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LT3502/LT3502A

3502fc

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 VIN and SW pins, turning the switch off when

this current reaches a level determined by the voltage at

VC. 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. 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 VIN 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 VIN 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 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.

OPERATION

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