LT3685 LINER [Linear Technology], LT3685 Datasheet - Page 11

• Manufacturer Part Number: LT3685
• Description: 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

OPERATION
The LT3690 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by R
enables an RS flip-flop, turning on the internal high side
(HS) power switch. An amplifier and comparator monitor
the current flowing between the V
ing the RS flip-flop and HS switch off when this current
reaches a level determined by the voltage at V
While the high side switch is off, the inductor current con-
ducts through the catch diode and the turned on low side
(LS) switch until either the next clock pulse of the oscillator
starts the next cycle, or the inductor current becomes too
low, as indicated by the zero crossing comparator. This
prevents the inductor from running reverse current.
An error amplifier measures the output voltage through
an external resistor divider tied to the FB pin and servos
the V
current is delivered to the output; if it decreases, less cur-
rent is delivered. An active clamp on the V
current limit.
The SS node acts as an auxiliary input to the error ampli-
fier. The voltage at FB will servo to the SS voltage until SS
goes above 0.8V. Soft-start is implemented by generating
a voltage ramp at the SS pin using an external capacitor
C
Alternatively, connecting the SS pin to a resistive divider
between the voltage to be tracked and ground provides a
tracking function.
An internal regulator provides power to the control cir-
cuitry. The bias regulator normally draws power from the
V
voltage higher than 3V, bias power will be drawn from the
external source (typically the regulated output voltage).
This improves efficiency.
The EN pin is used to place the LT3690 in shutdown,
disconnecting the output and reducing the input current
to less than 1µA. A comparator monitors the voltage at
the UVLO input. A external resistive divider connected to
V
unused, connect the input to V
SS
IN
IN
pin, but if the BIAS pin is connected to an external
programs the wake up threshold and hysteresis. If
which is charged by an internal constant current.
C
pin. If the error amplifier’s output increases, more
IN
or above 1.5V.
IN
and SW pins, turn-
C
pin provides
C
.
T
,
The HS switch driver operates from either the input or from
the BOOST pin. An external capacitor is 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.
The synchronously driven N-channel transistor (LS switch)
in parallel with the catch diode reduces the overall solution
size and improves efficiency. Internal overload comparator
circuitry monitors the current through the LS switch and
delays the generation of new switch pulses if this current is
too high (above 5A nominal). This mechanism also protects
the part during short-circuit and overload conditions by
keeping the current through the inductor under control.
A short-circuit protected regulator at V
LS driver. The LS switch only operates at V
greater than 3.8V.
To further optimize efficiency, the LT3690 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down, reducing the input sup-
ply current to 70µA in a typical application. Pulling the
SYNC pin above 0.8V prevents Burst Mode operation. The
positive edge of an external clock signal at the SYNC pin
synchronizes the internal oscillator and therefore switching.
The oscillator reduces the LT3690’s operating frequency
when the voltage at the FB pin is low. This frequency fold-
back helps to control the output current during start-up
and overload conditions.
The LT3690 contains a power good comparator which trips
when the FB pin is at 90% of its regulated value. The PG
output is an open-collector transistor that is off when the
output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the LT3690 is
enabled and V
The LT3690 has an overvoltage protection feature which
disables switching action when V
during transients. When switching is disabled, the LT3690
can safely sustain transient input voltages up to 60V.
IN
is above 3.9V.
IN
goes above 38V (typical)
CCINT
CCINT
LT3690
supplies the
voltages
11
3690f