13
CMR with the LED On (CMR
H
)
A high CMR LED drive circuit must keep the LED on dur-
ing common mode transients. This is achieved by over-
driving the LED current beyond the input threshold so
that it is not pulled below the threshold during a tran-
sient. A minimum LED current of 8 mA provides ade-
quate margin over the maximum IFLH of 5 mA to achieve
10 kV/µs CMR.
CMR with the LED O (CMR
L
)
A high CMR LED drive circuit must keep the LED o (VF ≤
VF(OFF)) during common mode transients. For example,
during a -dVCM/dt transient in Figure 23, the current
owing through CLEDP also ows through the RSAT and
VSAT of the logic gate. As long as the low state voltage
developed across the logic gate is less than VF(OFF) the
LED will remain o and no common mode failure will oc-
cur.
The open collector drive circuit, shown in Figure 24, can
not keep the LED o during a +dVCM/dt transient, since
all the current owing through CLEDN must be supplied
by the LED, and it is not recommended for applications
requiring ultra high CMR1 performance. The alternative
drive circuit which like the recommended application
circuit (Figure 19), does achieve ultra high CMR perfor-
mance by shunting the LED in the o state.
IPM Dead Time and Propagation Delay Specications
The HCPL-314J includes a Propagation Delay Dierence
(PDD) specication intended to help designers minimize
“dead time” in their power inverter designs. Dead time
is the time high and low side power transistors are o.
Any overlap in Ql and Q2 conduction will result in large
currents owing through the power devices from the
high-voltage to the low-voltage motor rails. To minimize
dead time in a given design, the turn on of LED2 should
be delayed (relative to the turn o of LED1) so that un-
der worst-case conditions, transistor Q1 has just turned
o when transistor Q2 turns on, as shown in Figure 26.
The amount of delay necessary to achieve this condition
is equal to the maximum value of the propagation de-
lay dierence specication, PDD max, which is specied
to be 500 ns over the operating temperature range of
-40° to 100°C.
Delaying the LED signal by the maximum propaga-
tion delay dierence 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 is equivalent to the dierence between the
maximum and minimum propagation delay dier-
ence specication as shown in Figure 27. The maxi-
mum dead time for the HCPL-314J is 1 µs (= 0.5 µs
- (-0.5 µs)) over the operating temperature range of
-40°C to 100°C.
Note that the propagation delays used to calculate PDD
and dead time are taken at equal temperatures and test
conditions since the optocouplers under consideration
are typically mounted in close proximity to each other
and are switching identical IGBTs.
