LTC3854EMSE datasheet(13/28 Pages) LINER

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LTC3854

3854fa

applicaTions inForMaTion

An optional Schottky diode connected from GND (anode)

to the SW node (cathode) conducts during the dead time

between the conduction of the two power MOSFETs. This

prevents the body diode of the bottom MOSFET from turn-

ing on, storing charge during the dead time and requiring

a reverse recovery period that could cost as much as 3%

in efficiency at high VIN. A 1A to 3A Schottky is generally

a good size due to the relatively small average current.

Larger diodes result in additional transition losses due to

their larger junction capacitance.

Soft-Start

When the LTC3854 is configured to soft-start by itself,

a capacitor must be connected to the RUN/SS pin. The

LTC3854 is in the shutdown state if the RUN/SS pin volt-

age is below 1.2V. The RUN/SS pin has an internal 1.25µA

pull-upcurrentandshouldbeexternallypulledlow(<0.4V)

to keep IC in shutdown mode.

Once the RUN/SS pin voltage reaches 1.2V, the LTC3854

is enabled. As the RUN/SS pin moves from 1.2V to 2V the

LTC3854 operates in a forced discontinuous mode with

the bottom gate turning on only one time for every four

clock cycles to allow the output to come up to its required

value. During this time the error amp compares the FB

pin to a level shifted version of the RUN/SS pin allowing

the output to come up in a controlled fashion. Current

foldback is disabled during this phase to ensure smooth

soft-start or tracking. Once the RUN/SS pin is greater

than 2V the LTC3854 operates in forced continuous mode.

The LTC3854 output voltage is soft-start controlled when

RUN/SS is between 1.2V and 2V. The total soft-start time

can be calculated as:

tSOFT-START = 0.8 •

CSS

1.25µA

If the RUN/SS pin is externally driven beyond 2V (5V

is recommended) the soft-start feature is disabled and

the LTC3854 will immediately go into forced continuous

mode. Care must be taken to insure the RUN/SS pin has

either a capacitor tied to it or is driven externally. Do not

let this pin float.

INTVCC Regulator

TheLTC3854featuresaPMOSlowdropoutlinearregulator

(LDO) that supplies power to INTVCC from the VIN supply.

INTVCCpowersthegatedriversandmuchoftheLTC3854’s

internal circuitry. The LDO regulates the voltage at the

INTVCC pin to 5V when VIN is greater than 6V.

The LDO supplies a peak current of 40mA and must be

bypassed to ground with a minimum of 2.2μF low ESR

ceramic capacitor. Good bypassing is needed to supply

the high transient currents required by the MOSFET gate-

drivers.

High input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the maxi-

mum junction temperature rating for the LTC3854 to be

exceeded. The INTVCC current, which is dominated by the

gate-charge current, is supplied by the 5V LDO.

Power dissipation for the IC in this case is highest and is

equaltoVIN•IINTVCC.Thegate-chargecurrentisdependent

on operating frequency (400kHz), and the QG of the power

MOSFETs, as discussed in the Efficiency Considerations

section. The junction temperature can be estimated by

using the equations given in Note 3 of the Electrical Char-

acteristics section. For example, if the LTC3854 INTVCC

current is limited to less than 17mA from a 36V supply in

the DFN package; then the junction temperature is:

TJ = 70°C + [(17mA•36V)•(76°C/W)] = 116.5°C

To prevent the maximum junction temperature from be-

ing exceeded, the input supply current must be checked

during operation at maximum VIN.

Topside MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor CB connected from the

BOOST pin to the SW pin and supplies the gate drive volt-

age for the topside MOSFET. Capacitor CB in the Functional

DiagramischargedthoughexternaldiodeDBfromINTVCC

when the SW pin is low. When the topside MOSFET is to

be turned on, the driver places the CB voltage across the

gate source of the MOSFET. This enhances the MOSFET

and turns on the topside switch. The switch node volt-

age, SW, rises to VIN and the BOOST pin follows. With the

topside MOSFET on, the boost voltage is above the input

supply: VBOOST = VIN + VINTVCC. The value of the boost

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