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ACPL-W343-500E Datasheet, PDF (18/20 Pages) AVAGO TECHNOLOGIES LIMITED – 4.0 Amp Output Current IGBT Gate Drive Optocoupler
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LED Drive Circuit Considerations for High CMR Performance
Figure 32 shows the recommended drive circuit for the
ACPL-P343/W343 that gives optimum common-mode
rejection. The two current setting resistors balance the
common mode impedances at the LED’s anode and
cathode. Common-mode transients can be capacitive
coupled from the LED anode, through CLA (or cathode
through CLC) to the output-side ground causing current
to be shunted away from the LED (which is not wanted
when the LED should be on) or conversely cause current
to be injected into the LED (which is not wanted when the
LED should be off ).
Table 8 shows the directions of ILP and ILN depend on the
polarity of the common-mode transient. For transients
occurring when the LED is on, common-mode rejection
(CMH, since the output is at “high” state) depends on
LED current (IF). For conditions where IF is close to the
switching threshold (IFLH), CMH also depends on the
extent to which ILP and ILN balance each other. In other
words, any condition where a common-mode transient
causes a momentary decrease in IF (i.e. when dVCM/dt > 0
and |ILP| > |ILN|, referring to Table 8) will cause a common-
mode failure for transients which are fast enough.
if an imbalance between ILP and ILN results in a transient
IF equal to or greater than the switching threshold of the
optocoupler, the transient “signal” may cause the output
to spike above 1 V, which constitutes a CML failure. The
balanced ILED-setting resistors help equalize the common
mode voltage change at the anode and cathode. The
shunt drive input circuit will also help to achieve high CML
performance by shunting the LED in the off state.
VDD = 5.0 V:
R1 = 205 7 ±1%
+5 V
R2 = 137 7 ±1%
R1/R2 ≈ 1.5
R1 ANODE
ILP
1
CLA
6 VCC
2
ILN
R2
3
CATHODE
CLC
5 VOUT
4 VEE
Likewise for a common-mode transient that occurs when
the LED is off (i.e. CML, since the output is at “low” state), Figure 32. Recommended high-CMR drive circuit for the ACPL-P343/W343
Table 8. Common Mode Pulse Polarity and LED current Transients
dVCM/dt
Positive (>0)
Negative(<0)
ILP Direction
Away from LED anode
through CLA
Toward LED anode
through CLA
ILP Direction
Away from LED cathode
through CLC
Toward LED cathode
through CLC
If |ILP| < |ILN|,
IF is momentarily
Increase
Decrease
If |ILP| > |ILN|,
IF is momentarily
Decrease
Increase
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