HCPL-4506 Datasheet, PDF (13/14 Pages) Agilent(Hewlett-Packard) – Intelligent Power Module and Gate Drive Interface Optocouplers

I_LED1
I_LED1
V_OUT1
V_OUT2
Q1 ON
Q2 OFF
Q1 OFF
Q2 ON
I_LED2
t_{PLH MAX.}
t_PHL
MIN.
PDD* MAX. =
(t_PLH-t_PHL) _{MAX. =} t_{PLH MAX. -} t_{PHL MIN.}
*PDD = PROPAGATION DELAY DIFFERENCE
NOTE: THE PROPAGATION DELAYS USED TO CALCULATE
PDD ARE TAKEN AT EQUAL TEMPERATURES.
Figure 29. Minimum LED Skew for Zero Dead Time.
V_OUT1
V_OUT2
Q1 ON
Q2 OFF
Q1 OFF
Q2 ON
I_LED2
t_PLH
MIN.
t_PLH
MAX.
PDD*
MAX.
t_PHL
MIN.
t_PHL
MAX.
MAX.
DEAD TIME
MAXIMUM DEAD TIME (DUE TO OPTOCOUPLER)
= (t_{PLH MAX. -} t_{PLH MIN.}) + (t_{PHL MAX. -} t_{PHL MIN.})
= (t_{PLH MAX. -} t_{PHL MIN.}) - (t_{PLH MIN. -} t_{PHL MAX.})
= PDD* MAX. - PDD* MIN.
*PDD = PROPAGATION DELAY DIFFERENCE
NOTE: THE PROPAGATION DELAYS USED TO CALCULATE THE MAXIMUM
DEAD TIME ARE TAKEN AT EQUAL TEMPERATURES.
Figure 30. Waveforms for Dead Time Calculation.
LED Drive Circuit
Considerations for Ultra
High CMR Performance
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
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
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
recommended application circuit
(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.
Another cause of CMR failure for
a shielded optocoupler is direct
coupling to the optocoupler
output pins through C_LEDO1 and
C_LEDO2 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 (C_L).
Techniques to keep the LED in
the proper state and minimize the
effect of the direct coupling are
discussed in the next two
sections.
CMR with the LED On
(CMR_L )
A high CMR LED drive circuit
must keep the LED on during
common mode transients. This is
achieved by overdriving the LED
current beyond the input
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 I_TH of 5.0 mA (see
Figure 5) to achieve 15 kV/µs
CMR. Capacitive coupling is
higher when the internal load
resistor is used (due to C_LEDO2)
and an I_F = 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
(I_F) is reduced from its DC value by
an amount equal to the current that
flows through C_LEDP and C_LEDO1.
The situation is made worse
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