PDF CS51031 Data sheet ( Hoja de datos )

Número de pieza	CS51031
Descripción	Fast PFET Buck Controller Does Not Require Compensation
Fabricantes	Cherry Semiconductor Corporation
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No Preview Available ! CS51031 Fast PFET Buck Controller Does Not Require Compensation Description Features The CS51031 is a switching con- troller for use in DC-DC converters. It can be used in the buck topology with a minimum number of exter- nal components. The CS51031 con- sists of a VCC monitor for control- ling the state of the device, 1.0A power driver for controlling the gate of a discrete P-channel transis- tor, fixed frequency oscillator, short circuit protection timer, pro- grammable soft start, precision ref- erence, fast output voltage monitor- ing 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 8L SO and 8L PDIP plastic packages. s 1A Totem Pole Output Driver s High Speed Oscillator (700kHz max) s No Stability Compensation Required s Lossless Short Circuit Protection s VCC Monitor s 2% Precision Reference s Programmable Soft Start Typical Application Diagram 5V - 12V CIN 47mF VGATE VGATE VC COSC 150pF PGnd CS CS51031 COSC VCC Gnd VFB 20W MP1 IRF7416 RVCC 10W CVCC 100mF 100 .01mF CS 0.1mF MBRS360 D1 L 4.7mH RB 2.5kW VO 3.3V @ 3A RA 1.5kW CRR 0.1mF CO 100mF ´ 2 Package Options 8 Lead SO Narrow & PDIP VGATE 1 PGnd COSC Gnd VC CS VCC VFB Rev. 2/13/98 Cherry Semiconductor Corporation 2000 South County Trail, East Greenwich, RI 02818 Tel: (401)885-3600 Fax: (401)885-5786 Email: [email protected] Web Site: www.cherry-semi.com 1 A ¨ Company 1 page Circuit Description: continued 2.4V, the CS charge sense comparator (A4) sets the VFB comparator reference to 1.25V completing the startup cycle. Lossless Short Circuit Protection The CS51031 has ÒlosslessÓ short circuit protection since there is no current sense resistor required. When the volt- age at the CS pin (the fault timing capacitor voltage ) reach- es 2.5V during startup, the fault timing circuitry is enabled. During normal operation the CS voltage is 2.6V. During a short circuit or a transient condition, the output voltage moves lower and the voltage at VFB drops. If VFB drops below 1.15V, the output of the fault comparator goes high and the CS51031 goes into a fast discharge mode. The fault timing capacitor, CS, discharges to 2.4V. If the VFB voltage is still below 1.15V when the CS pin reaches 2.4V, 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 1.5V. The CS51031 then enters a normal startup routine. If the fault is still present when the fault timing capacitor voltage reaches 2.5V, the fast and slow discharge cycles repeat as shown in figure 2. If the VFB voltage is above 1.15V when CS reaches 2.4V a fault condition is not detected, normal operation resumes and CS charges back to 2.6V. This reduces the chance of erroneously detecting a load transient as a fault condition. Buck Regulator Operation VIN CIN Q1 L D1 Control R1 CO R2 RLOAD Feedback Figure 3. Buck regulator block diagram. A block diagram of a typical buck regulator is shown in Figure 3. 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 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 ÒonÓ 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. 2.6V VCS 2.4V 1.5V 0V S1 TSTART START S2 S1 S2 S3 S2 2.5V S3 S1 S3 S3 NORMAL OPERATION td1 tFAULT tRESTART td2 FAULT tFAULT 0V VGATE 1.25V 1.15V VFB Figure 2. Voltage on start capacitor (VGS ), the gate (VGATE ), and in the feedback loop (VFB), during startup, normal and fault conditions. Applications Information CS51031 Design Example Specifications 12V to 5V, 3A Buck converter VIN = 12V ±20% (i.e. 14.4V max., 12Vnom., 9.6V min.) VOUT = 5V ±2% IOUT = 0.3A to 3A Output ripple voltage < 50mV max. Efficiency > 80% fSW = 200kHz 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 continu- ous conduction mode is given by: D = VOUT + VF VIN - VSAT 5 5 Page

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