CS51033 Fast PFET Buck Controller The CS51033 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 CS51033 consists of a 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 PFET duty cycle to approximately 1/30 of its normal cycle during short circuit conditions. The CS51033 is available in an 8–Lead SO package. http://onsemi.com MARKING DIAGRAM 8 SO–8 D SUFFIX CASE 751 8 1 51033 ALYW 1 Features • 1.0 A Totem Pole Output Driver • High Speed Oscillator (700 kHz max) • No Stability Compensation Required • Lossless Short Circuit Protection • 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 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* Device Package Shipping CS51033YD8 SO–8 95 Units/Rail CS51033YDR8 SO–8 2500 Tape & Reel CS51033GD8 SO–8 95 Units/Rail CS51033GDR8 SO–8 2500 Tape & Reel *Additional ordering information can be found on page 9 of this data sheet. Semiconductor Components Industries, LLC, 2002 October, 2002 – Rev. 8 1 Publication Order Number: CS51033/D CS51033 3.3VIN CIN 100 µF D2 1N4148 RC 10 Ω D4 1N5818 C1 D3 1N4148 0.1 µF RG VC V GATE VCC COSC U1 CS51033 10 Ω IRF7404 0.1 µF VFB 4.7 µH 1.5VOUT @ 3.0 Amp 100 C2 1.0 µF C3 100 µF COSC 150 pF GND PGND CS 0.1 µF CS C0 100 µF 0.1 µF 100 µF C4 0.1 µF D1 1N5821 GND GND RA 1.5 k RB 300 Note: Capacitors C2, C3, and C4, are low ESR tantalum caps used for noise reduction. Figure 1. Typical Application Diagram MAXIMUM RATINGS* Rating Value Unit Power Supply Voltage, VCC 5.0 V Driver Supply Voltage, VC 20 V Driver Output Voltage, VGATE 20 V COSC, CS, VFB (Logic Pins) 5.0 V Peak Output Current 1.0 A Steady State Output Current 200 mA Operating Junction Temperature, TJ 150 °C –65 to 150 °C 2.0 kV 45 165 °C/W °C/W 230 peak °C Storage Temperature Range, TS ESD (Human Body Model) Package Thermal Resistance: Junction–to–Case, RθJC Junction–to–Ambient, RθJA Lead Temperature Soldering: Reflow (SMD styles only) (Note 1) 1. 60 sec. max above 183°C. *The maximum package power dissipation must be observed. http://onsemi.com 2 CS51033 ELECTRICAL CHARACTERISTICS (Specifications apply for 3.135 ≤ VCC ≤ 3.465, 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 0.70 0.85 1.40 ms Start Fault Inhibit Time – 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 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 VFB Comparators VCOSC = VCS = 2.0 V Regulator Threshold Voltage TJ = 25°C (Note 2) 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 2) TJ = –40 to 125°C 1.12 1.10 1.15 1.15 1.17 1.19 V V Threshold Line Regulation 3.135 V ≤ VCC ≤ 3.465 – 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 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 ICC 3.135 V < VCC < 3.465 V, Gate switching – 3.5 6.0 mA IC 3.0 V < VC < 16 V, Gate non–switching – 2.7 4.0 mA Current Drain 2. Guaranteed by design, not 100% tested in production. http://onsemi.com 3 CS51033 PACKAGE PIN DESCRIPTION PIN NUMBER 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 VC VCC RG IC Oscillator 7IC VGATE VGATE Flip–Flop + Comparator A1 – COSC G1 R Q F2 Q 2.5 V – – + – + 1.5 V PGND S G2 VFB Comparator A6 + 0.7 V – + + VCC – + Hold Off Comp VFB 1.25 V – VCC – 1.15 V CS Charge Sense Comparator G3 + A4 IT CS Comparator + A2 – R G5 2.4 V 2.3 V Q F1 2.5 V – + 1.5 V – + IT 5 – + IT 55 – + CS – + G4 – + Fault Comp – A3 + Slow Discharge Comparator S Q Slow Discharge Flip–Flop GND Figure 2. Block Diagram http://onsemi.com 4 CS51033 CIRCUIT DESCRIPTION THEORY OF OPERATION 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 2.4 V, the CS charge sense comparator (A4) sets the VFB comparator reference to 1.25 V completing the startup cycle. Control Scheme The CS51033 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 of the charge time. The PFET gets turned off and remains off during the oscillator’s discharge cycle time with the maximum duty cycle to 80%. It requires 7.0 mV typical, and 20 mV maximum ripple on the VFB pin to operate. This method of control does not require any loop stability compensation. Lossless Short Circuit Protection The CS51033 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, the fault timing circuitry is enabled. 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 CS51033 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 CS51033 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. Startup The CS51033 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: 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 2.6 V VCS S2 2.4 V S2 S1 S3 S3 S1 2.5 V S2 S3 S1 S3 1.5 V 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 CS51033 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)/Load. The duty cycle (or “on” time) for the CS51033 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. Charge Pump Circuit turns on, it’s drain voltage will be approximately equal to VIN. Since the voltage across C1 can not change instantaneously, D2 is reverse biased and the anode voltage rises to approximately 2.0 × 3.3 V – VD2. C1 transfers some of its stored charge C2 via D3. After several cycles there is sufficient gate drive voltage. (Refer to the CS51033 Application Diagram on page 2). An external charge pump circuit is necessary when the VC input voltage is below 5.0 V to ensure that there is suffifient gate drive voltage for the external FET. When VIN is applied, capacitors C1 and C2 will be charged to a diodes drop below VIN via diodes D2 and D4, respectively. When the PFET APPLICATIONS INFORMATION DESIGNING A POWER SUPPLY WITH THE CS51033 In this case we can assume that VD = 0.6 V and VSAT = 0.6 V so the equation reduces to: Specifications • • • • • V D OUT VIN VIN = 3.3 V ±10% (i.e. 3.63 V max., 2.97 V min.) VOUT = 1.5 V ±2.0% IOUT = 0.3 A to 3.0 A Output ripple voltage < 33 mV. FSW = 200 kHz From this, the maximum duty cycle DMAX is 53%, this occurs when VIN is at it’s minimum while the minimum duty cycle DMIN is 0.35%. 2) Switching Frequency and On and Off Time Calculations 1) Duty Cycle Estimates Since the maximum duty cycle D, of the CS51033 is limited to 80% min., it is best to estimate the duty cycle for the various input conditions to see that the design will work over the complete operating range. The duty cycle for a buck regulator operating in a continuous conduction mode is given by: FSW = 200 kHz. The switching frequency is determined by COSC, whose value is determined by: V VD D OUT VIN VSAT T 1.0 5.0 s FSW COSC where: VSAT = RDS(ON) × IOUT Max. http://onsemi.com 6 95 SW 3010 FSW 1 310 6 FSW F 3 2 470 pF CS51033 TON(MAX) 5.0 s 0.53 2.65 s 5) VFB Divider VOUT 1.25 V R1 R2 1.25 V R1 1.0 R2 R2 TON(MIN) 5.0 s 0.35 1.75 s 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 gives a divider current of 1.0 mA and simplifies the calculations. TOFF(MAX) 5.0 s 0.7 s 4.3 s 3) Inductor Selection Pick the inductor value to maintain continuous mode operation down to 0.3 Amps. The ripple current ∆I = 2 × IOUT(MIN) = 2 × 0.3 A = 0.6 A. 1.5 V R1 R2 1.5 k 1.0 mA 2.1 V 4.3 s V VD TOFF(MAX) LMIN OUT 15 H I 0.6 A Let R2 = 1.0 k Rearranging the divider equation gives: The CS51033 will operate with almost any value of inductor. With larger inductors the ripple current is reduced and the regulator will remain in a continuous conduction mode for lower values of load current. A smaller inductor will result in larger ripple current. The core must not saturate with the maximum expected current, here given by: OUT 1.0 V1.25 R1 R2 1.0 k 1.5 V 200 1.25 6) Divider Bypass Capacitor CRR 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 so that the output ripple is not attenuated by the divider network. The ripple voltage frequency is equal to the switching frequency so we choose CRR so that: I I IMAX OUT 3.0 A 0.6 A2.0 3.3 A 2.0 4) Output Capacitor The output capacitor limits the output ripple voltage. The CS51033 needs a maximum of 15 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: XC 1.0 2fC is negligible at the switching frequency. In this case FSW is 200 kHz if we allow XC = 3.0 Ω then: I CO 8.0 FSW V C 1.0 0.265 F 2f3 0.6 A 11.4 F 8.0 (200 103 Hz) (33 103 V) 7) Soft Start and Fault Timing Capacitor CS The minimum ESR needed to limit the output voltage ripple to 50 mV peak to peak is: CS performs several important functions. First it provides a dead 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 to rise slowly and finally it controls the hiccup short circuit protection circuitry. This function reduces the PFET’s duty cycle to 2.0% of the CS period. The most 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. 3 ESR V 50 10 55 m 0.6 A I The output capacitor should be chosen so that its ESR is at least half of the calculated value and the capacitance is at least ten times the calculated value. It is often advisable to use several capacitors in parallel to reduce ESR. Low impedance aluminum electrolytic, tantalum or organic semiconductor capacitors are a good choice for an output capacitor. Low impedance aluminum are the cheapest but are not available in surface mount at present. Solid tantalum chip capacitors are available from a number of suppliers and offer the best choice for surface mount applications. The capacitor working voltage should be greater than the output voltage in all cases. http://onsemi.com 7 CS51033 If we choose an arbitrary startup time of 200 µs, we calculate the value of CS from: A larger value of CS will increase the fault time out time but will also increase the Soft Start time. T CS 2.5 V ICHARGE 8) Input Capacitor The input capacitor reduces the peak currents drawn from the input supply and reduces the noise and ripple voltage on 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. CIN should be a low ESR capacitor of at least 100 µF. A ceramic surface mount capacitor should also be connected between VCC and ground to prevent spikes. 200 s 264 A CS(MIN) 0.02 F 2.5 V Use 0.1 µF. The fault time out time is the sum of the slow discharge time the fast discharge time and the recharge time and is obviously 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: 9) MOSFET Selection The CS51033 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. Choose a PFET with a continuous drain current (ID) rating greater than the maximum output current. RDS(ON) should be less than CS (2.4 V 1.5 V) TSLOWDISCHARGE IDISCHARGE where IDISCHARGE is 6.0 µA typical. TSLOWDISCHARGE CS 1.5 V 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 RDS CS (2.5 V 2.4 V) IFASTDISCHARGE 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 IFASTDISCHARGE is 66 µA typical. TFASTDISCHARGE CS 1515 The recharge time is the time for CS to charge from 1.5 V to 2.5 V. TCHARGE 0.6 V 167 m IOUT(MAX) PD IOUT2 RDS(ON) D The power dissipation due to the switching losses is given by: CS (2.5 V 1.5 V) ICHARGE PD 0.5 VIN IOUT (TRr TF) FSW where ICHARGE is 264 µA typical. where TR = Rise Time and TF = Fall Time. TCHARGE CS 3787 The fault time out time is given by: 10) 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 (3787 1515 1.5 105) TFAULT CS (1.55 105) For this circuit TFAULT 0.1 106 1.55 105 0.0155 PD IOUT VD (1.0 DMIN) http://onsemi.com 8 CS51033 ORDERING INFORMATION Device CS51033YD8 CS51033YDR8 CS51033GD8 CS51033GDR8 Operating Temperature Range Package Shipping 95 Units/Rail –40°C 40°C < TA < 85°C SO 8 SO–8 0°C < TA < 70°C 2500 Tape & Reel 95 Units/Rail 2500 Tape & Reel http://onsemi.com 9 CS51033 PACKAGE DIMENSIONS SO–8 D SUFFIX CASE 751–07 ISSUE AA 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. 6. 751-01 THRU 751-06 ARE OBSOLETE. NEW STANDAARD IS 751-07 –X– A 8 5 0.25 (0.010) S B 1 M Y M 4 K –Y– G C N X 45 SEATING PLANE –Z– 0.10 (0.004) H D 0.25 (0.010) M Z Y S X M S http://onsemi.com 10 J DIM A B C D G H J K M N S 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 CS51033 Notes http://onsemi.com 11 CS51033 ON Semiconductor and are registered trademarks 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 2–9–1 Kamimeguro, Meguro–ku, Tokyo, Japan 153–0051 Phone: 81–3–5773–3850 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 CS51033/D