Evaluates: MAX8550/MAX8550A/MAX8551
falling transitions and may interfere with circuit perfor-
mance and generate EMI. To dampen this ringing, an
optional series RC snubber circuit is added across the
low-side switch. Below is a simple procedure for select-
ing the value of the series RC for the snubber circuit:
1) Connect a scope probe to the LX node labeled on
the MAX8550 EV kit schematic and observe the ring-
ing frequency, f
R
.
2) Estimate the circuit parasitic capacitance (C
PAR
) at
LX by first finding a capacitor value, which, when
connected from LX to PGND1, reduces the ringing
frequency by half. C
PAR
can then be approximated
as 1/3 the value of the capacitor value found.
3) Estimate the circuit parasitic inductance (L
PAR
) from
the equation:
4) Calculate R12 for critical dampening from the equation:
R12 = 2π×f
R
x L
PAR
Adjust the resistor value up or down to tailor the
desired damping and the peak voltage excursion.
5) Capacitor C15 should be at least 2 to 4 times the
value of C
PAR
to be effective.
The power loss of the snubber circuit (PWR_SNUB) is
dissipated in the resistor and can be calculated as:
PWR_SNUB = C15 x VIN
2
x f
SW
where VIN is the input voltage and f
SW
is the switching
frequency. Choose the power rating of R12 according
to the specific application’s derating rule for the power
dissipation calculated in the equation above.
Recommended snubber values for this EV kit are 3Ω
(R12) and 2.2nF (C15).
