AN2208 datasheet(18/51 Pages) STMICROELECTRONICS

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4 Load switching tests

AN2208

18/51

Load switching tests

Many different types of loads can be found in an industrial environment. Typical loads have

inductive or resistive characteristics. Applications compliant with UL 508 (48

Ω and 1.15H)

specifications are generally considered as the worst case.

A basic description of typical switching inductor loads is given in Figure 16. The supply voltage

is nominally 24V but can rise up to 30.5V. In this application, 24V filament lamps are used with

130mH/48

Ω inductors as the loads. The V

CC supply condition is between 18.5V and 28.5V DC.

The VCLAMP voltage value decides the tOFF demagnetization duration: the faster you want to

switch off the circuit, the bigger | VCLAMP| compared with | VCC| has to be.

Note:

Typical VCLAMP value for VN808 is 52V.

STMicroelectronics’ Intelligent Power Switches (IPSs) provide a “fast demagnetization” output

structure, an integrated solution for fast switch-off of inductive loads.

IPSs are basically a Zener diode with a 52V breakdown (approx.) and high power dissipation

capability connected between an output and VCC as shown in Figure 17. In most applications,

the output voltage is then clamped at VOUT = VCC - 52, and is therefore dependent on the

supply voltage.

The integrated clamping structure saves on components and space. Internal demagnetization

can be used only if thermal behavior and load conditions are well known to designers.

Therefore a detailed analysis of thermal behavior related to inductive load switching is

mandatory to prevent improper utilization of the IPSs.

Figure 16. Description of the switching inductor loads

VINx

Load x

VN808

VINx

Load x

VINx

Load x

VINx

OUT

CLAMP

OUT

OFF

Ai11608

AN2208 Datasheet(PDF) 18 Page - STMicroelectronics