LT3752 datasheet(27/52 Pages) AD

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LT3752/LT3752-1

Rev. C

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APPLICATIONS INFORMATION

application. Simple integer turns ratios can be used which

allowoff-the-shelftransformers(seeexamplecircuitsinthe

Typical Applications section). Turns ratios can be chosen

on the basis of desired duty cycle. However, the input and

output levels, turns ratio and flyback leakage spike must

be considered for the breakdown rating of the MOSFET.

Transformer leakage inductance causes a voltage spike to

occur after the switch turns off. In some cases a snubber

circuit will be required to limit this spike.

Housekeeping: Operating Without This Supply

The housekeeping supply is highly recommended for

providing local bias voltages for both the primary and

secondary sides (to improve efficiency, simplify the main

transformer design and ensure all ICs are activated even

for VOUT = 0V). The LT3752 (not LT3752-1) housekeeping

supply components can be omitted (not populated) if an

extra winding already exists from the main transformer

to create an auxiliary supply. Care must be taken that the

auxiliary supply (for either the primary side or secondary

side or both) does not affect proper operation. A resistor

divider (Figure 8) should now be connected directly from

INTVCC to supply the HFB pin with a ratio :

R1/R2 = 3

(Example : R1 = 10k, R2 = 3.32k).

This ratio ensures HFB >> 0.96V (typical PGOOD level to

enable SS1 and the forward converter).

(a) At INTVCC = 4.75V (UVLO(+)), HFB = 1.2V.

(b) At INTVCC = 7V (Regulated), HFB = 1.7V.

Care should be taken not to exceed HFB = 3V.

FORWARD CONTROLLER

The LT3752/LT3752-1 are primary side, current mode,

PWM controllers optimized for use in a synchronous

forward converter with active clamp reset. Each IC can

be used in a fully regulated forward converter application.

In addition, they can still operate if damage occurs to

the feedback path—no secondary side error amplifier or

opto-coupler—by using an accurate, programmable volt-

secondclamptoregulatedutycycleinverselyproportional

to transformer input voltage.

Adaptive Leading Edge Blanking Plus Programmable

Extended Blanking

The LT3752/LT3752-1 provide a ±2A gate driver at the

OUT pin to control an external N-channel MOSFET for

main power delivery in the forward converter (Figure 10).

During gate rise time and sometime thereafter, noise can

be generated in the current sensing resistor connected

to the source of the MOSFET. This noise can potentially

cause a false trip of sensing comparators resulting in

early switch turn off and in some cases re-soft-start of

the system. To prevent this, LT3752/LT3752-1 provide

adaptive leading edge blanking of both OC and ISENSEP

signals to allow a wide range of MOSFET QG ratings. In

addition, a resistor RTBLNK connected from TBLNK pin to

analog ground (Pin 18) programs an extended blanking

duration (Figure 9).

Figure 9. Adaptive Leading Edge Blanking Plus

Programmable Extended Blanking

(ADAPTIVE)

LEADING

EDGE

BLANKING

(PROGRAMMABLE)

EXTENDED

BLANKING

7.32k ≤ RTBLNK ≤ 249k

tBLNK = 50ns + (2.2ns • RTBLNK)

CURRENT

SENSE

DELAY

220ns

3752 F09

OUT

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