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CS51031YN8 数据表(PDF) 5 Page - Cherry Semiconductor Corporation |
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CS51031YN8 数据表(HTML) 5 Page - Cherry Semiconductor Corporation |
5 / 8 page 5 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. Figure 2. Voltage on start capacitor (VGS ), the gate (VGATE ), and in the feedback loop (VFB), during startup, normal and fault conditions. Buck Regulator Operation 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. Control Feedback VIN L D1 R1 R2 RLOAD CO Q1 CIN 2.5V 0V FAULT NORMAL OPERATION START TSTART td1 td2 tFAULT tRESTART tFAULT S2 S2 S2 S3 S3 S3 S3 S1 S1 S1 2.6V 2.4V 1.5V 0V 1.25V 1.15V VCS VGATE VFB Circuit Description: continued Applications Information 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 CS51031 Design Example |
类似零件编号 - CS51031YN8 |
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类似说明 - CS51031YN8 |
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