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LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

Setting RTIMER in Preactive Mode

In preactive mode, the TIMER pin resistor, RTIMER, pro-

grams the maximum period that can elapse between two

CSW rising edges before a timeout period is triggered.

Timeout allows the LT8311 to stop all synchronous activ-

ity in the event that the primary-side IC stops switching.

Since CSW rising edges represent primary-side switching

activity, timeout of CSW rising edges is interpreted as

stoppage of switching—at which point the LT8311 ceases

all secondary-side synchronous switching, and starts its

evaluation period. Refer to the Operation section for de-

tails on the evaluation period. Secondary-side switching

resumes when all conditions within the evaluation period

are satisfied. Timeout also ensures that switching activity

within preactive mode occurs at a frequency that is within

preactive mode’s operating frequency range.

As shown in Figure 18, every time the CSW pin voltage

is detected to rise past 1.2V from a voltage level below

–150mV, the LT8311 resets its internal timeout signal.

The gate of the catch MOSFET, CG turns on (after some

propagation delay) when CSW is detected to fall below

–150mV. Upon CG going high, the catch MOSFET turns

on and pulls its drain voltage (CSW) close to its source

voltage, which is tied to GND. CG turns off predictively

in CCM before an anticipated CSW rising edge. If a CSW

rising edge (rising from below –150mV to above 1.2V)

does not come along in time to reset the timeout signal,

the signal eventually charges up to voltage VREF_TIMEOUT

and triggers an internal timeout condition. Consequently,

the LT8311 shuts down all synchronous conduction and

startstheevaluationperiod.Theevaluationperiodendsonly

when the four conditions listed in the Operation section,

including the timely reset of the internal timeout signal,

aresatisfiedforthreeconsecutiveCSWrisingedges.Upon

completion of the evaluation period, the LT8311 restarts

synchronous control.

Figure 18. If Timeout Is Triggered in Preactive Mode, LT8311 Shuts Down All Synchronous Conduction and

Starts the Evaluation Period (Note: CSW's ringing waveform is caused by the inductor current getting to 0A.)

8311 F18

LT8311 INTERNAL

TIMEOUT SIGNAL

CG PREDICTIVE

TURN-OFF

TIMEOUT RESETS ONLY WHEN

CSW RISES PAST 1.2V, PRECEDED BY

FALLING BELOW –150mV

TIMEOUT

CSW

(PREACTIVE MODE)

TIME

–150mV

1.2V

CG TURNS ON WHEN CSW < –150mV

EVALUATION PERIOD

VREF_TIMEOUT

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