Data Sheet ADuM3224/ADuM4224
Rev. B | Page 17 of 19
POWER CONSUMPTION
The supply current at a given channel of the ADuM3224/
ADuM4224 isolator is a function of the supply voltage,
channel data rate, and channel output load.
During the driving of a MOSFET gate, the driver must dissipate
power. This power is not insignificant and can lead to thermal
shutdown (TSD) if considerations are not made. The gate of a
MOSFET can be simulated approximately as a capacitive load.
Due to Miller capacitance and other nonlinearities, it is common
practice to take the stated input capacitance, CISS, of a given
MOSFET and multiply it by a factor of 5 to arrive at a conservative
estimate to approximate the load being driven. With this value,
the estimated total power dissipation per channel due to
switching action is given by
PDISS = CEST × (VDDx)2 × fS
where:
CEST = CISS × 5.
fS is the switching frequency.
Alternately, use the gate charge to obtain a more precise value
for PDISS.
PDISS = QGATE × VDDx × fS
where:
QGAT E is the gate charge for the MOSFET.
fS is the switching frequency.
This power dissipation is shared between the internal on
resistances of the internal gate driver switches and the external
gate resistances, RGON and RGOFF. The ratio of the internal gate
resistances to the total series resistance allows the calculation of
losses seen within the ADuM3224/ADuM4224 chips per channel.
PDISS_IC = PDISS × ½ × (RDSON_P/(REXT_X + RDSON_P) +
RDSON_N/(REXT_X + RDSON_N))
Taking the power dissipation found inside the chip and
multiplying it by θJA gives the rise above ambient temperature
that the ADuM3224/ADuM4224 experiences, multiplied by two
to reflect that there are two channels.
TJ = θJA × 2 × PDISS_IC + TAMB
For the device to remain within specification, TJ must not
exceed 125°C. If TJ exceeds 150°C (typical), the device enters TSD.
Quiescent power dissipation may also be added to give a more
accurate number for temperature rise, but the switching power
losses are often the largest source of power dissipation, and
quiescent losses can often be ignored. To calculate the total
supply current, the quiescent supply currents for each input and
output channel corresponding to IDD1(Q), IDDA(Q), and IDDB(Q) are
added. The full equation for the TJ becomes
TJ = θJA × (2 × PDISS_IC + VDD1 × IDD1(Q) + VDDA × IDDA(Q) +
VDDB × IDDB(Q)) + TAMB
Figure 9 provides total input IDD1 supply current as a function of
data rate for both input channels. Figure 10 provides total IDDA
or IDDB supply current as a function of data rate for both outputs
loaded with 2 nF capacitance.
INSULATION LIFETIME
All insulation structures eventually break down when subjected to
voltage stress over a sufficiently long period. The rate of insulation
degradation is dependent on the characteristics of the voltage
waveform applied across the insulation. In addition to the testing
performed by the regulatory agencies, Analog Devices carries out
an extensive set of evaluations to determine the lifetime of the
insulation structure within the ADuM3224/ADuM4224.
Analog Devices performs accelerated life testing using voltage
levels higher than the rated continuous working voltage. Accel-
eration factors for several operating conditions are determined.
These factors allow calculation of the time to failure at the actual
working voltage.
The values shown in Table 12 summarize the peak voltage for
50 years of service life for a bipolar ac operating condition and
the maximum CSA/VDE approved working voltages. In many
cases, the approved working voltage is higher than the 50-year
service life voltage. Operation at these high working voltages
can lead to shortened insulation life in some cases.
The insulation lifetime of the ADuM3224/ADuM4224 depends
on the voltage waveform type imposed across the isolation barrier.
The iCoupler insulation structure degrades at different rates
depending on whether the waveform is bipolar ac, unipolar ac,
or dc. Figure 25, Figure 26, and Figure 27 illustrate these different
isolation voltage waveforms.
A bipolar ac voltage environment is the worst case for the
iCoupler products and is the 50-year operating lifetime that
Analog Devices recommends for maximum working voltage. In
the case of unipolar ac or dc voltage, the stress on the insulation
is significantly lower. This allows operation at higher working
voltages while still achieving a 50-year service life. Treat any
cross insulation voltage waveform that does not conform to
Figure 26 or Figure 27 as a bipolar ac waveform, and limit its
peak voltage to the 50-year lifetime voltage value listed in Table 12.
Note that the voltage presented in Figure 26 is shown as sinusoidal
for illustration purposes only. It is meant to represent any voltage
waveform varying between 0 V and some limiting value. The limiting
value can be positive or negative, but the voltage cannot cross 0 V.
Figure 25. Bipolar AC Waveform
Figure 26. Unipolar AC Waveform
Figure 27. DC Waveform
0V
RATED PEAK VOLTAGE
11791-009
0V
RATED PEAK VOLTAGE
11791-010
0V
RATED PEAK VOLTAGE
11791-011