LT3976
11
3976f
For more information www.linear.com/3976
operaTion
The LT3976 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
sets an RS flip-flop, turning on the internal power switch.
An amplifier and comparator monitor the current 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
(see Block Diagram). 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 ampli-
fier’s output increases, more current is delivered to the
output; if it decreases, less current is delivered. An active
clamp on the V
C
pin provides current limit. The V
C
pin is
also clamped by the voltage on the SS pin; soft-start is
implemented by generating a voltage ramp at the SS pin
using an external capacitor.
An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the V
IN
pin, but if the OUT pin is connected to an external volt-
age higher than 3.2V, bias power will be drawn from the
external source (typically the regulated output voltage).
This
improves efficiency.
If
the EN pin is low, the LT3976 is shut down and draws
700nA from the input. When the EN pin falls below 1.02V,
the switching regulator will shut down, and when the EN
pin rises above 1.08V, the switching regulator will become
active. This accurate threshold allows programmable
undervoltage lockout.
The switch driver operates from either V
IN
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.
To further optimize efficiency, the LT3976 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 supply
current to 1.7μA. In a typical application, 3.3μA will be
consumed from the supply when regulating with no load.
The oscillator reduces the LT3976’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during start-
up and overload.
The LT3976 can provide up to 5A of output current. A
current limit
foldback feature throttles back the current
limit during overload conditions to limit the power dis-
sipation. When SS is below 2V, the LT3976 overrides
the current limit foldback circuit to avoid interfering with
start-up. Thermal shutdown further protects the part from
excessive power dissipation, especially in elevated ambient
temperature environments.
If the input voltage decreases towards the programmed
output voltage, the LT3976 will start to skip switch-off
times and decrease the switching frequency to maintain
output regulation. As the input voltage decreases below
the programmed output voltage, the output voltage will be
regulated 500mV below the input voltage. This enforced
minimum dropout voltage limits the duty cycle and keeps
the boost capacitor charged during dropout conditions.
Since sufficient boost voltage is maintained, the internal
switch can fully saturate yielding low dropout performance.
The LT3976 contains a power good comparator which
trips when the FB pin is at 91.6% of its regulated value.
The PG output is an open-drain 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 V
IN
is
above 2V. When the LT3976 is shut down the PG pin is
actively pulled low.
