MIC4721 datasheet(9/19 Pages) MICREL

9 / 19 page

Micrel, Inc.

MIC4721

May 2007

M9999-052907-A

Application Information

The MIC4721 is a 1.5A PWM non-synchronous buck

regulator. A regulated DC output voltage is obtained by

switching an input voltage supply, and filtering the

switched voltage through an inductor and capacitor.

Figure 1 shows a simplified example of a non-synch-

ronous buck converter and its input/output voltage.

Figure 1. Simplified Buck Converter

For a non-synchronous buck converter, there are two

modes of operation; continuous and discontinuous. The

mode refers to the state of current in the inductor. If

current

continuously

flows

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.

OUT

CRITICAL

OUT

−

Where: fS is the switching frequency (2MHz for the

MIC4721).

L is the output inductance (Henry).

When IOUT is less than IOUT_CRITICAL, the buck converter

operates in discontinuous mode and the inductor current

goes to zero before the end of each switching cycle.

When IOUT is greater than IOUT_CRITICAL, the converter

operates in continuous mode and current always flows in

the inductor. Continuous or discontinuous operation

determines how peak inductor current is calculated.

Continuous Operation

Figure 2 illustrates the switch voltage and inductor

current during continuous operation.

Figure 2. Continuous Operation

The

output

voltage

regulated

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.

As seen in Figure 3, the high side switch is turned on

(on-time) and current flows from the input supply through

the inductor and to the output.

Figure 3. On-Time

The inductor current is 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:

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