lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 9

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

OPERATION
The LT3686A is a current mode step-down regulator.
The EN/UVLO pin is used to place the LT3686A in shut-
down. The 1.27V 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 1.27V, an internal regulator provides power to the
control circuitry. This regulator includes both overvoltage
and undervoltage lockout to prevent switching when V
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
sets an RS fl ip-fl op, turning on the internal 1.2A power
switch Q1. An amplifi er and comparator monitor the cur-
rent fl owing 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 amplifi er measures the output
C
node. If the error amplifi er’s output
IN
and SW pins, turning the
T
. The oscillator
IN
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
effi cient operation.
A comparator monitors the current fl owing 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.
C
node provides current limit.
LT3686A
IN
or from the
3686af
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