MIC4722 datasheet(9/20 Pages) MICREL

9 / 20 page

Micrel, Inc.

MIC4722

September 2007

M9999-090507-B

Application Information

The MIC4722 is a 3A PWM non-synchronous buck

regulator. By switching an input voltage supply, and

filtering the switched voltage through an Inductor and

capacitor, a regulated DC voltage is obtained. Figure 1

shows a simplified example of a non-synchronous buck

converter.

Figure 1. Example of non-synchronous buck converter

For a non-synchronous buck converter, there are two

modes of operation; continuous and discontinuous.

Continuous or discontinuous refer to the inductor

current. If current is continuously flowing through the

inductor throughout the switching cycle, it is in

continuous operation. If the inductor current drops to

zero during the off time, it is in discontinuous operation.

Critically continuous is the point where any decrease in

output current will cause it to enter discontinuous

operation. The critically continuous load current can be

calculated as follows;

2.7MHz

OUT

⎥

⎦

⎤

⎢

⎣

⎡

−

Continuous or discontinuous operation determines how

we calculate peak inductor current.

Continuous Operation

Figure 2 illustrates the switch voltage and inductor

current during continuous operation.

Figure 2. Continuous Operation

The output voltage is regulated by pulse width

modulating (PWM) the switch voltage to the average

required output voltage. The switching can be broken up

into two cycles; On and Off.

During the on-time,

the high side switch is turned on,

current flows from the input supply through the inductor

and to the output. The inductor current is

Figure 3. On-Time

charged at the rate;

(

)

OUT

IN −

To determine the total on-time, or time at which the

inductor charges, the duty cycle needs to be calculated.

The duty cycle can be calculated as;

OUT

and the On time is;

2.7MHz

TON =

Therefore, peak to peak ripple current is;

()

2.7MHz

OUT

−

Since the average peak to peak current is equal to the

load current. The actual peak (or highest current the

inductor will see in a steady state condition) is equal to

the output current plus ½ the peak-to-peak current.

()

2.7MHz

OUT

−

Figure 4 demonstrates the off-time. During the off-time,

the high-side internal P-channel MOSFET turns off.

Since the current in the inductor has to discharge, the

current flows through the free-wheeling Schottky diode

to the output. In this case, the inductor discharge rate is

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