Micrel, Inc.
MIC4720
Application Information
The MIC4720 is a 2A 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.
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. 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;
⎡
V
OUT 2
⎤
⎢
V
OUT
−
⎥
V
IN
⎥
⎢
⎣
⎦
I
OUT
=
2.0MHz
×
2
×
L
Continuous or discontinuous operation determines how
we calculate peak inductor current.
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
=
V
OUT
V
IN
and the On time is;
Continuous Operation
Figure 2 illustrates the switch voltage and inductor
current during continuous operation.
T
ON
=
D
2.0MHz
Therefore, peak to peak ripple current is;
I
pk
−
pk
=
(
V
IN
−
V
OUT
)
×
V
OUT
V
IN
2.0MHz
×
L
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.
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.
I
pk
=
I
OUT
+
(
V
IN
−
V
OUT
)
×
V
OUT
V
IN
2
×
2.0MHz
×
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
10
M9999-051707
May 2007
MICREL [ MICREL SEMICONDUCTOR ]
