Datasheet

LT3686A
9
3686afa
The LT3686A is a current mode step-down regulator.
The EN/UVLO pin is used to place the LT3686A in shut-
down. The 1.28V threshold on the EN/UVLO pin can be
programmed by an external resistor divider (R4, R5) to
disable the LT3686A. When the EN/UVLO pin is driven
above 1.28V, an internal regulator provides power to the
control circuitry. This regulator includes both overvoltage
and undervoltage lockout to prevent switching when V
IN
is more than 37V or less than 3.6V.
Tracking soft-start is implemented by providing constant
current via the SS pin to an external soft-start capacitor
(C4) to generate a voltage ramp. FB voltage is regulated
to the voltage at the SS pin until it exceeds 0.8V; FB
is then regulated to the reference 0.8V. Soft-start also
reduces the oscillator frequency to avoid hitting current
limit during start-up. The SS capacitor is reset during
fault events such as overvoltage, undervoltage, thermal
shutdown and startup.
An oscillator is programmed by resistor R
T
. The oscillator
sets an RS flip-flop, turning on the internal 1.2A power
switch Q1. An amplifier and comparator monitor the cur-
rent flowing 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 amplifier measures the output
voltage through an external resistor divider tied to the FB
pin and servos the V
C
node. If the error amplifier’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
node provides current limit.
The switch driver operates from either V
IN
or from the
BOOST pin. An external capacitor and the internal boost
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
efficient operation.
A comparator monitors the current flowing through the
catch diode via the DA pin and reduces the LT3686A’s
operating frequency when the DA pin current exceeds the
1.7A valley current limit. This helps to control the output
current in fault conditions such as shorted output with high
input voltage. The DA comparator works in conjunction
with the switch peak current limit comparator to determine
the maximum deliverable current of the LT3686A.
The SYNC/MODE pin doubles as mode select for the
BD active load circuit. The active load is enabled when
SYNC/MODE is driven with sync pulses or tied above
0.8V and disabled when SYNC/MODE is tied below 0.4V.
The LT3686A will prevent pulse skipping at light loads
by regulating the active load. The active load will assist
startup by guaranteeing a minimum load to charge the
boost capacitor. It also hastens the recharge of boost
capacitor when operating beyond maximum duty cycle.
The active load works only when the BD pin is less than 6V.
operAtion