LT3752 datasheet(26/48 Pages) LINER

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LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

the time taken by the buck converter output voltage to

get to its target, relative to the forward converter’s main

outputvoltage,oftenasimpletechniqueistoslowdown

the main output voltage start-up time by increasing the

soft-start capacitor on the primary-side IC.

INTVCC BIAS SUPPLY

The INTVCC pin powers the catch and forward MOSFET

gate drivers of the LT8311. Two configurations exist for

biasing up the INTVCC pin, as shown in Figure 14:

1. In the first configuration, the LT8311’s on-chip LDO

regulates the INTVCC pin voltage from the VIN supply.

When the VIN pin voltage is low, the internal LDO will

operate in drop-out, driving the INTVCC pin to about

400mV below the VIN pin voltage. When the VIN pin

voltage is high, the internal LDO will regulate INTVCC’s

voltage to 7V. Ensure that VIN’s supply voltage does not

exceed VIN’s abs max voltage of 30V. If INTVCC drops

below its UVLO voltage (4.6V rising and 4.3V falling), all

synchronous switching will be stopped. The maximum

guaranteed current that the INTVCC LDO can source is

Figure 13. Buck Circuit Generates VAUX Supply, Which

Powers LT8311’s VIN and INTVCC Pins

Figure 14. VIN and INTVCC Pin Configurations

8311 F13

VIN

BUCK AUXILLIARY SUPPLY

•

VOUT

VAUX

LT8311

•

NAUX

VIN

INTVCC

VAUX = VOUT •

NAUX









MCG

MFG

40mA. Ensure that the total gate charge (Qg) current

required by both secondary MOSFETs, MCG and MFG,

is less than 40mA:

IMOSFET_TOTAL = fSW • (Qg_MCG + Qg_MFG) < 40mA

where fSW is the converter’s switching frequency,

Qg_MCGisthegatecharge(Qg)ratingofMCGandQg_MFG

is the gate charge (Qg) rating of MFG.

This configuration, utilizing the LT8311’s internal LDO,

willsufficeformostapplications,limitedonlybythermal

considerations related to the LDO’s power dissipation.

Keeping the power dissipation to a minimum will help

lowertheoperatingjunctiontemperatureoftheLT8311,

potentially allowing the system to operate over a wider

ambient temperature range:

LDOPowerDissipation=(VIN–INTVCC)•IMOSFET_TOTAL

LT8311 Operating Junction Temperature ≈

θJA • (VIN • 4.5mA + LDO Power Dissipation + VIN •

IOPTO) + TA

where

θJA is LT8311’s junction-to-ambient thermal

resistance and is typically 38°C/W; IOPTO is the current

8311 F14

INTVCC

VIN

VIN < 30V

REGULATED to 7V

LT8311

LDO

4.7µF

INTVCC

VIN

VIN < 16V

LT8311

LDO

4.7µF

LT3752 Datasheet(PDF) 26 Page - Linear Technology

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