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LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

VIN BIAS SUPPLY

The LT8311’s VIN pin can be powered in various ways.

Place at least a 2.2µF ceramic bypass capacitor close to

the pin.

PickinganappropriatebiassupplytopoweruptheLT8311

requires consideration of the following criteria:

1. The VIN pin, in certain configurations, may be the only

supply to the LT8311’s INTVCC pin, which provides

gate drive to the catch and forward MOSFETs. In such

situations, VIN’s bias supply must be high enough to

provide adequate gate-drive voltage (typically 5V to 7V)

for both synchronous MOSFETs.

2. VIN’s bias supply must be able to source:

a. LT8311’s VIN current (4.5mA typical)

b. INTVCC gate-drive current when using VIN to sup-

ply the INTVCC pin (typically 10mA to 30mA)

c. Opto-driver source current (typically 1mA to 5mA)

3. VIN start-up and short-circuit conditions:

a. VIN must come up in reasonable time to allow the

LT8311 to begin synchronous and opto-coupler

control. While synchronous control is shut off, the

secondary-side current will flow through the body

diodes of the secondary synchronous MOSFETs.

While opto-control is off, the forward converter

will operate open-loop, using a volt-second clamp

to control VOUT if operating with LT3752, LT3752-1

or LT3753 on the primary side.

b. VIN may be shorted to GND during transient events.

For instance, VIN powered from the output voltage,

will be driven to 0V during an output short-circuit.

The forward converter must be able to ride through

the momentary loss of power to the LT8311, which

is often easily accomplished by appropriately

configuring soft-start control on the primary-side

ICs. Refer to the LT3752/LT8310 data sheets for

details on configuring soft-start control on the

primary-side IC.

With the previous criteria in mind, there are three meth-

ods (1-3), listed below, for powering up the LT8311. For

preactive mode, use method 1, 2 or 3. For SYNC mode

FCM, use method 1 or 3; for DCM, use method 1, 2 or 3.

1. Power from the LT3752’s housekeeping supply (see

Figure 21 in the Typical Application section). Being

a flyback converter rather than a LDO, the LT3752’s

housekeeping supply is an efficient supply source. It

can be connected through an external winding to the

LT8311’s VIN and INTVCC pins, and can be set high

enough to provide adequate gate drive for the catch

and forward MOSFETs, but low enough to minimize

efficiencyandthermallosses.Thehousekeepingsupply

comes up as soon as the LT3752 receives input power,

so power is delivered to the LT8311 without delay.

2. Power directly from VOUT. At output voltages lower

than 10V, careful consideration must be given to the

output voltage start-up time, ensuring that the LT8311

can turn on and provide synchronous/opto control well

before the output voltage approaches regulation. It is

alsoimportanttoensure,attheseloweroutputvoltages,

that sufficient gate drive voltage can be provided to the

external MOSFETs. At higher VOUT voltages, efficiency

and thermal considerations related to the IC’s internal

power dissipation can become important criteria. In

addition, at higher VOUT voltages, it is important to

ensure that voltage transients on the VIN pin do not

exceed the pin’s abs max rating of 30V.

3. Use a buck circuit from an auxiliary transformer wind-

ing, as shown in Figure 13. This circuit has the benefit

of being highly efficient, and is fairly simple to design.

It is particularly useful for low output voltage applica-

tions (3.3V or 5V) that do not have an external house-

keeping supply, and where powering directly from the

output voltage is inadequate. In this configuration, the

buck circuit’s output voltage derives its energy from

secondary-sideswitchingpulsesthatalsosourceenergy

to the forward converter’s main output voltage, VOUT.

Careful consideration must be given to ensure that the

buck output voltage comes up well in time, and turns

ontheLT8311toprovidesynchronousandoptocontrol

before the forward converter’s actual output voltage

gets close to regulation. If there is a need to speed up

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