AD5100
Rev. A | Page 22 of 36
PROTECTION
For automotive applications, proper external protections on the
AD5100 are needed to ensure reliable operation. The V
1MON
is
likely to be used for battery monitoring. The V
2MON
is likely to
be used for ignition switch or other critical inputs. As a result,
these inputs may need additional protections such as EMI, load
dump, and ESD protections. In addition, battery input requires
reverse battery protection and short-circuit fuse protection (see
Figure 19).
Overcurrent Protection
If the V
1MON
is shorted internally in the AD5100 to GND, the
short-circuit protection kicks in and limits subsequent current
to 150 mA in normal operation.
Thermal Shutdown
When the AD5100 junction temperature is near the junction
temperature limit, it automatically shuts down and cuts out the
power from V
1MON
. The part resumes operation when the device
junction temperature returns to normal.
ESD Protection
It is common to require a contact rating of ±8 kV and a no
contact or air rating of ±15 kV ESD protection for the
automotive electronics. As a result, an ESD-rated protection
device must be used, such as MMBZ27VCL, a dual 40 W
transient voltage suppressor (TVS) at the V
1MON
and V
2MON
.
Load Dump Protection
A load dump is a severe overvoltage surge that occurs when the
car battery is being disconnected from a spinning alternator and
a resulting long duration, high voltage surge is introduced into
the supply line. Therefore, external load dump protection is
recommended. Typically, the load dump overvoltage lasts for a
few hundred milliseconds and peaks at around 40 V to 70 V,
while current can be as high as 1 A. As a result, a load dump-
rated TVS D1 and D2, such as SMCJ17, are used to handle the
surge energy. A series resistor is an inline current limiting
resistor; it should be adequate to limit the current without
significant drop and yet small enough to not affect the input
monitoring accuracy.
Reverse Battery Protection
Reverse battery protection can be provided by a regular diode
if the battery monitoring accuracy can be relaxed. Otherwise,
a 60 V P-channel power MOSFET, like the NDT2955, can be
used. Because of the MOSFET internal diode, the battery first
conducts through the P1 body diode as soon as the voltage reaches
its source terminal. The voltage divider provides adequate gate-
to-source voltage to turn on P1, and the voltage drop across the
FET is negligible. The resistor divider values are chosen such
that the maximum V
GS
of the P1 is not violated and the current
drawn through the battery is only a few microamps.
EMI Protection
For EMI protection, a ferrite bead or EMC rated inductor, such
as DR331-7-103, can be used.
05692-020
DIGIPOT
DIGIPOT
V
2MON
V
1MON
D4
SMCJ17 MMBZ27VCL
D3
D2
SMCJ17
D1
R4
2.2Ω
R3
2.2Ω
VREF
V
REG
EN
R1
2MΩ
R2
1.5MΩ
C1
0.1µF
NDT2955
L1
10µH
DR331-7-103
F1
C2
0.1µF
C3
10µF
IGNITION SWITCH
L1
VMAIN
+–
B+
P1
AD5100
Figure 19. Protection Circuits
