CS51031YD8 View Datasheet(PDF) - Cherry semiconductor

Part Name

Description

View to exact match

CS51031YD8

Fast PFET Buck Controller Does Not Require Compensation

Cherry semiconductor 

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

Q1

VIN

CIN

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

0V

td1

tFAULT

tRESTART td2

tFAULT

NORMAL OPERATION

FAULT

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

Share Link: