LT3505
BLOCK DIAGRAM
V
IN
V
3
IN
C2
INT REG
AND
UVLO
D2
BOOST
Σ
1
2
ON OFF
SLOPE
COMP
R
S
Q
Q
R3
SHDN
C3
L1
4
DRIVER
Q1
C4
SW
OSC
V
OUT
C1
D1
FREQUENCY
FOLDBACK
V
C
g
m
780mV
GND
V
C
FB
R
T
5
8
7
6
R1
3505 BD
R2
OPERATION (Refer to Block Diagram)
The LT3505 is a constant frequency, current mode step-
downregulator.Aresistor-programmedoscillatorenables
an RS flip-flop, turning on the internal 1.4A power switch
Q1. An amplifier and comparator monitor the current
flowing between the VIN and SW pins, turning the switch
off when this current reaches a level determined by the
voltage at the VC pin. An error amplifier measures the
output voltage through an external resistor divider tied to
the FB pin and servos the VC 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
(not shown) on the VC node provides current limit. The
VC node is also clamped to the voltage on the SHDN pin;
soft-start is implemented by generating a voltage ramp at
the SHDN pin using an external resistor and capacitor.
Aninternalregulatorprovidespowertothecontrolcircuitry.
Thisregulatorincludesanundervoltagelockouttoprevent
switching when VIN is less than ~3.4V. The SHDN pin is
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 efficient opera-
tion.
When the FB pin is low, the voltage at the RT pin decreases
toreducetheoscillatorfrequency.Thisfrequencyfoldback
helps to control the output current during start-up and
overload.
3505fc
8
Linear [ Linear ]
