B340A 查看數據表（PDF） - Linear Technology

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B340A

42V, 2.5A, 2MHz Step-Down Switching Regulator with 2.7µA Quiescent Current

Linear Technology 

LT3975

OPERATION

The LT3975 is a constant frequency, current mode step-

down regulator. An oscillator, with frequency set by RT,

sets an RS flip-flop, turning on the internal power switch.

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 (see Block Diagram). 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 ampli-

fier’s output increases, more current is delivered to the

output; if it decreases, less current is delivered. An active

clamp on the VC pin provides current limit. The VC pin is

also clamped by the voltage on the SS pin; soft-start is

implemented by generating a voltage ramp at the SS pin

using an external capacitor.

An internal regulator provides power to the control circuitry.

The bias regulator normally draws power from the VIN

pin, but if the OUT pin is connected to an external volt-

age higher than 3.2V, bias power will be drawn from the

external source (typically the regulated output voltage).

This improves efficiency.

If the EN pin is low, the LT3975 is shut down and draws

700nA from the input. When the EN pin falls below 1.02V,

the switching regulator will shut down, and when the EN

pin rises above 1.08V, the switching regulator will become

active. This accurate threshold allows programmable

undervoltage lockout.

The switch driver operates from either VIN or from the

BOOST pin. An external capacitor is 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.

To further optimize efficiency, the LT3975 automatically

switches to Burst Mode operation in light load situations.

Between bursts, all circuitry associated with controlling

the output switch is shut down reducing the input supply

current to 1.7μA. In a typical application, 2.7μA will be

consumed from the supply when regulating with no load.

The oscillator reduces the LT3975’s operating frequency

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

foldback helps to control the output current during start-

up and overload.

The LT3975 can provide up to 2.5A of output current.

A current limit foldback feature throttles back the cur-

rent limit during overload conditions to limit the power

dissipation. When SS is below 2V, the LT3975 overrides

the current limit foldback circuit to avoid interfering with

start-up. Thermal shutdown further protects the part from

excessive power dissipation, especially in elevated ambient

temperature environments.

If the input voltage decreases towards the programmed

output voltage, the LT3975 will start to skip switch-off

times and decrease the switching frequency to maintain

output regulation. As the input voltage decreases below

the programmed output voltage, the output voltage will be

regulated 500mV below the input voltage. This enforced

minimum dropout voltage limits the duty cycle and keeps

the boost capacitor charged during dropout conditions.

Since sufficient boost voltage is maintained, the internal

switch can fully saturate yielding low dropout performance.

The LT3975 contains a power good comparator which

trips when the FB pin is at 91.6% of its regulated value.

The PG output is an open-drain transistor that is off when

the output is in regulation, allowing an external resistor

to pull the PG pin high. Power good is valid when VIN is

above 2V. When the LT3975 is shut down the PG pin is

actively pulled low.

3975f

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