LT3502/LT3502A
9
3502fd
The LT3502/LT3502A are constant frequency, current
mode step-down regulators. An oscillator enables an RS
flip-flop, turning on the internal 500mA power switch Q1.
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
. 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 (not shown) on the V
C
node provides current limit. The V
C
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. The SHDN pin voltage
during soft-start also reduces the oscillator frequency to
avoid hitting current limit during start-up.
An internal regulator provides power to the control cir
-
cuitry. This regulator includes an undervoltage lockout to
prevent switching when V
IN
is less than ~3V. The SHDN
pin is used to place the LT3502/LT3502A in shutdown,
disconnecting the output and reducing the input current
to less than 2µA.
The switch driver operates from either V
IN
or from the
BOOST pin. An external capacitor and the internal 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 operation.
A comparator monitors the current flowing through
the catch diode via the DA pin and reduces the LT3502/
LT3502A’s operating frequency when the DA pin current
exceeds the 650mA valley current limit. This frequency
foldback helps to control the output current in fault
conditions such as shorted output with high input volt-
age. The DA comparator works in conjunction with the
switch peak current limit comparator to determine the
maximum deliverable current of the L
T3502/LT3502A. The
peak current limit comparator is used in normal current
mode operations and is used to turn off the switch. The DA
valley current comparator monitors the catch diode current
and will delay switching until the catch diode current is
below the 650mA limit. Maximum deliverable current to
the output is therefore limited by both switch peak current
limit and DA valley current limit.
operation
