L9954LXP datasheet(23/35 Pages) STMICROELECTRONICS

23 / 35 page

L9954LXP

Application information

Doc ID 16186 Rev 3

23/35

3.13

Programmable soft start function to drive loads with higher

inrush current

Loads with start-up currents higher than the overcurrent limits (e.g. inrush current of lamps,

start current of motors and cold resistance of heaters) can be driven by using the

programmable soft start function (i.e. overcurrent recovery mode). Each driver has a

corresponding over-current recovery bit. If this bit is set, the device switchs automatically on

the outputs again after a programmable recovery time. The duty cycle in over-current

condition can be programmed by the SPI interface to be about 15 %...25 %. The PWM

modulated current provides sufficient average current to power up the load (e.g. heat up the

bulb) until the load reaches operating condition. The PWM frequency settles at 1.5 kHz or

3 kHz. The device itself cannot distinguish between a real overload and a non linear load

like a light bulb. A real overload condition can only be qualified by time. As an example the

microcontroller can switch on light bulbs by setting the over-current recovery bit for the first

50ms. After clearing the recovery bit the output is automatically disabled if the overload

condition still exits.

Figure 9.

Programmable soft start function for inductive loads and incandescent bulbs

Load Current

Overcurrent

detection

Unlimited Inrush Current

Limited Inrush Current in

overcurrent recovery mode

with inductive load

Load Current

Overcurrent

detection

Unlimited Inrush Current

Limited Inrush Current in

overcurrent recovery mode with

incandescent bulb

L9954LXP Datasheet(PDF) 23 Page - STMicroelectronics