IC-HD7_14 Datasheet, PDF(6/8 Page) IC-Haus GmbH – QUAD DIFFERENTIAL LINE DRIVER

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IC-HD7_14 Datasheet, PDF (6/8 Pages) IC-Haus GmbH – QUAD DIFFERENTIAL LINE DRIVER

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iC-HD7

QUAD DIFFERENTIAL LINE DRIVER

APPLICATION NOTE

Rev A6, Page 6/8

Reverse polarity and circuit protection

For reverse polarity protection electronic circuitries 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.

If the supply voltage Vsupply 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 result in capacitor voltage exceeding max-

imum ratings, leading to malfunction or damage of the

iC-HD7 and associated parts. Therefore EMC protec-

tion will require additional circuitry when a reverse po-

larity diode is used.

Figure 3 shows the iC-HD7 with the diode D for reverse

polarity protection and additional protective devices TS

and ZD.

to remaining circuitry

iC-HD7 VB

internal

ESD diodes

Vsupply (24V)

capacitive crosstalk

NO1

LINE

GND

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 the supply line, injected ex-

ternally into the cable. Clamp voltage of the diode

TS should be rated slightly above maximum speciï¬ed

supply voltage. To protect the driver outputs against

fast transients, additional parallel capacitors Cp in the

range of about 1 nF are recommended and should be

connected from each output to ground. Further over-

voltage protection elements like damping chokes, tran-

sient suppressor diodes or varistors may be needed to

comply with application speciï¬c EMC directives.

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-HD7 connected directly to the cable. These

currents can be passed to ground or to VB by the in-

ternal ESD diodes of the iC-HD7. 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 ï¬g. 4.

Suggested protective devices

As stated above, diode D must withstand at least twice

the maximum operating voltage. Assuming VBmax

speciï¬ed to be 30V, reverse voltage VR,D of the diode

D then should be at least 60 V. Current rating de-

pends on total power consumption of the circuitry, but

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