AND8116/D

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2

Standard Discrete RELAY DRIVERS

For both type of applications industrial and automotive,

the most traditional and popular relay drivers are the ones

formed discretely with a bipolar transistor, two bias resistors

and a free−wheeling diode. In some cases, it is required to

add extra components such as MOVs (metal oxide varistors)

and extra diodes to ensure proper protection. Figure 1 shows

a typical discrete relay driver with the extra protection

devices. Diode D1 provides reverse supply protection and

diode D2 provides a clamp function to suppress the voltage

spike generated by the relay’s coil during the turn−off

interactions (V = Ldi/dt). A power MOV device is used to

limit positive transients to within the bipolar transistor’s

breakdown voltage. The saturation voltage of the bipolar

transistor (typically over 1.0 V) causes high power

dissipation which in some cases eliminates the option to use

inexpensive surface mount package devices such as

SOT−23 or smaller, therefore the need for bigger packages

such as TO220 is always present. The resulting discrete

circuit is expensive because it takes several components and

a big space in the PC board.

Figure 1. Typical Discrete Relay Driver

+12 V

VARISTOR

0

RELAY

D1

Q1

R2

R1

+5 V/3.3 V

D2

LOGIC

ON Semiconductor’s RELAY DRIVERS

The ON Semiconductor’s relay drivers portfolio is

divided in two main categories:

Industrial Version

Figure 2 describes the industrial relay driver version

(devices NUD3105, NUD3112). This device integrates

several discrete components in a single SOT−23 three

leaded surface mount package to achieve a simpler and more

efficient solution than the conventional discrete relay

drivers. The characteristics of the integrated devices are

listed below:

between gate and source)

relay’s coils, and 14 V for 12 V coils)

Figure 2. Industrial Relay Driver Description

(NUD3105 and NUD3112 Devices)

Drain (3)

1.0 k

300 k

Gate (1)

Source (2)

ESD

Zener

7 V

ESD

Zener

7 V

Clamp Zener

7 V or 14 V

Clamp Zener

7 V or 14 V