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LT3436

3436fa

amplifier commands current to be delivered to the output

rather than voltage. A voltage fed system will have low

phase shift up to the resonant frequency of the inductor

and output capacitor, then an abrupt 180

° shift will occur.

The current fed system will have 90

° phase shift at a much

lower frequency, but will not have the additional 90

° shift

until well beyond the LC resonant frequency. This makes

it much easier to frequency compensate the feedback loop

and also gives much quicker transient response.

A comparator connected to the shutdown pin disables the

internal regulator, reducing supply current.

The LT3436 is a constant frequency, current-mode boost

converter. This means that there is an internal clock and

two feedback loops that control the duty cycle of the power

switch. In addition to the normal error amplifier, there is a

current sense amplifier that monitors switch current on a

cycle-by-cycle basis. A switch cycle starts with an oscilla-

tor pulse which sets the RS flip-flop to turn the switch on.

When switch current reaches a level set by the inverting

input of the comparator, the flip-flop is reset and the

switch turns off. Output voltage control is obtained by

using the output of the error amplifier to set the switch

current trip point. This technique means that the error

Figure 1. Block Diagram

BLOCK DIAGRAM

–

INPUT

2.5V BIAS

REGULATOR

800kHz

OSCILLATOR

GND

3436 F01

SLOPE COMP

0.005

Ω

INTERNAL

VCC

CURRENT SENSE

AMPLIFIER VOLTAGE

GAIN = 40

SYNC

SHDN

SHUTDOWN

COMPARATOR

CURRENT

COMPARATOR

ERROR

AMPLIFIER

gm = 850µMho

FLIP-FLOP

DRIVER

CIRCUITRY

0.3V

POWER

SWITCH

1.2V

–

1.35V

3µA

µA

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