L99DZ120 datasheet(83/162 Pages) STMICROELECTRONICS

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L99DZ120

Application information

161

4.22

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) can be driven by using the programmable soft-start function (i.e.

overcurrent recovery mode). Each driver has a corresponding overcurrent recovery bit

OUTx_OCR (CR 7). If this bit is set, the device automatically switches the outputs on again

after a programmable recovery time. The PWM modulated current will provide sufficient

average current to power up the load (e.g. heat up the bulb) until the load reaches operating

condition.

The PWM frequency is defined by setting OCR_FREQ (CR7).

The device itself cannot distinguish between a real overload (e.g. short-circuit condition) and

a load characterized by operation currents exceeding the short-circuit threshold.

Examples are non-linear loads like a light bulb used on the HS outputs or a motor used on

the half bridge output with inrush and stall currents that shall be limited by the auto recovery

feature.

For the bulb, a real overload condition can only be qualified by time. For overload detection

the microcontroller can switch on the light bulbs by setting the overcurrent recovery bit for

the first e.g. 50 ms. After clearing the recovery bit, the output will be switched off

automatically if the overload condition remains.

For the half bridges the high current can be present during all motor activation and another

SW strategy must be applied to identify a SC to GND or Supply. Before running the motor

e.g. with a first SPI command all bridges LS are switched on (without auto recovery

functionality / cleared overcurrent recovery bit), all HS are switched off and a SC to Battery

can be diagnosed. With a next SPI command, all HS are switched on (without auto recovery

functionality/ cleared overcurrent recovery bit) and all LS are switched off. In this sequence,

a short to GND can be diagnosed. If in both sequences no overload condition is identified,

the motor can be run by switching on the relevant HS and LS each configured in auto

recovery mode (see Figure 35). Such sequence can be applied before any motor activation

to identify SC just before operating the motor (in case the delay due to the 2 additional SPI

commands is not limiting the application) or in case of power up of the system resp. applied

on a certain time base.

L99DZ120 Datasheet(PDF) 83 Page - STMicroelectronics