IC-HD2 Datasheet, PDF (6/8 Pages) IC-Haus GmbH – QUAD DIFFERENTIAL LINE DRIVER

iC-HD2
QUAD DIFFERENTIAL LINE DRIVER
APPLICATION NOTE
Rev A5, Page 6/8
Reverse polarity and circuit protection
For reverse polarity protection electronic circuitry are
usually powered via a diode D in the supply line. Un-
der normal operating conditions, this diode will not af-
fect function of the circuitry when the additional forward
voltage drop across the diode is accounted for operat-
ing voltage specification.
If the supply voltage V_supply is suddenly reversed, a
load capacitor C may be still fully charged. Therefore,
the diode D has to be selected to withstand a voltage
difference of at least twice the maximum supply volt-
age.
Since the reverse polarity protection diode D prevents
discharging of the load capacitor C, especially at low
power consumption injected charge through distur-
bances may in general result in capacitor voltage ex-
ceeding maximum ratings, leading to malfunction or
destruction of circuitry and associated parts. Thus
EMC requirements will afford more external circuitry
due to the introduction of a reverse polarity diode.
Figure 3 shows the iC-HD2 with the diode D for reverse
polarity protection and additional protective devices TS
and ZD.
Figure 3: Circuit schematic showing protective devices
D: reverse polarity protective diode; TS: bidirectional suppressor diode;
ZD: supply voltage limiting zener diode
For over-voltage protection, the suppressor diode TS
absorbs transients on supply line injected externally
on the cable. Clamp voltage of the diode TS should
be rated slightly above maximum specified supply volt-
age.
Due to capacitive crosstalk between the wires in the
cable of the supply line, additional currents may be in-
jected into the circuitry during transients via the driver
pins of iC-HD2 connected directly to the cable. These
currents can be passed to ground or to VB by the in-
ternal ESD diodes of the iC-HD2. Whereas negative
current injection will simply be drained off to ground,
positive current injection will charge capacitor C fur-
ther to higher voltages.
By introducing an additional Zener diode ZD in parallel
to capacitor C, excessive charge can be drained off,
thus limiting circuitry supply voltage to a safe value, as
shown in fig. 4.
Suggested protective devices
As stated above, diode D must withstand at least twice
the maximum operating voltage. Assuming VB_max
specified to be 30V, reverse voltage V_R,D of the diode
D then should be at least 60 V. Current rating de-
pends on total power consumption of the circuitry, but
is usually below 1 amps. Therefore, typical 1 amps
rated rectifier diodes like 1N4002 (with V_R,D = 100 V)
through 1N4007 (with V_R,D = 1000 V) or equivalent
types (BA157 through BA159) can be used. At VB_max
of 30V, neither the suppressor diode TS nor the Zener