LT3686 LINER [Linear Technology], LT3686 Datasheet - Page 9

• Manufacturer Part Number: LT3686
• Manufacturer: LINER [Linear Technology]

operAtion
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
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
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
switch off when this current reaches a level determined by
the voltage at V
voltage through an external resistor divider tied to the FB
pin and servos the V
increases, more current is delivered to the output; if it
C
. An error amplifier measures the output
C
node. If the error amplifier’s output
IN
and SW pins, turning the
IN
is more than 37V
T
. The oscillator
decreases, less current is delivered. An active clamp (not
shown) on the V
The switch driver operates from either V
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
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.
C
node provides current limit.
OUT
IN
LT3686
. The LT3686
or from the
3686fa