UM1556 datasheet(39/87 Pages) STMICROELECTRONICS

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UM1556

Switching inductive loads

Doc ID 023520 Rev 2

39/87

Switching inductive loads

Switching inductive loads such as relays, solenoids, motors etc. can generate transient

voltages of many times the steady-state value. For example, turning off a 12 volt relay coil

can easily create a negative spike of several hundred volts. The M0-5/M0-5E high side

drivers are well designed to drive such loads, in most cases without any external protection.

Nevertheless, there are physical limits for each component that have to be respected in

order to decide whether external protection is necessary or not.

An attractive feature of the M0-5/M0-5E drivers is that a relatively high output voltage

clamping leads to a fast demagnetization of the inductive load.

The purpose of this chapter is to provide a simple guide on how to check the conditions

during demagnetization and how to select a proper HSD (and the external clamping if

necessary) according to the given load.

4.1

Turn-on phase behaviour

Figure 35.

Inductive load – HSD turn-on phase

When an HSD turns on an inductive load, the current increases with a time constant given

by L/R values, so the nominal load current is not reached immediately. This fact should be

considered in diagnostics software (i.e. to avoid false open load detection).

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