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LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

voltage (VOPTO(MAX)). The opto-coupler’s input LED has

a turn-on voltage of 1.2V:

RD =

VOPTO(MAX) − 1.2V − 0.5V

IF_HIGH

The extra 0.5V in the equation is margin to account for the

OPTO pin's linear range. The maximum OPTO pin voltage

is 6V (minimum guaranteed), when the LT8311’s VIN pin

is at 8V or higher. At lower VIN pin voltages, VOPTO_MAX

is VIN – 1.7V.

The previous equations show how RD and RE ought to

be calculated for large signal characteristics of an opto-

coupler-based voltage feedback loop. The final values

chosen for RD and RE may need to be tweaked from the

valuescalculatedheretoachieveasatisfactorycompromise

between the large and small signal characteristics of the

voltage feedback loop.

Picking Soft-Start Capacitor (CSS) for Output Soft-Start

The Operation section explained how the LT8311’s SS

pin helps with output soft-start at start-up, with output

overshoot control during short-circuit recovery, and to

prebias the voltage feedback loop during start-up of the

LT8311’s opto-control scheme. The soft-start capacitor,

CSS, is charged by the LT8311’s internally trimmed 10µA

current source at start-up. Since the FB pin voltage tracks

theSSpinvoltagewhenthevoltageonSSisbelow1.227V,

setting the SS pin’s slew rate will set the FB pin’s slew

rate, setting the time taken by the output to come up to

its regulation voltage. It is important to recognize that the

tracking between the SS pin’s slew rate and the FB’s pin

slew rate is only valid as long as the LT8311’s soft-start of

outputvoltageisslowerthantheprimary-sideIC’ssoft-start

of output voltage, as explained in the Operation section.

By observing this criteria, the following equation applies:

∂VOUT

∂t

= ∂

VFB

∂t

10µA

CSS

where CSS is the capacitor from the LT8311’s SS pin to

GND, VOUT is the output voltage of the forward converter,

and VFB is the LT8311’s FB pin voltage.

In steady state, the SS pin voltage is clamped to a maxi-

mum of 2V by an internal clamp.

LT8311 SYNCHRONOUS CONTROL FUNDAMENTALS

Catch and Forward MOSFET Selection

Whenselectingthesecondary-sidesynchronousMOSFETs,

it is important to choose the following parameters care-

fully to ensure robust operation of the system: maximum

drain-source voltage, maximum drain-source current and

maximum gate-source voltage. Furthermore, to maximize

systemefficiency,itisimportanttolowerpowerdissipation

intheMOSFETsbyminimizingtheiron-resistance(RDS(ON))

and gate charge (Qg). Please use the following guidelines

to choose appropriate catch and forward MOSFETs for a

specific application:

1. Maximum VDS Rating

The maximum voltage seen on the drain of the catch MOS-

FET is a function of the maximum input voltage (VIN(MAX))

of the system, and the transformer turns ratio (NS/NP).

Catch MOSFET VDS(MAX) = VIN(MAX) •

• Margin

where Margin is a number from 1 to 3 (typically 1.5 to 2),

allowing a certain safety margin in the catch MOSFET’s

VDS(MAX)equation.Thiswillaccountforvoltagespikesas-

sociated with the leakage inductance of the transformer’s

secondary winding. Using a snubber on the drain of the

catch MOSFET will minimize leakage inductance spikes

and allow Margin to approach the lower end of its range.

The maximum voltage seen on the drain of the forward

MOSFET is a function of the reset mechanism used on

the primary side of the forward converter to reset the

transformer’s magnetic flux.

When using active clamp reset:

Forward MOSFET VDS(MAX) ≈

VOUT

−

VOUT

VIN(MIN) •

whereVIN(MIN)istheminimuminputvoltageofthesystem,

and VOUT is the forward converter’s output voltage. Note

that this equation for the forward MOSFET’s VDS(MAX)

assumes that the primary side’s active clamp capacitor

(CCL) is large enough to be treated as a voltage source.

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