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LT1506

BLOCK DIAGRAM

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.

High switch efficiency is attained by using the BOOST pin

to provide a voltage to the switch driver which is higher

than the input voltage, allowing switch to be saturated.

This boosted voltage is generated with an external capaci-

tor and diode. Two comparators are connected to the

shutdown pin. One has a 2.38V threshold for undervoltage

lockout and the second has a 0.4V threshold for complete

shutdown.

The LT1506 is a constant frequency, current mode buck

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

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

Figure 1. Block Diagram

–

INPUT

2.9V BIAS

REGULATOR

500kHz

OSCILLATOR

FREQUENCY

SHIFT CIRCUIT

VSW

GND

1506 BD

SLOPE COMP

0.01

Ω

INTERNAL

VCC

CURRENT

SENSE

AMPLIFIER

VOLTAGE GAIN = 20

SYNC

SHDN

SHUTDOWN

COMPARATOR

LOCKOUT

COMPARATOR

CURRENT

COMPARATOR

ERROR

AMPLIFIER

gm = 2000µMho

FOLDBACK

CURRENT

LIMIT

CLAMP

BOOST

FLIP-FLOP

DRIVER

CIRCUITRY

0.9V

POWER

SWITCH

PARASITIC DIODES

DO NOT FORWARD BIAS

2.42V

–

0.4V

3.5

µA

2.38V

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