Datasheet

LT3686
9
3686fc
The LT3686 is a current mode step-down regulator. The
EN/UVLO pin is used to place the LT3686 in shutdown. The
1.27V threshold on the EN/UVLO pin can be programmed
by an external resistor divider (R4, R5) to disable the
LT3686. When the EN/UVLO pin is driven above 1.27V, 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 dur-
ing 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 LT3686’s op-
erating 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 LT3686.
The active load is enabled when MODE is tied above 0.8V
and disabled when the MODE pin is below 0.4V. To use the
active load, the BD pin should be tied to V
OUT
. The LT3686
will prevent pulse skipping at light loads by regulating the
active load. The active load will assist startup by guarantee-
ing 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
5.2V.
operAtion