MIC4744 Datasheet, PDF (10/23 Pages) Micrel Semiconductor – 4 MHz Dual 2A Integrated Switch Buck Regulator

Micrel, Inc.
Application Information
The MIC4744 is a dual 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.
MIC4744
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, Figure 3 illustrates the high side
switch is turned on, current flows from the input supply
through the inductor and to the output. The inductor
current is charged at the rate;
(V_IN − V_OUT )
L
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;
I_OUT
=
⎡
⎢V_OUT
⎢⎣
−
V_OUT2
V_IN
fsw × 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
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
fsw
Therefore, peak to peak ripple current is;
I_{pk − pk}
=
(V_IN−V_OUT )×
fsw × L
V_OUT
V_IN
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 1/2 the peak-to-peak current.
March 2009
10
M9999-030209-C