ADuM7223

Data Sheet

The following equation defines the Q factor of the RLC circuit,

which indicates how the ADuM7223 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.

GS

TRACE

GATE

SW

C

L

R

R

Q

×

+

=

)

(

1

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.

Figure 17. RLC Model of the Gate of an N-Channel MOSFET

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 ADuM7223 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

ADuM7223 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

where:

β is the magnetic flux density (gauss).

N is the number of turns in the receiving coil.

Given the geometry of the receiving coil in the ADuM7223 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 18.

Figure 18. Maximum Allowable External Magnetic Flux Density

For example, at a magnetic field frequency of 1 MHz, the maxi-

mum allowable magnetic field of 0.2 kgauss induces a voltage

of 0.25 V at the receiving coil. This is about 50% of the sensing

threshold and does not cause a faulty output transition. Simi-

larly, 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 thresh-

old of the decoder.

The preceding magnetic flux density values correspond to

specific current magnitudes at given distances away from the

ADuM7223 transformers. Figure 19 expresses these allowable

current magnitudes as a function of frequency for selected

distances. As shown, the ADuM7223 is immune and only

affected by extremely large currents operated at a high frequency

and near the component. For the 1 MHz example, place a 0.5 kA

current 5 mm away from the ADuM7223 to affect the operation

of the component.

Figure 19. Maximum Allowable Current for Various

Current to ADuM7223 Spacings

ADuM7223

MAGNETIC FIELD FREQUENCY (Hz)

100

0.001

1M

10

0.01

1k

10k

10M

0.1

1

100M

100k

MAGNETIC FIELD FREQUENCY (Hz)

1000

100

10

1

0.1

0.01

1k

10k

100M

100k

1M

10M

DISTANCE = 5mm

DISTANCE = 1m

DISTANCE = 100mm

Rev. A | Page 12 of 16