LT3752 datasheet(38/48 Pages) LINER

38 / 48 page

LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

SYNC MODE SYNCHRONOUS CONTROL

Picking the Pulse Transformer and High Pass Filter

In SYNC Mode, the LT8311 determines the turn-on/off

timings of the catch and forward MOSFETs based on

voltage signals on its SYNC pin. Figure 7 in the Operation

section shows a typical circuit used to communicate syn-

chronous control signals from the primary-side IC’s SOUT

pin to the LT8311’s SYNC pin. This circuit utilizes a pulse

transformer (T2 in Figure 7) to provide isolation between

the primary and secondary sides, and a high pass filter

(RSYNC and CSYNC). CSYNC blocks DC signals from being

applieddirectlytoT2.EliminatingtheDCcomponentofthe

SOUTsignal,throughthehighpassfilter(RSYNCandCSYNC),

allows the SYNC pin signal to go positive or negative at

the rising and falling edges of SOUT, as shown in Figure

19. Positive and negative signals of equal magnitudes and

duration allow equal positive and negative volt-seconds

to be maintained on transformer T2, preventing any net

magnetizing current build-up.

Appropriate values of RSYNC and CSYNC must be chosen

to satisfy all of the following criteria:

1. The RSYNC • CSYNC time constant must be large

enough to allow a sufficiently long pulse width to be

generated on the SYNC pin with sufficient overdrive

voltage. This is shown in Figure 19 where t1 must be

at least 50ns at a SYNC voltage of ±2V (or greater

over drive) to trip the SYNC comparators. Using this

constraint, the equation below sets a limit on the

minimum RSYNC • CSYNC product required:



RSYNC • CSYNC ≥

50ns

−1•

n

VMAX





















where VMAX is the maximum SOUT voltage, as shown

in Figure 19.

2. RSYNC must be small enough to ensure that the

SYNC signal is sufficiently damped. An underdamped

SYNC signal can cause ringing large enough to cause

false triggering of the SYNC detection comparators,

which may lead to improper secondary-synchronous

control. The equation used to calculate RSYNC for

optimal damping is given by:

RSYNC ≤

2 •









•

CSYNC

where

ζ is the damping factor and should typically be

chosen to be about 1. Lmisthemagnetizinginductance

of the pulse transformer’s primary winding.

Choosing Lm to be larger allows the damping factor

to increase, so it would be wise to choose a pulse

transformer with a larger primary winding inductance

to increase the damping of the SYNC signal.

Smaller RSYNC values also reduce the sensitivity of

the highpass filter to stray signals (parasitic magnetic

fields) that may couple in.

Figure 19. Positive and Negative SYNC Edges Are Generated on the Rising and Falling Edges of SOUT, Respectively.

The LT8311 Requires Pulse Width Time, t1, to Be at Least 50ns (Typical) with the SYNC Pin Voltage at ±2V (or Greater

Overdrive) to Trigger the Internal SYNC Detect Comparators.

8311 F19

SOUT

SYNC

TIME

VMAX

–2V

–VMAX

LT3752 Datasheet(PDF) 38 Page - Linear Technology

LT3752 Datasheet(HTML) 38 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Datasheet Download

Link URL