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LT8311

8311f

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LT8311 SYNCHRONOUS CONTROL SCHEMES

The LT8311 offers two modes of synchronous control:

1. Preactive Mode: No pulse transformer needed; DCM

operation at light load. Enabled by tying the PMODE pin

to 0V. Use a Schottky diode across MCG (Figure 20).

2. SYNC Mode: Pulse Transformer needed; FCM or DCM

operation at light load. Enabled by tying the PMODE

pin to INTVCC.

PREACTIVE MODE SYNCHRONOUS CONTROL

MCG Turn-On/Off Timings in Preactive Mode

"Preactive" is short for "predictive" + "reactive". In preactive

mode, the LT8311 controls the secondary synchronous

MOSFETs without any communication from the primary-

side IC. In preactive mode, the catch MOSFET, MCG, is

turned on (CG rising edge in Figure 5) when the voltage

on its drain, CSW, is detected to be below –150mV, and

the forward MOSFET, MFG, is detected to be off. MCG is

turned off when the first of two events after MCG’s turn-

on occurs:

• Predictive MCG Turn-Off (Figure 5): In predictive turn-

off, the LT8311 predicts when M1 will turn on in the

next cycle, and turns off MCG 100ns prior to this event.

Predictive turn-off of MCG prevents cross conduction

between MCG and M1. M1’s turn-on timings are pre-

dicted by phase locking to the rising edge of present

and past CSW cycles. Predictive turn-off relies on the

periodicity of M1’s turn-on edge, an inherent aspect of

fixed-frequency operation. Furthermore, the predictive

turn-off is designed to be independent of the duty cycle

of the system, which allows MCG to be correctly turned

off, even during load/line transients. Predictive turn-off

will typically be the dominant turn-off mechanism for

MCG in CCM.

• Reactive MCG Turn-Off (Figure 6): Reactive turn-off

forces the forward converter to operate in DCM at light

load. In reactive turn-off, the LT8311 turns off MCG

when the current in MCG (IMCG) trips the LT8311’s

internal current comparator. The inputs to this current

comparator are the CSP and CSN pins. Typically, the

CSP and CSN pins will be configured to trip at almost

zero current in MCG, which should correspond to nearly

zero current in the output inductor. Reactive turn-off

will typically be the dominant turn-off mechanism for

MCG in DCM.

The LT8311’s seamless transition between predictive

and reactive portions of preactive mode allows the catch

MOSFET to be turned off at the correct time to avoid cross

conduction or avalanching.

MFG Turn-On/Off in Preactive Mode

In preactive mode, MFG is turned on after MCG’s turn-off

edgeisdetected,andthevoltageonthedrainoftheforward

MOSFET, FSW, is detected to be below 1.2V. Waiting for

FSW to fall below 1.2V ensures that transformer reset is

close to completion. MFG is turned off when the voltage

on CSW is detected to be below –150mV.

Since preactive mode requires each MOSFET to be turned

on only after the other MOSFET’s turn-off edge is detected,

the system requires a start point where one of the two

MOSFETs begins switching. Preactive mode’s start point

happens by turning on MCG first to commence switching.

Preactive Mode Shutdown and Start-Up

Preactivemodeisdesignedwithmanyfeaturestofacilitate

smooth start-up of synchronous control and shut down of

the scheme when necessary. Prior to starting switching

activity, the LT8311 evaluates conditions on the forward

converter’s secondary side to determine if switching can

commence. The evaluation period ends when four specific

conditions, are satisfied for a period of three continuous

CSW switching cycles (rising edge to rising edge). If

any of the conditions are violated, the evaluation period

is reset, and switching activity is kept shut off. During

this evaluation period, the secondary side current will

flow through the body diodes of MCG and MFG. The four

conditions are:

1. VIN must be greater than its UVLO voltage

2. INTVCC must be within its UVLO/OVLO limits

3. The TIMER pin should not have timed out. This feature

exists to ensure that the LT8311 ceases switching in

the event that the primary side stops switching.

OPERATION

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