LMQ61460-Q1: How to add an RC snubber circuit to the Inverting Buck Boost application to reduce the ringing of the SW node ?

The RC snubber goes into position A.

Your value of RSNUB is what provides the damping, the CSNUB is a DC blocking components so as to not dissipate too much energy for the insertion of RSNUB.


The larger the CSNUB, the more power you will dissipate in CSNUB for a given frequency.

The power dissipated in RSNUCH for a given CSNUB/FSW is:
PRSNUB= CSNUB*VIN^2*FSW

The RC snubber is used to dampen the ringing cause by the parasitic Loop inductance and the capacitance at the switch node. RSNUB is selected as a optimized dampener, equal to SQRT(LPARA/CSWITCH). This is the RSENUCH value to use.

If we assume CSWITCH to be 10’s of pF’s of capacitance, the CSNUB will do well if its ~10X this, so I will say 100’pFs….

Note CSWITCH = COSS of MOSFET and parasitic C of switch node.

Now all you have to do is to select a RSNUB. Typically the parasitic L is low nH, say 1nH .

Lets just assume some numbers based of above assumptions.

SQRT (1nH/50pF) = 4.4ohm = RSNUB

Make CSNUB – 470pF. ~10 x the capacitance of CSWITCH

If switching at say 400kHz.and 12V in 27mW
At 2.2MHx will be = 150mW Approx..

If this does not snub well it typically means our estimation of CSWTICH and LPARA are a little off, usually lowering RSNUB helps this assumes LPARA is more than typical…


Hope this helps.

Thank you for your reply!