LT3999 switch node voltage

Pradeep.C on Nov 17, 2020

Hi All,

I am using LT3999MP part in my design. As per datasheet, SW node voltages (at SWA and SWB) should be between -0.3V and 80V (screenshot below).

The transformer that I am using has leakage inductance of 0.45uH. Based on simulations with LT3999, I am observing ringing at the switch node during turn off and for brief amount of time during start-up (upto 5 msec), the switch node voltage goes below -0.3V up-to -6V. Will this damage the IC?

I am assuming that LT3999MP uses MOSFETs as switches and body diodes of MOSFET should have capability to handle this voltages. Please let me know if the device can sustain negative voltage spike of less than 10V for few msec during start-up or not?

Thanks

C.Pradeep

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Wenwei
Nov 30, 2020 in reply to Pradeep.C +1 verified
Hello,

It is recommended to use a snubber in this case. Too much of negative voltage might affect the IC ground and its internal logic.

Thanks,

Wenwei.

0 Pradeep.C on Nov 23, 2020 6:05 AM

Hi All, please provide your support on the same.

Further update on simulations using LT3999:

Addition of a R-C snubber from SWA to ground and SWB to ground is helping us to ensure that under steady-state, switch nodes (SWA, SWB) are not seeing negative voltages. However, during start-up (up-to 1msec post powerup), we are still seeing negative voltage of ~-0.6V at SWA, SWB during switch turn-off. Can the device sustain these voltages?

If the device can sustain negative voltage of (-6V), we want to eliminate snubbers in our design. Your response will help us finalize the design.

Thanks

C.Pradeep
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0 Wenwei

on Nov 30, 2020 3:25 PM in reply to Pradeep.C
Hello,

It is recommended to use a snubber in this case. Too much of negative voltage might affect the IC ground and its internal logic.

Thanks,

Wenwei.
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0 Pradeep.C on Dec 2, 2020 2:01 AM in reply to Wenwei

Thank you Wenwei. We will go ahead with snubber solution. During power up, we see small negative voltage (-1V) for < 1msec. Will that be ok?
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