1-62
because the current through C
LEDO1
has the effect of trying to pull the 
output high (toward a CMR failure) 
at the same time the LED current is 
being reduced. For this reason, the 
recommended LED drive circuit 
(Figure 19) places the current setting resistor in series with the LED 
cathode. Figure 24 is the AC equivalent circuit for Figure 19 during 
common mode transients. In this 
case, the LED current is not 
reduced during a +dVcm/dt transient because the current flowing 
through the package capacitance is 
supplied by the power supply. 
During a -dVcm/dt transient, however, the LED current is reduced by 
the amount of current flowing 
through C
LEDN
. But, better CMR 
performance is achieved since the 
current flowing in C
LEDO1
 during a 
negative transient acts to keep the 
output low.
Coupling to the LED and output 
pins is also affected by the connection of pins 1 and 4. If CMR is 
limited by perturbations in the LED 
on current, as it is for the recommended drive circuit (Figure 19), 
pins 1 and 4 should be connected to 
the input circuit common. However, 
if CMR performance is limited by 
direct coupling to the output when 
the LED is off, pins 1 and 4 should 
be left unconnected.
CMR with the LED Off 
(CMRH)
A high CMR LED drive circuit must 
keep the LED off (V
F
≤ V
F(OFF)
) 
during common mode transients. 
For example, during a +dVcm/dt 
transient in Figure 24, the current 
flowing through C
LEDN
 is supplied 
by the parallel combination of the 
LED and series resistor. As long as 
the voltage developed across the 
resistor is less than V
F(OFF)
 the LED 
will remain off and no common 
mode failure will occur. Even if the 
LED momentarily turns on, the 100 
pF capacitor from pins 6-5 will 
keep the output from dipping below 
the threshold. The recommended 
LED drive circuit (Figure 19) provides about 10 V of margin between 
the lowest optocoupler output 
voltage and a 3 V IPM threshold
during a 15 kV/µs transient with 
V
CM
= 1500 V. Additional margin 
can be obtained by adding a diode 
in parallel with the resistor, as 
shown by the dashed line connection in Figure 24, to clamp the 
voltage across the LED below 
V
F(OFF)
.
Since the open collector drive circuit, shown in Figure 25, cannot 
keep the LED off during a +dVcm/ 
dt transient, it is not desirable for 
applications requiring ultra high 
CMR
H
 performance. Figure 26 is 
the AC equivalent circuit for Figure 
25 during common mode 
transients. Essentially all the 
current flowing through C
LEDN
during a +dVcm/dt transient must 
be supplied by the LED. CMR
H
failures can occur at dV/dt rates 
where the current through the LED 
and C
LEDN
 exceeds the input 
threshold. Figure 27 is an 
alternative drive circuit which does 
achieve ultra high CMR 
performance by shunting the LED 
in the off state.
IPM Dead Time and 
Propagation Delay 
Specifications
The HCPL-4506, HCPL-0466 and 
HCNW4506 include a Propagation 
Delay Difference specification 
intended to help designers minimize 
“dead time” in their power inverter 
designs. Dead time is the time 
period during which both the high 
and low side power transistors (Q1 
and Q2 in Figure 28) are off. Any 
overlap in Q1 and Q2 conduction 
will result in large currents flowing 
through the power devices between 
the high and low voltage motor rails.
To minimize dead time the designer 
must consider the propagation 
delay characteristics of the optocoupler as well as the characteristics of the IPM IGBT gate drive 
circuit. Considering only the delay 
characteristics of the optocoupler 
(the characteristics of the IPM 
IGBT gate drive circuit can be 
analyzed in the same way) it is 
important to know the minimum 
and maximum turn-on (t
PHL
) and
turn-off (t
PLH
) propagation delay
specifications, preferably over the 
desired operating temperature 
range.
The limiting case of zero dead time 
occurs when the input to Q1 turns 
off at the same time that the input 
to Q2 turns on. This case 
determines the minimum delay 
between LED1 turn-off and LED2 
turn-on, which is related to the 
worst case optocoupler propagation 
delay waveforms, as shown in 
Figure 29. A minimum dead time of 
zero is achieved in Figure 29 when 
the signal to turn on LED2 is 
delayed by (t
PLH max
 - t
PHL min
) from 
the LED1 turn off. Note that the 
propagation delays used to calculate PDD are taken at equal temperatures since the optocouplers under 
consideration are typically mounted 
in close proximity to each other. 
(Specifically, t
PLH max
 and t
PHL min
in the previous equation are not the 
same as the t
PLH max
 and t
PHL min
, 
over the full operating temperature 
range, specified in the data sheet.) 
This delay is the maximum value for 
the propagation delay difference 
specification which is specified at 
450 ns for the HCPL-4506, HCPL0466 and HCNW4506 over an 
operating temperature range of
-40°C to 100°C.
Delaying the LED signal by the 
maximum propagation delay difference ensures that the minimum 
dead time is zero, but it does not 
tell a designer what the maximum 
dead time will be. The maximum 
dead time occurs in the highly 
unlikely case where one optocoupler with the fastest t
PLH
 and another
with the slowest t
PHL
 are in the 
same inverter leg. The maximum 
dead time in this case becomes the 
sum of the spread in the t
PLH
 and
t
PHL
 propagation delays as shown in 
Figure 30. The maximum dead time 
is also equivalent to the difference 
between the maximum and minimum propagation delay difference 
specifications. The maximum dead 
time (due to the optocouplers) for 
the HCPL-4506, HCPL-0466 and 
HCNW4506 is 600 ns (= 450ns -
(-150 ns)) over an operating
temperature range of -40°C to 
100°C.