LT3681
W
BLOCK DIAGRA
V
IN
V
IN
13
C1
BIAS
–
+
INTERNAL 1.265V REF
2
BD
10
11
RUN/SS
RT
14
6
SLOPE COMP
SWITCH
LATCH
BOOST
C3
R
OSCILLATOR
300kHz–2.8MHz
Q
S
L1
SW
DC
DC
DA
V
R
OUT
12
9
T
DISABLE
C2
SOFT-START
BurstMode
DETECT
16
8
PG
1
V
C
CLAMP
ERROR AMP
+ 1.12V
–
+
–
V
C
5
C
C
C
F
R
C
GND GND GND
FB
4
7
15
3
R2
R1
3681 BD
OPERATION
The LT3681 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
The 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.
between the V and SW pins, turning the switch off when
IN
this current reaches a level determined by the voltage at
To further optimize efficiency, the LT3681 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 55µA in a typical application.
V .Anerroramplifiermeasurestheoutputvoltagethrough
C
an external resistor divider tied to the FB pin and servos
the V pin. If the error amplifier’s output increases, more
C
current is delivered to the output; if it decreases, less cur-
rent is delivered. An active clamp on the V pin provides
C
The oscillator reduces the LT3681’s operating frequency
when the voltage at the FB pin is low. This frequency
foldbackhelpstocontroltheoutputcurrentduringstartup
and overload.
current limit. The V pin is also clamped to the voltage on
C
the RUN/SS pin; soft-start is implemented by generating a
voltage ramp at the RUN/SS pin using an external resistor
and capacitor.
TheLT3681containsapowergoodcomparatorwhichtrips
when the FB pin is at 91% 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 LT3681 is
An internal regulator provides power to the control cir-
cuitry. The bias regulator normally draws power from the
IN
voltage higher than 3V bias power will be drawn from the
external source (typically the regulated output voltage).
This improves efficiency. The RUN/SS pin is used to place
the LT3681 in shutdown, disconnecting the output and
reducing the input current to less than 1µA.
V
pin, but if the BIAS pin is connected to an external
enabled and V is above 3.6V.
IN
The LT3681 integrates a high quality, 36V, 2A power
Schottky diode to reduce the overall solution size.
3681f
8
Linear [ Linear ]
