MIC2208 Datasheet, PDF (9/19 Pages) Micrel Semiconductor – 3mmx3mm 1MHz 3A PWM Buck Regulator

Micrel, Inc.
Applications Information
The MIC2208 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.
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;
I_OUT
=
⎡
⎢V_OUT
⎢⎣
-
V_OUT2
V_IN
⎤
⎥
⎥⎦
1MHz × 2 × L
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.
MIC2208
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;
(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;
T_ON
=
D
1MHz
September 2005
9
M9999-092905
www.micrel.com