Datasheet

D =
V
OUT
+ V
SWL
V
IN
- V
SWH
+ V
SWL
(I
LIM
x R
DS(ON)
) ± (I
SEN
x R
CS
)
R
CLF
=
R
CS
x V
OUT
V
IN
± 9.5V
R
CS
10 mA
t
P
LIM
x R
DS(ON)
= R
CS
I
SEN
R
CLF
V
OUT
R
CS
SW
+
-
40 PA
ILIM
I
SEN
LM2743
SNVS276G –APRIL 2004–REVISED MARCH 2013 www.ti.com
Figure 33. Foldback Current Limit Circuit
Obtain the R
CS
with the following equation:
where I
SEN
= 40 μA. If the switch node goes above 9.5V the following criterion must be satisfied:
The equation for calculating the foldback resistance value is:
SHUTDOWN
If the shutdown pin is pulled low, (below 0.8V) the LM2743 enters shutdown mode, and discharges the soft-start
capacitor through a MOSFET switch. The high and low-side MOSFETs are turned off. The LM2743 remains in
this state as long as V
SD
sees a logic low (see the Electrical Characteristics table). To assure proper IC start-up
the shutdown pin should not be left floating. For normal operation this pin should be connected directly to V
CC
or
to another voltage between 1.3V to V
CC
(see the Electrical Characteristics table).
DESIGN CONSIDERATIONS
The following is a design procedure for all the components needed to create the Figure 1. This design converts
3.3V (V
IN
) to 1.2V (V
OUT
) at a maximum load of 4A with an efficiency of 89% and a switching frequency of 300
kHz. The same procedures can be followed to create many other designs with varying input voltages, output
voltages, and load currents.
Duty Cycle Calculation
The complete duty cycle for a buck converter is defined with the following equation:
where V
SWL
and V
SWH
are the respective forward voltage drops that develop across the low side and high side
MOSFETs. Assuming the inductor ripple current is 20% to 30% of the output current, therefore:
V
SWL
= I
OUT
x R
DS(ON)LOW
(Low-Side MOSFET)
V
SWH
= I
OUT
x R
DS(ON)HIGH
(High-Side MOSFET)
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