I
LED1
Q1 OFF
Q2 ON
Q1 ON
I
V
V
LED1
OUT1
OUT2
Q2 OFF
Q1 OFF
Q2 ON
Q1 ON
V
V
OUT1
OUT2
I
LED2
Q2 OFF
t
PLH
MIN.
t
PLH
MAX.
I
LED2
t
PHL
PDD*
MAX.
t
PLH MAX.
MIN.
t
t
PHL
PHL
MIN.
MAX.
MAX.
DEAD TIME
PDD* MAX. =
(t
t
)
t
t
PLH- PHL MAX. = PLH MAX. - PHL MIN.
MAXIMUM DEAD TIME (DUE TO OPTOCOUPLER)
*PDD = PROPAGATION DELAY DIFFERENCE
= (t
= (t
t
t
) + (t
) - (t
t
)
PLH MAX.
-
-
PLH MIN.
PHL MAX.
-
t
PHL MIN.
)
PLH MAX.
PHL MIN.
PLH MIN.
-
PHL MAX.
NOTE: THE PROPAGATION DELAYS USED TO CALCULATE
PDD ARE TAKEN AT EQUAL TEMPERATURES.
= PDD* MAX. - PDD* MIN.
*PDD = PROPAGATION DELAY DIFFERENCE
Figure 29. Minimum LED Skew for Zero Dead Time.
NOTE: THE PROPAGATION DELAYS USED TO CALCULATE THE MAXIMUM
DEAD TIME ARE TAKEN AT EQUAL TEMPERATURES.
Figure 30. Waveforms for Dead Time Calculation.
(Figure 19), can achieve 15 kV/µs
CMR while minimizing component
complexity. Note that a CMOS
gate is recommended in Figure 19
to keep the LED off when the gate
is in the high state.
achieved by overdriving the LED
current beyond the input
LED Drive Circuit
Considerations for Ultra
High CMR Performance
threshold so that it is not pulled
below the threshold during a
transient. The recommended
minimum LED current of 10 mA
provides adequate margin over
the maximum ITH of 5.0 mA (see
Figure 5) to achieve 15 kV/µs
CMR. Capacitive coupling is
higher when the internal load
Without a detector shield, the
dominant cause of optocoupler
CMR failure is capacitive coupling
from the input side of the opto-
coupler, through the package, to
the detector IC as shown in
Another cause of CMR failure for
a shielded optocoupler is direct
coupling to the optocoupler
output pins through CLEDO1 and
CLEDO2 in Figure 21. Many factors
influence the effect and magni-
tude of the direct coupling includ-
ing: the use of an internal or
external output pull-up resistor,
the position of the LED current
setting resistor, the connection of
the unused input package pins,
and the value of the capacitor at
the optocoupler output (CL).
Figure 20. The HCPL-4506,
HCPL-0466 and HCNW4506
improve CMR performance by
using a detector IC with an optic-
ally transparent Faraday shield,
which diverts the capacitively
coupled current away from the
sensitive IC circuitry. However,
this shield does not eliminate the
capacitive coupling between the
LED and the optocoupler output
pins and output ground as shown
in Figure 21. This capacitive
resistor is used (due to CLEDO2
)
and an IF = 16 mA is required to
obtain 10 kV/µs CMR.
The placement of the LED current
setting resistor effects the ability of
the drive circuit to keep the LED on
during transients and interacts with
the direct coupling to the
optocoupler output. For example,
the LED resistor in Figure 22 is
connected to the anode. Figure 23
shows the AC equivalent circuit for
Figure 22 during common mode
transients. During a +dVcm/dt in
Figure 23, the current available at
the LED anode (Itotal) is limited by
the series resistor. The LED current
(IF) is reduced from its DC value by
an amount equal to the current that
Techniques to keep the LED in
the proper state and minimize the
effect of the direct coupling are
discussed in the next two
sections.
coupling causes perturbations in
the LED current during common
mode transients and becomes the
major source of CMR failures for
a shielded optocoupler. The main
design objective of a high CMR
LED drive circuit becomes keep-
ing the LED in the proper state
(on or off) during common mode
transients. For example, the
CMR with the LED On
(CMRL)
A high CMR LED drive circuit
must keep the LED on during
common mode transients. This is
flows through CLEDP and CLEDO1
The situation is made worse
.
recommended application circuit
1-61
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