L9800 datasheet(29/55 Pages) STMICROELECTRONICS

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7.1.3

Maximum load inductance

The maximum energy, which is converted into heat, is limited by the thermal design of the component. The EAR

value provided in Features assumes that all channels can dissipate the same energy when the inductances

connected to the outputs are demagnetized at the same time.

7.2

Bulb inrush mode

Although the device is optimized for relays and LED, it may be necessary to use one or more of the outputs to

drive small lamps (typically 2 W) or electronic loads with a high input capacitor. In such operative conditions at the

switch on, an inrush current may appear, reaching the overload current threshold, which latches the channel off.

In normal operation the device waits until the microcontroller sends an SPI command to clear the latches (register

DIAG_OVC_OVT) allowing the channel to turn on again. Usually this delay is too long to transfer enough energy

to the load. If the corresponding bit BIM.OUTn is set to “1”, in case the channel reaches the overload current

threshold and latches off, it restarts automatically after a time tINRUSH, allowing the load to go out of the inrush

phase. A time diagram is shown in the Figure 22.

The channel configured with the BIM option can be either driven by an SPI command, PWM command (internal

generator) or INn parallel input. Once programmed to “1” via SPI, the BIM.OUTn bits are set back to “0” at the end

of BIM time(tBIM), unless a reset condition occurs (VDDIO(UV), hardware, or software reset): an eventual switch off

of the channel commanded by the user has no impact on BIM.OUTn configuration.

When BIM.OUTn are set to “1”, eventual overcurrent conditions flag DIAG_OVC_OVT.OUTn bit, but the affected

channels will not be permanently switched OFF (automatic restart after TINRUSH). Once asserted, the eventual

SPI attempt to clear DIAG_OVC_OVT.OUTn is ignored, until the BIM counter is expired. To resume the normal

behavior when the BIM counter is expired, the DIAG_OVC_OVT.OUTn should be cleared. If OVT condition

happens before OVC condition, the flag is set and the channel is turned off but it could be turned on again by

clearing register DIAG_OVC_OVT.OUTn

An internal timer set the bit BIM.OUTn back to “0” after 40 ms (parameter tBIM) starting from the first OVC event

latched to prevent an excessive thermal stress to the channel, especially in case of short circuit at the output.

Another countermeasure against overheating is that, in case the load current is higher than IL(OVL0) or IL(OVL1),

the overloaded channel is immediately switched OFF, without waiting for digital filter time tOFF(OVL) expiration. The

overload threshold is IL(OVL0) between first channel switch on and tOVLIN (in BIM mode tOVLIN is not reset every

time the channel is switched off by overcurrent); IL(OVL1) from tOVLIN expiration to the end of bulb inrush mode.

The device allows a per-channel selection of bulb inrush mode (BIM) in order to be fully flexible without any

additional reliability risk.

Figure 22. Bulb mode

L9800

Power stages

DS14041 - Rev 2

page 29/55

L9800 Datasheet(PDF) 29 Page - STMicroelectronics