MIC4721 Datasheet, PDF (9/19 Pages) Micrel Semiconductor – 1.5A 2MHz Integrated Switch Buck Regulator

Micrel, Inc.
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.
MIC4721
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.
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 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.
I_{OUT _ CRITICAL}
V_OUT
=
− ^{VOUT 2}
V_IN
2 × L × f_S
Where: f_S is the switching frequency (2MHz for the
MIC4721).
L is the output inductance (Henry).
When I_OUT is less than I_{OUT_CRITICAL}, the buck converter
operates in discontinuous mode and the inductor current
goes to zero before the end of each switching cycle.
When I_OUT is greater than I_{OUT_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 3. On-Time
The inductor current is charged at the rate:
(V_IN − V_OUT )
L
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:
D = ^VOUT
V_IN
and the On time is:
May 2007
9
M9999-052907-A