Datasheet

LT3971A/LT3971A-5
9
3971af
BLOCK DIAGRAM
+
–
+
–
+
–
OSCILLATOR
200kHz TO 2MHz
Burst Mode
DETECT
V
C
CLAMP
V
C
SLOPE COMP
R
V
IN
V
IN
EN
BOOST
SW
SHDN
SWITCH
LATCH
SS
1µA
V
OUT
C2
C3
C4
R3
L1
D1
BD
RT
R2
GND
ERROR AMP
R1
R2
FB
LT3971A
ONLY
LT3971A-5
ONLY
LT3971A-5: R1 = 7.62M, R2 = 2.38M
R
T
C1
PG
1.09V
1V
S
Q
V
OUT
3971A BD
INTERNAL 1.19V REF
SYNC
R1
Σ
+
–
SHDN
C5
C5
OPERATION
The LT3971A 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 amplifier’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered. An active clamp
on the V
C
node provides current limit. The V
C
node 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.
If the EN pin is low, the LT3971A is shut down and draws
700nA from the input. When the EN pin exceeds 1V, the
switching regulator will become active.
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 LT3971A 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 1.7A. In a typical application, 2.8A will be
consumed from the supply when regulating with no load.
The oscillator reduces the LT3971A’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.
The LT3971A contains a power good comparator which
trips when the FB pin is at 91% 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 the LT3971A
is enabled and V
IN
is above 4.3V.