Datasheet

ADM3052 Data Sheet
Rev. A | Page 16 of 20
THERMAL SHUTDOWN
The ADM3052 contains thermal shutdown circuitry that protects
the part from excessive power dissipation during fault conditions.
Shorting the driver outputs to a low impedance source can result
in high driver currents. The thermal sensing circuitry detects the
increase in die temperature under this condition and disables the
driver outputs. This circuitry is designed to disable the driver
outputs when a junction temperature of 150°C is reached. As
the device cools, the drivers reenable at a temperature of 140°C.
LINEAR REGULATOR
The linear regulator takes the V
+
bus power (ranging between
11 V to 25 V) and regulates this voltage to 5 V to provide power
to the internal bus-side circuitry (iCoupler isolation, V
+SENSE
, and
transceiver circuits). The linear regulator uses two regulation
loops to share the power dissipation between the internal die and
an external resistor. This reduces the internal heat dissipation in
the package. The 300 Ω external resistor should be capable of
dissipating 750 mW of power and have a tolerance of 1%.
MAGNETIC FIELD IMMUNITY
The limitation on the magnetic field immunity of the iCoupler
is set by the condition in which an induced voltage in the receiving
coil of the transformer is large enough to either falsely set or
reset the decoder. The following analysis defines the conditions
under which this may occur. The 3 V operating condition of the
ADM3052 is examined because it represents the most susceptible
mode of operation.
The pulses at the transformer output have an amplitude greater
than 1 V. The decoder has a sensing threshold of about 0.5 V,
thus establishing a 0.5 V margin in which induced voltages can
be tolerated.
The voltage induced across the receiving coil is given by
∑
π






β−
=
2
n
r
dt
d
V
;
Nn ,...,2,1=
where:
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
n
is the radius of the n
th
turn in the receiving coil (cm).
Given the geometry of the receiving coil 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 can be determined using Figure 31.
MAGNETIC FIELD FREQUENCY (Hz)
1k 10k 100k 100M1M 10M
100
10
1
0.1
0.01
0.001
MAXIMUM ALLOWABLE MAGNETIC
FLUX DENSITY (kGAUSS)
09292-012
Figure 31. 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 is about 50% of the
sensing threshold and does not cause a faulty output transition.
Similarly, if such an event occurs during a transmitted pulse and
is the worst-case polarity, it reduces the received pulse from
>1.0 V to 0.75 V, still well above the 0.5 V sensing threshold of
the decoder.
Figure 32 shows the magnetic flux density values in terms of
more familiar quantities, such as maximum allowable current
flow at given distances away from the ADM3052 transformers.
MAGNETIC FIELD FREQUENCY (Hz)
1k 10k 100k 100M1M 10M
DISTANCE = 1m
DISTANCE = 100mm
DISTANCE = 5mm
1000
100
0.1
1
10
0.01
MAXIMUM ALLOWABLE CURRENT (kA)
09292-013
Figure 32. Maximum Allowable Current for
Various Current-to-ADM3052 Spacings
With combinations of strong magnetic field and high frequency,
any loops formed by PCB traces can induce error voltages large
enough to trigger the thresholds of succeeding circuitry. Care
should be taken in the layout of such traces to avoid this possibility.