LT3752 datasheet(39/48 Pages) LINER

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LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

3. RSYNC must be large enough to limit the amount of

source/sink current required each time a positive or

negative SYNC pin voltage signal is generated. The

SOUT pin’s gate drivers offer limited source current

capability; RSYNC must be large enough to ensure

that this constraint in current-drive is not violated.

For instance, the LT3752’s SOUT drivers are rated for

a maximum current of about 100mA. This results in:

RSYNC ≥

VMAX

100mA

VMAX is the SOUT gate driver high voltage, which is

typically about 8V to 12V for the LT3752.

The following steps can be used as guidelines to calculate

RSYNC and CSYNC values:

Step 1: Choose Pulse Transformer. A typically recom-

mendedchoiceisthePE-68386NLfromPulseElectronics.

Step 2: Determine the primary-side IC’s maximum SOUT

signalmagnitude,VMAX(seeFigure19).Thissetsthemaxi-

mum magnitude of the signal on the LT8311’s SYNC pin.

Step 3: Guess a capacitance value for CSYNC. A good

starting value might be between 220pF and 1nF.

Step 4: Pick RSYNC based on constraint shown in the

following equation:

•

CSYNC









≥ RSYNC ≥ MAX

50ns

CSYNC • −1 • IN(2V / VMAX)





, VMAX

IMAX

























where IMAXisthemaximumcurrentsource/sinkcapability

of the primary-side IC’s SOUT pin (LT3752’s maximum

capabilityisabout100mAandLT8310’smaximumcurrent

capability is about 300mA). It is recommended to design

for an IMAX that is lower than the maximum recommended

source current specified, to allow for design margin over

process and temperature.

If the RSYNC calculation in Step 4 yields an unreasonable

resistance value, go back to steps 1 to 3, and change either

Lm, VMAX, or CSYNC. Recalculate RSYNC in Step 4 until all

criteria are satisfied.

Design Example

In a LT3752-LT8311 forward converter design, pulse

transformer PE-68386NL is chosen for communication

of LT3752 SOUT signals, through a highpass filter, to the

LT8311’s SYNC pin.

Step 1: This transformer has a magnetizing inductance

of Lm = 785µH.

Step 2: LT3752’s VMAX = 12V.

Step 3: Choose CSYNC = 220pF

Step 4: Designing for IMAX = 70mA, Lm = 785µH, CSYNC

= 220pF, VMAX = 12V, results in the following calculation

for RSYNC:

944Ω ≥ RSYNC ≥ Max {127Ω, 171Ω}

Conclusion

In this example, RSYNC=560ΩischosenalongwithCSYNC

= 220pF as the highpass filter to be used along with pulse

transformer, PE-68386NL to communicate the LT3752’s

SOUT signals to the LT8311’s SYNC pin.

LT3752 Datasheet(PDF) 39 Page - Linear Technology

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