CS51031GD8G datasheet(6/10 Pages) ONSEMI

6 / 10 page

CS51031

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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

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 “on” 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.

Figure 4. Buck Regulator Block Diagram

RLOAD

Feedback

Control

CIN

VIN

APPLICATIONS INFORMATION

CS51031 DESIGN EXAMPLE

Specifications 12 V to 5.0 V, 3.0 A Buck Controller

• VIN = 12 V ±20% (i.e. 14.4 V max, 12 V nom, 9.6 V

min)

• VOUT = 5.0 V ±2%

• IOUT = 0.3 A to 3.0 A

• Output ripple voltage < 50 mV max

• Efficiency > 80%

• fSW = 200 kHz

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:

VOUT ) VF

VIN * VSAT

where:

VSAT = RDS(ON) × IOUT max and RDS(ON) is the value at

TJ 100°C.

If VF = 0.60 V and VSAT = 0.60 V then the above equation

becomes:

DMAX +

5.6

9.0

+ 0.62

DMIN +

5.6

13.8

+ 0.40

2) Switching Frequency and On and Off Time

Calculations

Given that fSW = 200 kHz and DMAX = 0.80

+ 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

The switching frequency is set by COSC, whose value is

given by:

COSC in pF +

10)6

FSW 1 )

FSW

106 *

103

FSW

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