BM28723AMUV
Selection of Components Externally Connected - continued
4 Snubber Circuit Constant
When overshoot of PWM Output exceeds absolute maximum rating, and when VCCP1, VCCP2>22V, or when overshoot of PWM
output negatively affects EMI, or when ringing deteriorates the audio characteristic of the PWM output, snubber circuit is used
as shown below.
1. Measure the ringing resonance frequency f1 of PWM output waveform (when rising) by using low capacitance Probe
(e.g. FET probe) at the OUT pin. (Figure 76)
Shorten GND lead of FET probe and monitor as near as possible to output pin.
2. Measure the resonance frequency f2 of the ringing as the snubber-circuit Rsnb=0Ω
(capacitor is connected to GND)
Adjust the value of the capacitor C until it becomes half of f1 (2f2=f1).
The value of C that becomes (2f2=f1) is 3 times of the parasitic capacity Cp that a ringing is formed. (C=3Cp)
3. Parasitic inductance Lp is calculated using the next formula.
1
Lp
2ππ1 2C p
4. The characteristics impedance Z of resonance is calculated from the parasitic capacity Cp and the parasitic inductance Lp
using the next formula.
Lp
Z
C p
5. Set snubber circuit Rsnb same as the character impedance Z.
Set snubber circuit Csnb 4 to 10 times of the parasitic capacity Cp (Csnb=4Cp to 10Cp).
If Csnb value is set large, switching current will possibly increase.
Therefore, please decide it by the trade-off with the characteristic.
VCCP1
VCCP2
Snubber
LC Filter
Ringing resonance
frequency
Driver
OUT1N
OUT1P
OUT2N
OUT2P
Csnb
Rsnb
5ns/div
GNDP1
GNDP2
Figure 76. PWM Output Waveform
Figure 77. Snubber Schematic
(Measure of spike resonance frequency)
Following presents Snubber filter constants with the recommendation value at 22V<VCCP1, VCCP2≤24V, RL=8Ω,
Po=10W+10W, ROHM 4 layer board(Note 12).
Csnb
Rsnb
680pF 50V CH (±5%)
5.6Ω 1/4W J (±5%)
Following presents Snubber filter constants with the recommendation value at VCCP1, VCCP2≤22V, RL=8Ω,
Po=10W+10W, ROHM 4 layer board(Note 12).
Csnb
Rsnb
470pF 50V CH (±5%)
0Ω or 5.6Ω 1/8W J (±5%)
(Note 12) 100mmx100mmx1.6mm FR4 4-layer glass epoxy board Cu Thickness 35μm/70μm/70μm/35μm For Application Evaluation Board
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