LT3752 datasheet(20/52 Pages) AD

20 / 52 page

LT3752/LT3752-1

Rev. C

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OPERATION

the start-up waveforms in Figure 4. The LT3752-1 starts

up by using a high valued resistor from system input to

charge up the input capacitor at the VIN pin. If system

input is already high enough to generate UVLO_VSEC

above 1.25V, then the part turns on once VIN pin charges

past VIN(ON) (9.5V). If system input is not high enough

to generate UVLO_VSEC above 1.25V, the VIN pin charges

towards system input until it reaches an internal 16V, 8mA

clamp. The part turns on when system input becomes

high enough to generate UVLO_VSEC above 1.25V. As the

supply current of the part discharges the VIN capacitor a

bootstrap supply must be generated to prevent VIN pin

from falling below VIN(OFF) (7.6V).

The LT3752-1 uses the housekeeping controller to provide

the bootstrap bias to the VINpinduringRCstart-upinstead

ofwaitingfortheforwardconvertertoalsostart.Thismeth-

od is more efficient, requires a smaller VIN input capacitor

and avoids the need for an auxiliary winding in the main

transformer. The part’s low start-up current at the VIN pin

allowstheuseofalargestart-upresistortominimizepower

lossfromsysteminput.TheVINcapacitorvaluerequiredfor

proper start-up is minimized by providing a large VIN(ON)-

VIN(OFF) hysteresis, a low VIN IQ and a fast start-up time for

the housekeeping controller. In contrast to the LT3752, the

LT3752-1 housekeeping gate driver (HOUT) runs from the

VIN pin instead of INTVCC. This avoids having to use cur-

rent from the VINpintochargetheINTVCCcapacitorduring

initialstart-up.Thismeanstheregulated10VINTVCConthe

LT3752-1 does not wake up until the housekeeping supply

is valid. Start-up from this point is similar to the LT3752.

The housekeeping supply and forward converter switch

together with a soft-started frequency and volt-second

clamp.Theforwardconverterpeakinductorcurrentisalso

soft started similar to the LT3752.

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