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LT8311

8311f

For more information www.linear.com/LT8311

4. The CSP and CSN pins must not trip the internal

current comparator within a 150ns period of time

called "current sample window." This function helps

the LT8311 detect very light load conditions, dur-

ing which time it will keep synchronous conduc-

tion shut off, thereby improving system efficiency.

How the current sample window works:

Thecurrentsamplewindowexistsregardlessofwhether

MCG is turned on or not, in any given cycle. When CSW

is detected to fall below –150mV, the LT8311 starts a

blanktimeof200ns.Uponcompletionofthisblanktime,

theLT8311startsa150nscurrentsamplewindow.Ifthe

CSP/CSNpininputscausetheinternalcurrentcompara-

tor to trip during this 150ns window, the LT8311 will

interpret this as a condition of very light load, at which

point it will stop synchronous conduction and start the

evaluation period again. Please see "Configuring CSP/

CSN Inputs of Current Sense Comparator in Preactive

Mode” in the Applications Information section.

When all four conditions are valid for three continuous

CSW cycles, the evaluation period ends and the LT8311

gets ready to start switching. Switching commences with

the LT8311 turning on MCG for its minimum on-time. If

any of the four conditions listed are violated at any point

during switching activity, the LT8311 will shut down

all synchronous conduction and restart the evaluation

period.

Duringpreactivemodestart-up,theLT8311internallysoft-

starts the on-time of MCG, allowing the forward converter

to gradually transition from full cycles of nonsynchronous

MCG conduction (secondary-side current flowing through

body diode of MCG) to full cycles of synchronous MCG

conduction.

SYNC MODE SYNCHRONOUS CONTROL

SYNC mode allows the LT8311 to operate in forced con-

tinuous mode (FCM) at light loads. In SYNC mode, a pulse

transformer (see T2 in Figure 7) is required to allow the

LT8311 to receive synchronization control signals from

the primary-side IC. These control signals are interpreted

digitally (high or low) by the LT8311 to turn on/off the

catch and forward MOSFETs.

FCM operation allows the forward converter to avoid

operation in discontinuous conduction mode (DCM) at

light loads, by letting the inductor current go negative.

Hence, even at zero load, the inductor current remains

continuous and the converter runs at a fixed frequency.

MCG Turn-On/Off Timings in SYNC Mode

In SYNC mode, MCG turns on when the signal on the SYNC

pin is higher than 1.2V. MCG turns off when the signal on

the SYNC pin is lower than –1.2V.

MFG Turn-On/Off Timings in SYNC Mode

In SYNC mode, MFG turns on when the signal on the SYNC

pin is lower than –1.2V. MFG turns off when the signal on

the SYNC pin is higher than 1.2V.

The RSYNC and CSYNC time constant must be appropriately

chosen to generate a sufficient pulse width at a particular

overdrive voltage (see "Picking Pulse Transformer and

High Pass Filter" in the Applications Information section).

Typical values for CSYNC and RSYNC are 220pF and 560Ω,

respectively.

OPERATION

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