ADUM3123 Datasheet, PDF (13/15 Pages) Analog Devices – Isolated Precision Gate Driver, 4.0 A Output

Data Sheet
The following equation defines the Q factor of the RLC circuit,
which indicates how the ADuM3123 output responds to a step
change. For a well damped output, Q is less than one. Adding a
series gate resistance dampens the output response.
Q=
1
× ^LTRACE
(R_SW + R_GATE )
C _GS
To reduce output ringing, add a series gate resistance to dampen
the response. For applications using a load of 1 nF or less, add a
series gate resistor of about 5 Ω. It is recommended that the Q
factor be below 1, which results in a damped system, with a
value of 0.7 as the recommended target.
DC CORRECTNESS AND MAGNETIC FIELD
IMMUNITY
Positive and negative logic transitions at the isolator input cause
narrow (~1 ns) pulses to be sent to the decoder via the transformer.
The decoder is bistable and is, therefore, either set or reset by
the pulses, indicating input logic transitions. In the absence of
logic transitions of more than 1 µs (typical) at the input, a periodic
set of refresh pulses indicative of the correct input state are sent
to ensure dc correctness at the output.
If the decoder receives no internal pulses for more than about
3 µs (typical), the input side is assumed to be unpowered or
nonfunctional, in which case, the isolator output is forced to a
default low state by the watchdog timer circuit. In addition, the
outputs are in a low default state while the power is coming up
before the UVLO threshold is crossed.
The limitation on the ADuM3123 magnetic field immunity is
set by the condition in which induced voltage in the
transformer receiving coil is sufficiently large to either falsely
set or reset the decoder. The following analysis defines the
conditions under which this can occur. The 3 V operating
condition of the ADuM3123 is examined because it represents
the most susceptible mode of operation. The pulses at the
transformer output have an amplitude greater than 1.0 V. The
decoder has a sensing threshold at about 0.5 V, therefore
establishing a 0.5 V margin in which induced voltages can be
tolerated. The voltage induced across the receiving coil is given by
V = (−dβ/dt) ∑π rn², n = 1, 2, ... , N
where:
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
rn is the radius of the nth turn in the receiving coil (cm).
ADuM3123
Given the geometry of the receiving coil in the ADuM3123 and
an imposed requirement that the induced voltage is at most
50% of the 0.5 V margin at the decoder, a maximum allowable
magnetic field is calculated, as shown in Figure 19.
100
10
1
0.1
0.01
0.001
1k
10k
100k
1M
10M
MAGNETIC FIELD FREQUENCY (Hz)
100M
Figure 19. Maximum Allowable External Magnetic Flux Density
For example, at a magnetic field frequency of 1 MHz, the
maximum allowable magnetic field of 0.2 kgauss induces a
voltage of 0.25 V at the receiving coil. This induced voltage level
is about 50% of the sensing threshold and does not cause a faulty
output transition. Similarly, if such an event were to occur
during a transmitted pulse (and had the worst-case polarity),
the received pulse is reduced from >1.0 V to 0.75 V, still well
above the 0.5 V sensing threshold of the decoder.
The preceding magnetic flux density values correspond to
specific current magnitudes at given distances away from the
ADuM3123 transformers. Figure 20 expresses these allowable
current magnitudes as a function of frequency for selected
distances. As shown, the ADuM3123 is immune and only
affected by extremely large currents operated at a high frequency
and near to the component. For the 1 MHz example, place a
0.5 kA current 5 mm away from the ADuM3123 to affect the
operation of the component.
1000
DISTANCE = 1m
100
10
DISTANCE = 100mm
1
DISTANCE = 5mm
0.1
0.01
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 20. Maximum Allowable Current for Various Current to ADuM3123
Spacings
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