LT8311_15 Datasheet, PDF (24/48 Pages) Linear Technology – Synchronous Rectifier Controller with Opto-Coupler Driver for Forward Converters

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
The LT8311 provides output overshoot control by activating
its soft-start pull-down amplifier (SS_DOWNAMP in the Block
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
above the FB pin voltage. During output short-circuit events,
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 C_SS and the
10µA charge current. This allows the output to recover
gradually from the short-circuit condition. Note that when
the LT8311 has its V_IN pin powered directly from the output
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.
PGOOD
10V/DIV
SS
500mV/DIV
FB
500mV/DIV
PGOOD
10V/DIV
SS
500mV/DIV
FB
500mV/DIV
1ms/DIV
8311 F12a
2ms/DIV
8311 F12b
(a) Output Overshoot Control with C_SS = 1nF. LT8311 V_IN
Powered from a 12V Housekeeping Supply, Which Also
Pulls Up on the PGOOD Pin Through a 100k External
Resistor
(b) Output Overshoot Control with C_SS = 33nF. LT8311 V_IN
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
8311f
24
For more information www.linear.com/LT8311