four. This would require that the output ripple be at least 60 mV (4.0 × 15 mV) to trip the feedback comparator. We use a capacitor CRR to act as an AC short. The ripple voltage frequency is equal to the switching frequency so we choose CRR = 1.0 nF. The output capacitor and the inductor form a low pass filter. The output capacitor should have a low ESL and ESR. Low impedance aluminum electrolytic, tantalum or organic semiconductor capacitors are a good choice for an output capacitor. Low impedance aluminum are less expensive. Solid tantalum chip capacitors are available from a number of suppliers and are the best choice for surface mount applications. The output capacitor limits the output ripple voltage. The CS51031 needs a maximum of 20 mV of output ripple for the feedback comparator to change state. If we assume that all the inductor ripple current flows through the output capacitor and that it is an ideal capacitor (i.e. zero ESR), the minimum capacitance needed to limit the output ripple to 50 mV peak to peak is given by: C The input bias current to the comparator is 4.0 µA. The resistor divider current should be considerably higher than this to ensure that there is sufficient bias current. If we choose the divider current to be at least 250 times the bias current this permits a divider current of 1mA and simplifies the calculations. 5.6 V 3.0 s 28 H 0.6 A This is the minimum value of inductor to keep the ripple current < 0.6 A during normal operation. A smaller inductor will result in larger ripple current. Ripple current at a minimum off time is (VOUT VF) TOFF(min) VOUT 1.25 V R1 R2 1.25 V R1 1.0 R2 R2 8) Soft Start and Fault Timing Capacitor CS CS performs several important functions. First it provides a delay time for load transients so that the IC does not enter a fault mode every time the load changes abruptly. Secondly it disables the fault circuitry during startup, it also provides soft start by clamping the reference voltage during startup, allowing it to rise slowly, and, finally it controls the hiccup short circuit protection circuitry. This reduces the duty cycle to approximately 0.035 during short circuit conditions. An important consideration in calculating CS is that it’s voltage does not reach 2.5 V (the voltage at which the fault detect circuitry is enabled) before VFB reaches 1.15 V otherwise the power supply will never start. If the VFB pin reaches 1.15 V, the fault timing comparator will discharge CS and the supply will not start. For the VFB voltage to reach 1.15 V the output voltage must be at least 4 × 1.15 = 4.6 V. If we choose an arbitrary startup time of 900 µs, the value of CS is: 0.6 A I 7.5F 8.0 fSW V 8.0 (200 103Hz) (50 103 V) The minimum ESR needed to limit the output voltage ripple to 50 mV peak to peak is: 3 ESR V 50 10 83 m 0.6 A I The output capacitor should be chosen so that its ESR is less than 83 mΩ. During the minimum off time, the ripple current is 0.4 A and the output voltage ripple will be: V ESR I 83m 0.4 33 mV tStartup CS 2.5 V ICharge CS min http://onsemi.com 7 900 s 264 A 950 nF 0.1 F 2.5 V CS51031 the VCC and VC pins. This capacitor must also ensure that the VCC remains above the UVLO voltage in the event of an output short circuit. A low ESR capacitor of at least 100 µF is good. A ceramic surface mount capacitor should also be connected between VCC and ground to filter high frequency noise. The fault time is the sum of the slow discharge time the fast discharge time and the recharge time. It is dominated by the slow discharge time. The first parameter is the slow discharge time, it is the time for the CS capacitor to discharge from 2.4 V to 1.5 V and is given by: tSlowDischarge(t) CS (2.4 V 1.5 V) IDischarge 10) MOSFET Selection The CS51031 drives a P–channel MOSFET. The VGATE pin swings from GND to VC. The type of PFET used depends on the operating conditions but for input voltages below 7.0 V a logic level FET should be used. A PFET with a continuous drain current (ID) rating greater than the maximum output current is required. The Gate–to–Source voltage VGS and the Drain–to Source Breakdown Voltage should be chosen based on the input supply voltage. The power dissipation due to the conduction losses is given by: where IDischarge is 6.0 µA typical. tSlowDischarge(t) CS 1.5 105 The fast discharge time occurs when a fault is first detected. The CS capacitor is discharged from 2.5 V to 2.4 V. tFastDischarge(t) CS (2.5 V 2.4 V) IFastDischarge where IFastDischarge is 66 µA typical. tFastDischarge(t) CS 1515 PD IOUT2 RDS(ON) D The recharge time is the time for CS to charge from 1.5 V to 2.5 V. tCharge(t) where RDS(ON) is the value at TJ 100°C CS (2.5 V 1.5 V) ICharge The power dissipation of the PFET due to the switching losses is given by: where ICharge is 264 µA typical. PD 0.5 VIN IOUT (tr) fSW tCharge(t) CS 3787 where tr = Rise Time. The fault time is given by: tFault CS (3787 1515 1.5 105) 11) Diode Selection The flyback or catch diode should be a Schottky diode because of it’s fast switching ability and low forward voltage drop. The current rating must be at least equal to the maximum output current. The breakdown voltage should be at least 20 V for this 12 V application. The diode power dissipation is given by: tFault CS (1.55 105) For this circuit tFault 0.1 106 1.55 105 15.5 s A larger value of CS will increase the fault time out time but will also increase the soft start time. PD IOUT VD (1.0 D min) 9) Input Capacitor The input capacitor reduces the peak currents drawn from the input supply and reduces the noise and ripple voltage on http://onsemi.com 8 CS51031 ORDERING INFORMATION Operating Temperature Range Package Shipping CS51031YD8 –40°C < TA < 85°C SO–8 95 Units/Rail CS51031YDR8 2500 Tape & Reel Device –40°C < TA < 85°C SO–8 CS51031GD8 0°C < TA < 70°C SO–8 95 Units/Rail CS51031GDR8 0°C < TA < 70°C SO–8 2500 Tape & Reel http://onsemi.com 9 CS51031 PACKAGE DIMENSIONS SO–8 D SUFFIX CASE 751–07 ISSUE V –X– NOTES: 1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982. 2. CONTROLLING DIMENSION: MILLIMETER. 3. DIMENSION A AND B DO NOT INCLUDE MOLD PROTRUSION. 4. MAXIMUM MOLD PROTRUSION 0.15 (0.006) PER SIDE. 5. DIMENSION D DOES NOT INCLUDE DAMBAR PROTRUSION. ALLOWABLE DAMBAR PROTRUSION SHALL BE 0.127 (0.005) TOTAL IN EXCESS OF THE D DIMENSION AT MAXIMUM MATERIAL CONDITION. A 8 5 0.25 (0.010) S B 1 M Y M 4 K –Y– G C N DIM A B C D G H J K M N S X 45 SEATING PLANE –Z– 0.10 (0.004) H D 0.25 (0.010) M Z Y S X M J S PACKAGE THERMAL DATA Parameter SO–8 Unit RΘJC Typical 45 °C/W RΘJA Typical 165 °C/W http://onsemi.com 10 MILLIMETERS MIN MAX 4.80 5.00 3.80 4.00 1.35 1.75 0.33 0.51 1.27 BSC 0.10 0.25 0.19 0.25 0.40 1.27 0 8 0.25 0.50 5.80 6.20 INCHES MIN MAX 0.189 0.197 0.150 0.157 0.053 0.069 0.013 0.020 0.050 BSC 0.004 0.010 0.007 0.010 0.016 0.050 0 8 0.010 0.020 0.228 0.244 CS51031 Notes http://onsemi.com 11 CS51031 ON Semiconductor is a trademark and is a registered trademark of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. 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PUBLICATION ORDERING INFORMATION Literature Fulfillment: Literature Distribution Center for ON Semiconductor P.O. Box 5163, Denver, Colorado 80217 USA Phone: 303–675–2175 or 800–344–3860 Toll Free USA/Canada Fax: 303–675–2176 or 800–344–3867 Toll Free USA/Canada Email: [email protected] JAPAN: ON Semiconductor, Japan Customer Focus Center 4–32–1 Nishi–Gotanda, Shinagawa–ku, Tokyo, Japan 141–0031 Phone: 81–3–5740–2700 Email: [email protected] ON Semiconductor Website: http://onsemi.com For additional information, please contact your local Sales Representative. N. American Technical Support: 800–282–9855 Toll Free USA/Canada http://onsemi.com 12 CS51031/D