Datasheet

LT1936
7
1936fd
OPERATION
(Refer to Block Diagram)
The LT1936 is a constant frequency, current mode step-
down regulator. A 500kHz oscillator enables an RS fl ip-fl op,
turning on the internal 1.9A power switch Q1. An ampli-
fi er and comparator monitor the current fl owing between
the V
IN
and SW pins, turning the switch off when this
current reaches a level determined by the voltage at V
C
.
An error amplifi er measures the output voltage through
an external resistor divider tied to the FB pin and servos
the V
C
pin. If the error amplifi er’s output increases, more
current is delivered to the output; if it decreases, less
current is delivered. An active clamp (not shown) on the
V
C
pin provides current limit. The V
C
pin is also clamped
to the voltage on the SHDN pin; soft-start is implemented
by generating a voltage ramp at the SHDN pin using an
external resistor and capacitor.
An internal regulator provides power to the control circuitry.
This regulator includes an undervoltage lockout to prevent
switching when V
IN
is less than ~3.45V. The SHDN pin is
used to place the LT1936 in shutdown, disconnecting the
output and reducing the input current to less than 2μA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate
the internal bipolar NPN power switch for effi cient opera-
tion.
The oscillator reduces the LT1936’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during startup
and overload.