lt3681 Linear Technology Corporation, lt3681 Datasheet - Page 8

• Manufacturer Part Number: lt3681
• Description: 36v, 2a, 2.8mhz Step-down Switching Regulator With Integrated Power Schottky Diode
• Manufacturer: Linear Technology Corporation

OPERATION
LT3681
The LT3681 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
sets an RS fl ip-fl op, turning on the internal power switch.
An amplifi er and comparator monitor the current fl owing
between the V
this current reaches a level determined by the voltage at
V
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 V
the RUN/SS pin; soft-start is implemented by generating a
voltage ramp at the RUN/SS pin using an external resistor
and capacitor.
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 effi ciency. The RUN/SS pin is used to place
the LT3681 in shutdown, disconnecting the output and
reducing the input current to less than 1µA.
BLOCK DIAGRA
8
C
IN
. An error amplifi er measures the output voltage through
pin, but if the BIAS pin is connected to an external
C
pin. If the error amplifi er’s output increases, more
V
IN
R
IN
C1
T
and SW pins, turning the switch off when
13
14
2
6
1
C
GND
V
BIAS
RUN/SS
RT
PG
IN
4
pin is also clamped to the voltage on
SOFT-START
GND
7
GND
15
W
INTERNAL 1.265V REF
+
–
R2
1.12V
3
FB
R1
ERROR AMP
+
–
C
pin provides
300kHz–2.8MHz
SLOPE COMP
OSCILLATOR
–
+
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 effi cient operation.
To further optimize effi ciency, 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.
The oscillator reduces the LT3681’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during startup
and overload.
The LT3681 contains a power good comparator which trips
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
enabled and V
The LT3681 integrates a high quality, 36V, 2A power
Schottky diode to reduce the overall solution size.
V
R
S
BurstMode
SWITCH
C
LATCH
DETECT
CLAMP
DISABLE
Q
IN
is above 3.6V.
BOOST
DC
DC
DA
SW
BD
V
C
10
11
12
16
9
8
5
C
R
C3
C
C
L1
C
F
C2
3681 BD
V
OUT
3681f