LT3752 datasheet(38/52 Pages) AD

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LT3752/LT3752-1

Rev. C

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ing accidentally discharged during missing OUT pulses

and/or causing reverse saturation of the transformer.

For SOUT control, this prevents the secondary side syn-

chronous rectifier controller from incorrectly switching

between forward FET and synchronous FET conduction.

The LT3752/LT3752-1 correctly re-establish the required

AOUT, SOUT control signals if the OUT signal is required

for the next cycle.

AOUT Timeout

During converter start-up in soft-start, the switching fre-

quency and maximum duty cycle clamp DVSEC are both

folded back. While this correctly reduces the effective

minimum on time of the OUT pin (to allow control of induc-

tor current for very low output voltages during start-up),

this means the AOUT pin on time duration can be large.

In order to ensure the active clamp switch controlled by

AOUTdoesnotstayontoolong,theLT3752/LT3752-1have

an internal 15µs timeout to turn off the AOUT signal. This

prevents the active clamp capacitor from being connected

across the transformer primary winding long enough to

create reverse saturation.

Main Transformer Selection

The LT3752/LT3752-1 simplify the design of the main

transformer and output inductor by removing the need for

any auxiliary windings. Any bootstrap supplies required

for the primary side or bias supplies required for the

secondary side can all be provided by the housekeeping

DC/DC controller included in the LT3752/LT3752-1. (see

Housekeeping Controller in the Applications Information

Section).

APPLICATIONS INFORMATION

Figure 18. SS1, SS2 and COMP Pin Voltages During Faults, Soft-Start and Soft-Stop

COMP

RANGE

2.6V

1.25V

0.15V

0.25V

COMP 1.25V

SWITCHING THRESHOLD

SS2 SOFT STARTS

COMP

SS2

SS1

2.6V

HARD

STOP

SOFT-START

(WHEN ALL CONDITIONS SATISFIED)

(1) UVLO_VSEC > 1.25V

(2) OVLO < 1.215V

(3) HFB > 0.96V

(4) OC < 96mV

(5) X < INTVCC < 16V

(6) TJ < 165°C

(7) VIN > Y

(X = 4.75V, Y = 5.8V: LT3752)

(Y = 7.0V, Y = 9.5V: LT3752-1)

SOFT-STOP

(0.4V < UVLO_VSEC < 1.25)

(1) EXTERNAL SOFT-STOP SHUTDOWN

(2) SYSTEM INPUT UVLO

HARD STOP

(FAULTS)

(1) UVLO_VSEC < 0.4V

(2) OVLO > 1.25V

(3) HFB < 0.92V

(4) OC > 96mV

(5) INTVCC < X, > 16.5V

(6) TJ > 170°C

(7) VIN < Y

(X = 4.6V, Y = 5.5V: LT3752)

(X = 6.8V, Y = 7.6V: LT3752-1)

3752 F18

SS1 SOFT STOPS

fOSC, DVSEC AND COMP

SS1 SOFT STARTS

fOSC AND DVSEC

SS1 LATCH

RESET THRESHOLD

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