LT3752 datasheet(23/52 Pages) AD

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LT3752/LT3752-1

Rev. C

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APPLICATIONS INFORMATION

Worst-case values should be used to calculate the CSTART

and RSTART required to guarantee start-up and to turn on

in the time required.

Example: (LT3752-1)

For VS(MIN) = 75V, VIN(ON)(MAX) = 10.4V

ISTART(MAX) = 265µA, IHKEEP(MAX) = 4.6mA

QG = 8nC (at VIN = 10V), fOSC = 150kHz

tHSS(MAX) = 4ms, VDROOP(MIN) = 1.61V

CSTART(MIN) = 4.6mA + 71kHz •8nC

(

)•

4ms

1.61V

= 12.8µF Choose 14.7µF

(

)

RSTART(MAX) =

75V – 10.4V

265µA • k

= 243k for k = 1.0

(

)

The RSTART(MAX) value should be chosen with higher k

values until the charge-up time for CSTART is acceptable.

In most cases, CSTART will be charged to the 16V clamp

on the LT3752-1 VIN pin before system input reaches its

UVLO(+) threshold (Figure 4). This will allow an extra

5.6V for VDROOP in the CSTART equation, allowing a smaller

CSTART value and hence a faster start-up time.

The trade-off of lower RSTART is greater power dissipa-

tion, given by:

PRSTART = (VS – VIN)2/RSTART

for RSTART = 200k, VS(MAX) = 150V, VIN = 10V (back

driven from housekeeping supply)

PRSTART = (150 – 10)2/200k = 98mW.

Programming Switching Frequency

The switching frequency for the housekeeping supply

and the main forward converter are programmed using a

resistor, RT, connected from analog ground (Pin 18) to the

RT pin. Table 1 shows typical fOSC vs RT resistor values.

The value for RT is given by:

RT = 8.39 • X • (1 + Y)

where,

X = (109/fOSC) – 365

Y = (300kHz – fOSC)/107 (fOSC < 300kHz)

Y = (fOSC – 300kHz)/107 (fOSC > 300kHz)

Example: For fOSC = 200kHz,

RT = 8.39 • 4635 • (1 + 0.01) = 39.28k (choose 39.2k)

The LT3752/LT3752-1 include frequency foldback at start-

up (see Figures 3 and 4). In order to make sure that a

SYNC input does not override frequency foldback during

start-up,theSYNCfunctionisignoreduntilSS1pinreaches

2.2V. Both the housekeeping and forward controllers run

synchronized to each other and in phase, with or without

the SYNC input.

Table 1. RT vs Switching Frequency (fOSC)

SWITCHING FREQUENCY (kHz)

RT (kΩ)

100

82.5

150

53.6

200

39.2

250

30.9

300

24.9

350

400

18.2

450

15.8

500

Synchronizing to an External Clock

The LT3752 / LT3752-1 internal oscillator can be synchro-

nized to an external clock at the SYNC pin. SYNC pin high

level should exceed 1.8V for at least 100ns and SYNC

pin low level should fall below 0.6V for at least 100ns.

The SYNC pin frequency should be set equal to or higher

than the typical frequency programmed by the RT pin.

An fSYNC/fOSC ratio of x (1.0 < x < 1.25) will reduce the

externally programmed slope compensation by a factor

of 1.2x. If required, the external resistor RISLP can be

reprogrammed higher by a factor of 1.2x. (see Current

Sensing and Programmable Slope Compensation).

LT3752 Datasheet(PDF) 23 Page - Analog Devices

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