AND8116/D

http://onsemi.com

4

Based on the relay coil specifications, the energy that is

transferred to the driver by the relay’s coils can be

theoretically calculated by using the formula E =

½ L I2,

which results in 0.331 mJ. The avalanche energy capability

of the NUD3105 and NUD3112 devices is 50 mJ, so the

0.331 mJ transferred by the OMRON relay only represents

a 0.65% of their energy capability. The same theoretical

principle (E =

½ L I2) can be used to find out the type of

relay’s coils that the NUD3105 and NUD3112 devices can

drive. For these purposes, one needs to know the inductance

and current characteristics of the relay’s coil to calculate the

energy that will be transferred. The resulting energy should

not exceed the 50 mJ at which the devices are rated.

Automotive Version

Figure 5 describes the automotive relay driver version

(device NUD3124).

This device also integrates several discrete components in

a single SOT−23 three leaded surface mount package to

achieve a simpler and even more robust solution than the

conventional discrete relay drivers. The characteristics of

the integrated devices are listed below:

gate and source)

an active clamp function.

Figure 5. Automotive Relay Driver Description

(NUD3124 Device)

10 k

100 k

ESD

Zener

14 V

ESD

Zener

14 V

Clamp Zener

28 V

Clamp Zener

28 V

Gate (1)

Drain (3)

Source (2)

ESD

Zener

14 V

ESD

Zener

14 V

The 40 V N−channel FET is designed to switch on and off

the relay’s coil for currents up to 200 mA. The clamping

protection Zener diodes (28 V) provides an active clamp

function to drain to ground the voltage spikes generated by

the relay’s coils during the turn−off interactions (V = Ldi/dt).

This function is achieved by partially activating the FET

through the clamp Zener diodes anytime the voltage across

them reaches their breakdown voltage level (28 V). The

ESD protection Zener diodes protects the gate−source

silicon junction against ESD conditions possibly induced by

persons during the handling or assembly of the device. And

the bias resistor provides the drive control signals to the FET.

Figure 6 illustrates the typical connection diagram of the

NUD3124 device.

Figure 6. Typical Connection Diagram for

Automotive Relay Driver (NUD3124 Device)

10 k

100 k

ESD

Zener

14 V

ESD

Zener

14 V

Clamp Zener

28 V

Clamp Zener

28 V

Gate (1)

+12 V (Car’s Battery)

Source (2)

ESD

Zener

14 V

ESD

Zener

14 V

RELAY

LOGIC

NUD3124

When positive logic voltage is applied to the gate of the

device (5.0 V/3.3 V), the FET is turned−on which activates

the relay. When the FET is turned−off, the relay’s coil is

deactivated which causes it to kickback and generates a high

voltage spike. This voltage spike causes the clamp Zener

diodes (28 V) to breakdown which partially activates the

FET to drain this condition to ground. This operation

sequence is repeated for all the on and off operations of the

relay driver.