Datasheet

-
D1
1
1
=
c
D
=
V
OUT
V
IN
¸
¹
·
¨
©
§
LM2735
SNVS485F –JUNE 2007–REVISED APRIL 2013 www.ti.com
CURRENT LIMIT
The LM2735 uses cycle-by-cycle current limiting to protect the internal NMOS switch. It is important to note that
this current limit will not protect the output from excessive current during an output short circuit. The input supply
is connected to the output by the series connection of an inductor and a diode. If a short circuit is placed on the
output, excessive current can damage both the inductor and diode.
Design Guide
ENABLE PIN / SHUTDOWN MODE
The LM2735 has a shutdown mode that is controlled by the Enable pin (EN). When a logic low voltage is applied
to EN, the part is in shutdown mode and its quiescent current drops to typically 80 nA. Switch leakage adds up to
another 1 µA from the input supply. The voltage at this pin should never exceed V
IN
+ 0.3V.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature
exceeds 160°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature
drops to approximately 150°C.
SOFT-START
This function forces V
OUT
to increase at a controlled rate during start up. During soft-start, the error amplifier’s
reference voltage ramps to its nominal value of 1.255V in approximately 4.0ms. This forces the regulator output
to ramp up in a more linear and controlled fashion, which helps reduce inrush current.
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (V
O
) to input voltage (V
IN
):
(1)
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