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|  | hbliek
on 09/21 at 01:15 PM
| Category(s): | power.national.com |
| Subject: | LM5642 Gate (+SW) ringing |
| Message: (Notified via email on every direct response) |
Hi,
I'm experiencing some excessive ringing (>150%)in a new design with the LM5642. The design uses dual N fets in SO8 and a 10uf ceramic for Cin. High and low connect directly to each other and to Cin, so that cannot be the source of the ringing.
While investigating, I realized that in previous designs I used fets with a gate capacitance of about 4.7nF (35nC). This dual fet has much lower capacitance, only 404pF (8.2nC).
I tried a gate resistor of 4.7ohms, but this hardly helps. Then I tried 5nF caps between the gate and source of the highside fets, and voila, all ringing disappeared!
My question is, am I missing something here? As I have never ever seen this trick as a way to get rid of ringing...
Of course it will increase dissipation in the gate drivers of the LM5642, but current increase when idling is only marginal. Besides, the other designs with all 4.7nF fets works fine too.
Thanks, Henk
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