lt3505ems8e-trpbf Linear Technology Corporation, lt3505ems8e-trpbf Datasheet - Page 8

• Manufacturer Part Number: lt3505ems8e-trpbf
• Description: 1.2a, Step-down Switching Regulator In 3mm X 3mm Dfn
• Manufacturer: Linear Technology Corporation

LT3505
OPERATION
The LT3505 is a constant frequency, current mode step-
down regulator. A resistor-programmed oscillator enables
an RS fl ip-fl op, turning on the internal 1.4A power switch
Q1. An amplifi er and comparator monitor the current
fl owing between the V
off when this current reaches a level determined by the
voltage at the V
output voltage through an external resistor divider tied to
the FB pin and servos the V
output increases, more current is delivered to the output;
if it decreases, less current is delivered. An active clamp
(not shown) on the V
V
soft-start is implemented by generating a voltage ramp at
the SHDN pin using an external resistor and capacitor.
BLOCK DIAGRAM
8
ON OFF
C
node is also clamped to the voltage on the SHDN pin;
V
IN
R3
C2
C4
3
4
V
SHDN
IN
C
pin. An error amplifi er measures the
5
C
IN
GND
(Refer to Block Diagram)
node provides current limit. The
INT REG
and SW pins, turning the switch
UVLO
AND
OSC
C
node. If the error amplifi er’s
SLOPE
COMP
8
Σ
V
C
V
C
g
m
R
S
Q
Q
780mV
An internal regulator provides power to the control circuitry.
This regulator includes an undervoltage lockout to prevent
switching when V
used to place the LT3505 in shutdown, disconnecting the
output and reducing the input current to less than 2µA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are 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 opera-
tion.
When the FB pin is low, the voltage at the R
to reduce the oscillator frequency. This frequency foldback
helps to control the output current during start-up and
overload.
7
R2
FB
R1
DRIVER
FREQUENCY
FOLDBACK
IN
is less than ~3.4V. The SHDN pin is
6
R
T
Q1
BOOST
SW
1
2
D1
T
C3
pin decreases
D2
L1
C1
3505 BD
3505fc
V
OUT