CS51031

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6

Buck Regulator Operation

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

nominal value. The current drawn by the load is supplied by

below the threshold established by the feedback resistors R1

Q1 switches on and current flows through the inductor to the

output. The inductor current rises at a rate determined by

CS51031 is limited to 80%. If output voltage remains higher

does not turn on, skipping the pulse.

Figure 4. Buck Regulator Block Diagram.

L

Feedback

Control

APPLICATIONS INFORMATION

CS51031 DESIGN EXAMPLE

Specifications 12 V to 5.0 V, 3.0 A Buck Controller

min.)

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

+

VOUT ) VF

VIN * VSAT

where:

100

°C.

becomes:

DMAX +

5.6

9.0

+ 0.62

DMIN +

5.6

13.8

+ 0.40

2) Switching Frequency and On and Off Time

Calculations

T

+ 1.0

fSW

+ 5.0 ms

TON(max) + T

DMAX + 5.0 ms

0.62

^ 3.0 ms

TON(min) + T

DMIN + 5.0 ms

0.40

^ 2.0 ms

TOFF(max) + TON(min) + 5.0 ms * 2.0 ms + 3.0 ms

3) Oscillator Capacitor Selection

given by:

COSC in pF +

95

10)6

FSW 1 )

FSW

3

30

103

FSW

2