MIC2208_10 Datasheet, PDF (9/18 Pages) Micrel Semiconductor – 3mm x 3mm 1MHz 3A PWM Buck Regulator

Micrel, Inc.
Application 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.
MIC2208
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 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_OUT ²
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.
Figure 2. Continuous Operation
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
Therefore, peak to peak ripple current is:
( ) _{IPK−PK =}
V_IN − V_OUT
× ^VOUT
V_IN
1MHz × L
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
May 2010
9
M9999-051410-D