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BD28620MUV Datasheet(PDF) 40 Page - Rohm |
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BD28620MUV Datasheet(HTML) 40 Page - Rohm |
40 / 49 page 40/46 BD28620MUV © 2014 ROHM Co., Ltd. All rights reserved. 16.Mar.2015 Rev.004 www.rohm.com TSZ22111・15・001 TSZ02201-0C1C0E900100-1-2 (2) Snubber circuit constant When overshoot/undershoot of PWM Output exceeds absolute maximum rating, or when overshoot/undershoot of PWM output negatively affects EMC, or when ringing deteriorates the audio characteristic of the PWM output, snubber circuit is used as shown below. (a) Measure the spike resonance frequency “f1” of PWM output waveform (when rising) by using FET probe at the OUT terminal. (Figure 73) Shorten GND lead of FET probe and monitor as near as possible to output pin. (b) Measure the resonance frequency “f2” of the spike as the snubber-circuit R value equals 0Ω (capacitor “C” is connected to GND) Adjust the value of the capacitor “C” until it becomes (2 x f2 = f1) The value of “C” that becomes (2xf2=f1) is 3 times of the parasitic capacity “Cp” that a spike is formed. (C=3Cp) (c) Parasitic inductance “Lp” is calculated using the next formula. (d) The character impedance Z of resonance is calculated from the parasitic capacity “Cp” and the parasitic inductance Lp using the next formula. (e) Set snubber circuit “R” same as the character impedance “Z”. Set snubber circuit “C” 4 to 10 times of the parasitic capacity “Cp”. If “C” is set larger than 10Cp, switching current will possibly increase. The following table shows ROHM recommended value of “Snubber filter constants” when using ROHM 4 layer board. RL C R 4Ω 680pF to 1200pF ,50V B(±10%) 5.6Ω,1/10W J(±5%) 6Ω 680pF to 1200pF ,50V B(±10%) 5.6Ω,1/10W J(±5%) 8Ω 680pF to 1200pF ,50V B(±10%) 5.6Ω,1/10W J(±5%) Figure74. Snubber Schematic Figure 73. PWM Output Waveform ( Measure of Spike Resonance Frequency) Spike resonance frequency 5nsec/div Driver GNDP VCCP R C Snubber LCfilter OUT p C f 2 1 p 2 1 L p p C L Z |
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