Datasheet

LT3663
7
3663fc
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operaTion
The LT3663 is a constant frequency, current mode step
down regulator. A switch cycle is initiated when the os
-
cillator enables the RS flip flop, turning on the internal
power switch, Q1. The sense amplifier (A1) monitors the
switch current via the voltage dropped across the current
sense resistor R
SENSE1
. The comparator compares the
amplified current signal with the output (V
C
) of the error
amplifier (G1). The switch is turned off when this current
exceeds a value determined by the V
C
voltage. The error
amplifier monitors the V
OUT
voltage through an internal
resistor divider and, when not driven externally, servos
the V
C
voltage to regulate V
OUT
. If the V
OUT
voltage drops,
the V
C
voltage will be driven higher increasing the output
current and V
OUT
voltage. An active clamp (not shown)
on the V
C
node provides current limit. The LT3663 is
internally compensated with a pole-zero combination on
the V
C
node.
An external capacitor and internal diode, D2, generate a
voltage at the BOOST pin that is higher than the input supply.
(Refer to Block Diagram)
This allows the driver to fully saturate the internal bipolar
NPN power switch for efficient operation. The switch driver
operates from either V
IN
or BOOST to ensure startup.
An internal regulator provides power to the control circuitry.
This regulator includes input undervoltage and overvoltage
protection which disables switching action when V
IN
is
out of range. When switching is disabled, the LT3663
safely sustains input voltages up to 60V. Note that while
switching is disabled the output will discharge.
Output current limiting is provided via the servo action of
amplifier G2. The voltage across sense resistor, R
SENSE2
,
is compared to a voltage programmed by external resistor
R
ILIM
on the I
LIM
pin. A capacitor averages the inductor
ripple current. If the average inductor current exceeds the
programmed value then the V
C
voltage is pulled low, reduc-
ing the current in the regulator. The output current limit
circuit allows for a lower current rated inductor and diode
and provides better control of system power dissipation.