LT3752 datasheet(22/52 Pages) AD

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LT3752/LT3752-1

Rev. C

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APPLICATIONS INFORMATION

Figure 6. Programming Overvoltage Lockout (OVLO)

LT3752-1 Micropower Start-Up from High System

Input Voltages

The LT3752-1 starts up from system input voltage levels

limited only by external components (Figure 7). The low

start-up current of the LT3752-1 allows a large start-up

resistor (RSTART) to be connected from system input volt-

age (VS) to the VIN pin.

Whensysteminputvoltageisapplied,thestart-upcapacitor

(CSTART) begins charging at the VIN pin. Once the VIN pin

exceeds 9.5V (and UVLO_VSEC>1.25V)thehousekeeping

controller will start to switch and VIN supply current will

begin to discharge CSTART. The CSTART capacitor value

should be chosen high enough to prevent the VIN pin

from falling below 7.6V before the housekeeping supply

can provide a bootstrap bias to the VIN pin. The LT3752-1

start-up architecture minimizes the value of CSTART by

activating only the house keeping controller for provid-

ing drive back to the VIN pin. The forward controller only

operates once the housekeeping supply is established. (If

a bootstrap diode is used from the housekeeping supply

back to INTVCC, this only uses current from system input

and not from the VIN pin).

1.250V

3752 F07

RSTART

VIN

16V

8mA

GND

CSTART

SYSTEM

INPUT (VS)

LT3752-1

VHK (HOUSEKEEPING SUPPLY OUTPUT)

Figure 7. Micropower Start-Up from High System Input

The start-up capacitor can be calculated as:

CSTART(MIN) = IHKEEP +IDRIVE

(

)(MAX),•

tHSS(MAX)

VDROOP(MIN)

where:

IHKEEP = Housekeeping IQ (not switching)

IDRIVE = (fOSC/2.13) • QG)

fOSC = full-scale controller switching frequency

QG = gate charge (VGS = VIN)(HOUT MOSFET)

tHSS = housekeeping output voltage soft-start time

VDROOP = 16V(clamp) – VIN(OFF) or VIN(ONOFFHYST)

The start-up resistor can be calculated as:

RSTART(MAX) =

VS(MAX) – VIN(ON)(MAX)

ISTART(MAX) •k

where:

VS(MAX) = Maximum system input voltage

VIN(ON)(MAX) = Maximum VIN pin turn on threshold

ISTART(MAX) = Maximum VIN IQ for part start-up

k > 1.0 reduces RSTART and VIN charge-up time

1.250V(+)

1.215V(–)

3752 F06

OVLO

UVLO_VSEC

SYSTEM

INPUT (VS)

LT3752/LT3752-1

–

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