LT1936H データシートの表示（PDF） - Linear Technology

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LT1936H

1.4A, 500kHz Step-Down Switching Regulator

Linear Technology 

LT1936

OPERATION (Refer to Block Diagram)

The LT1936 is a constant frequency, current mode step-

down regulator. A 500kHz oscillator enables an RS flip-flop,

turning on the internal 1.9A power switch Q1. An ampli-

fier 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 pin. 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 pin provides current limit. The VC pin 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.

An internal regulator provides power to the control circuitry.

This regulator includes an undervoltage lockout to prevent

switching when VIN is less than ~3.45V. The SHDN pin is

used to place the LT1936 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.

The oscillator reduces the LT1936’s operating frequency

when the voltage at the FB pin is low. This frequency

foldback helps to control the output current during startup

and overload.

1936fd

7

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