HCPL-3150-360 View Datasheet(PDF) - Avago Technologies
Part Name
Description
MFG CO.
'HCPL-3150-360' PDF : 21 Pages View PDF
LED Drive Circuit Considerations for Ultra High CMR Per-
formance
Without a detector shield, the dominant cause of opto-
coupler CMR failure is capacitive coupling from the in-
put side of the optocoupler, through the package, to the
detector IC as shown in Figure 29. The HCPL-3150/315J
improves CMR performance by using a detector IC with
an optically transparent Faraday shield, which diverts
the capacitively coupled current away from the sensitive
IC circuitry. How ever, this shield does not eliminate the
capacitive coupling between the LED and optocoupler
pins 5-8 as shown in Figure 30. This capacitive 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 ob-
jective of a high CMR LED drive circuit becomes keeping
the LED in the proper state (on or off ) during common
mode transients. For example, the recommended ap-
plication circuit (Figure 25), can achieve 15 kV/μs CMR
while minimizing component complexity.
Techniques to keep the LED in the proper state are dis-
cussed in the next two sections.
CMR with the LED On (CMRH)
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 10 mA provides ade-
quate margin over the maximum I of 5 mA to achieve
FLH
15 kV/μs CMR.
CMR with the LED Off (CMR )
L
A high CMR LED drive circuit must keep the LED off
(VF ≤VF(OFF)) during common mode transients. For exam-
ple, during a -dV /dt transient in Figure 31, the current
CM
flowing through C also flows through the R and V
LEDP
SAT
SAT
of the logic gate. As long as the low state voltage devel-
oped across the logic gate is less than V , the LED will
F(OFF)
remain off and no common mode failure will occur.
The open collector drive circuit, shown in Figure 32, can-
not keep the LED off during a +dV /dt transient, since
CM
all the current flowing through C must be supplied
LEDN
by the LED, and it is not recommended for applications
requiring ultra high CMR performance. Figure 33 is an
L
alternative drive circuit which, like the recommended
application circuit (Figure 25), does achieve ultra high
CMR performance by shunting the LED in the off state.
Under Voltage Lockout Feature
The HCPL-3150/315J contains an under voltage lockout
(UVLO) feature that is designed to protect the IGBT under
fault conditions which cause the HCPL-3150/315J supply
voltage (equivalent to the fully-charged IGBT gate volt-
age) to drop below a level necessary to keep the IGBT in
a low resistance state. When the HCPL-3150/315J output
is in the high state and the supply voltage drops below
the HCPL-3150/315J V threshold (9.5 <V <12.0),
UVLO-
UVLO-
the optocoupler output will go into the low state with
a typical delay, UVLO Turn Off Delay, of 0.6 μs. When the
HCPL-3150/315J output is in the low state and the supply
voltage rises above the HCPL-3150/315J V threshold
UVLO+
(11.0<V <13.5), the optocoupler will go into the high
UVLO+
state (assuming LED is “ON”) with a typical delay, UVLO
TURN On Delay, of 0.8 μs.
7
Qg = 100 nC
6
Qg = 250 nC
Qg = 500 nC
5
4
VCC = 19 V
VEE = -9 V
3
2
1
0
0
20
40 60 80 100
Rg – GATE RESISTANCE – Ω
Figure 27. Energy Dissipated in the HCPL-3150
for Each IGBT Switching Cycle.
18
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