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LT1375/LT1376

APPLICATIONS INFORMATION

LOAD CURRENT (A)

0.001

8.0

7.5

7.0

6.5

6.0

5.5

5.0

0.01

0.1

1375/76 F09

(A)

(C)

(D)

(A) MINIMUM VOLTAGE

TO START WITH

STANDARD CIRCUIT

(B) MINIMUM VOLTAGE

TO RUN WITH

STANDARD CIRCUIT

TO START WITH

PNP

(D) MINIMUM VOLTAGE

TO RUN WITH

PNP

(B)

Figure 9. Minimum Input Voltage

Figure 10. Reducing Minimum Input Voltage

OUTPUT

INPUT

1375/76 F10

0.1

µF

1N914

2N3905

1N914

BOOST

LT1376-5

VIN

VSW

SENSE

GND

Compensation section for a discussion of an entirely

different cause of subharmonic switching before assum-

ing that the cause is insufficient slope compensation.

Application Note 19 has more details on the theory of slope

compensation.

There is a sync-supply sequence issue with the LT1375. If

power is supplied to the regulator

after the external sync

signal is supplied, the regulator may not start. This is

caused by the internal frequency foldback condition that

occurs when the FB pin is below 1V (see block diagram

description in the data sheet). The oscillator tries to run at

100kHz when the FB pin is below 1V, and a high frequency

sync signal will then create an extremely low amplitude

oscillator waveform. This amplitude may be so low that the

switch logic is not triggered to create switching. Under the

normal regulated condition, the oscillator runs at much

higher amplitude with plenty of drive for the switch logic.

Note that for fixed voltage parts, the FB pin is replaced with

a SENSE pin, and the voltage divider resistors are internal.

In that case, the FB pin drops below 1V when the output

voltage is less than 40% of its regulated value.

There are no sequence problems if the power supply for

the sync signal comes from the output of the LT1375. If

this is not the case, and the sync signal could be present

when power is applied to the regulator, a gate should be

used to block sync signals as shown in Figure 11. Any

other technique which prevents sync signals when the

regulator output is low will work just as well. It does not

matter whether the sync signal is forced high or low; the

internal circuitry is edge triggered.

SYNCHRONIZING (Available on LT1375 Only)

The LT1375 has the BIAS pin replaced with a SYNC pin,

which is used to synchronize the internal oscillator to an

external signal. It is directly logic compatible and can be

driven with any signal between 10% and 90% duty cycle.

The synchronizing range is equal to

initial operating fre-

quency up to 900kHz. This means that

minimum practical

sync frequency is equal to the worst-case

high self-

oscillating frequency (560kHz), not the typical operating

frequency of 500kHz. Caution should be used when syn-

chronizing above 700kHz because at higher sync frequen-

cies the amplitude of the internal slope compensation

used to prevent subharmonic switching is reduced. This

type of subharmonic switching only occurs at input volt-

ages less than twice output voltage. Higher inductor

values will tend to eliminate problems. See Frequency

LT1375

SYNC

VOUT

1375/76 F11

VIN

FREQUENCY COMPENSATION

Loop frequency compensation of switching regulators

can be a rather complicated problem because the reactive

components used to achieve high efficiency also

Figure 11. Gating the Sync Signal

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