LT3752 datasheet(24/48 Pages) LINER

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LT8311

8311f

For more information www.linear.com/LT8311

OPERATION

The PGOOD comparator has ±3% hysteresis. Therefore,

when the FB pin voltage is driven away from its regulated

value of 1.227V by ±10%, the PGOOD pin’s internal pull-

down shuts off immediately. As a result, the pin is pulled

high by an external resistor or external current source

connected to a supply voltage. The PGOOD pin’s output

can be fed to a microcontroller that make decisions based

on the state of the output voltage.

Output Overshoot Control Helps with Short-Circuit

Recovery

TheLT8311providesoutputovershootcontrolbyactivating

its soft-start pull-down amplifier (SSDOWNAMPintheBlock

Diagram) any time the FB pin voltage is less than 50%

of the FB reference voltage (1.227V). This is particularly

helpful with output voltage recovery after the removal of

a short-circuit condition or after a heavy load transient.

The SS pull-down amplifier will sink whatever current is

necessary (up to its maximum sink capability of 13mA), to

ensure that the SS pin voltage gets no higher than 100mV

abovetheFBpinvoltage.Duringoutputshort-circuitevents,

when the FB pin voltage is pulled to ground, the SS pull-

down amplifier gets activated and pulls the SS pin voltage

to 100mV above the FB pin voltage. Eventually, when the

short-circuit condition is over, the FB pin voltage gradually

rises up with the SS pin at a slew rate set by CSS and the

10µA charge current. This allows the output to recover

gradually from the short-circuit condition. Note that when

theLT8311hasitsVINpinpowereddirectlyfromtheoutput

of the forward converter, it will lose all its brains during a

short-circuit event. Under this scenario, output overshoot

control will not be in effect until the LT8311 gets brains

again, until which point, the output inductor current and

the output voltage will be controlled by the primary-side

IC’s soft-start function.

(a) Output Overshoot Control with CSS = 1nF. LT8311 VIN

Powered from a 12V Housekeeping Supply, Which Also

Pulls Up on the PGOOD Pin Through a 100k External

Resistor

(b) Output Overshoot Control with CSS = 33nF. LT8311 VIN

Powered from a 12V Housekeeping Supply, Which Also Pulls

Up on the PGOOD Pin Through a 100k External Resistor

Figure 12. Output Overshoot Control at Start-Up

PGOOD

10V/DIV

500mV/DIV

8311 F12a

1ms/DIV

PGOOD

10V/DIV

500mV/DIV

8311 F12b

2ms/DIV

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