UM1556 datasheet(49/87 Pages) STMICROELECTRONICS

49 / 87 page

UM1556

Switching inductive loads

Doc ID 023520 Rev 2

49/87

The HSD channel is protected directly by the parallel transil. The clamping voltage of the

transil should be below the minimum clamping voltage of the HSD (41 V). Such transils

(VCL < 41 V) usually start conducting at 30 V, therefore there is a high probability that it is

damaged during the clamped load dump pulse (36 V). For that reason it is usually better to

use the previously described solution (1).

3) Freewheeling diode and reverse battery diode

A general purpose diode connected in parallel with the load provides a conductive path for

proper demagnetization. Relatively small demagnetization voltage (1 diode voltage drop in

forward direction) leads to a very slow demagnetization. This can have a negative influence

when driving the relay, for example. A slow movement of the armature (slow opening of

contacts) can have a negative influence on the lifetime of the relay contacts (depending on

the switching current).

In order to protect the freewheeling diode against the reverse battery condition, there is an

additional diode (Dr) required in series with the output. This circuitry is suitable only for small

loads because of permanent voltage drop (

→ power dissipation) on Dr during normal

operation.

4) Freewheeling diode and reverse battery P-channel MOS circuitry

Vbat

GND

OUT

Rgnd

Dgnd

GND

Clamp

Vout

Iout