LT3502 LINER [Linear Technology], LT3502 Datasheet - Page 9

• Manufacturer Part Number: LT3502
• Manufacturer: LINER [Linear Technology]

OPERATION
The LT3502/LT3502A are constant frequency, current
mode step-down regulators. An oscillator enables an RS
fl ip-fl op, turning on the internal 500mA power switch Q1.
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
current is delivered. An active clamp (not shown) on the V
node provides current limit. The V
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
pin is used to place the LT3502/LT3502A in shutdown,
disconnecting the output and reducing the input current
to less than 2μA.
C
. An error amplifi er measures the output voltage through
C
node. If the error amplifi er’s output increases, more
IN
and SW pins, turning the switch off when
IN
is less than ~3V. The SHDN
C
node is also clamped
C
The switch driver operates from either V
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
effi cient operation.
A comparator monitors the current fl owing 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 LT3502/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.
LT3502/LT3502A
IN
or from the
3502fc
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