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LT1766/LT1766-5

1766fa

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.

Most of the circuitry of the LT1766 operates from an

internal 2.9V bias line. The bias regulator normally draws

power from the regulator input pin, but if the BIAS pin is

connected to an external voltage higher than 3V, bias

power will be drawn from the external source (typically the

BLOCK DIAGRA

Figure 1. LT1766 Block Diagram

–

VIN

2.9V BIAS

REGULATOR

200kHz

OSCILLATOR

FREQUENCY

FOLDBACK

GND

1, 8, 9, 16, 17

1766 F01

SLOPE COMP

ANTISLOPE COMP

BIAS

INTERNAL

VCC

SYNC

0.4V

5.5

µA

CURRENT

COMPARATOR

RLIMIT

RSENSE

ERROR

AMPLIFIER

gm = 2000µMho

FOLDBACK

CURRENT

LIMIT

CLAMP

BOOST

FLIP-FLOP

DRIVER

CIRCUITRY

POWER

SWITCH

1.22V

SHDN 15

LOCKOUT

COMPARATOR

SHUTDOWN

COMPARATOR

2.38V

×1

VC(MAX)

CLAMP

regulated output voltage). This will improve efficiency if

the BIAS pin voltage is lower than regulator input voltage.

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

capacitor and diode. Two comparators are connected to

the shutdown pin. One has a 2.38V threshold for under-

voltage lockout and the second has a 0.4V threshold for

complete shutdown.

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