L9951XP datasheet(20/36 Pages) STMICROELECTRONICS

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Application information

L9951 / L9951XP

20/36

Doc ID 14173 Rev 8

3.5

Over-voltage and under-voltage detection

If the power supply voltage VS rises above the over-voltage threshold VSOV OFF (typical

21V), the outputs OUT1 to OUT5 are switched to high impedance state to protect the load

and the internal charge-pump is turned-off. When the voltage VS drops below the

undervoltage threshold VSUV OFF (UV-switch-OFF voltage), the output stages are switched

to the high impedance to avoid the operation of the power devices without sufficient gate

driving voltage (increased power dissipation). If the supply voltage VS recovers to normal

operating voltage the output stages return to the programmed state (input register 0: bit

12=0). If the undervoltage / overvoltage recovery disable bit is set, the automatic turn-on of

the drivers is deactivated. The microcontroller needs to clear the status bits to reactivate the

drivers.

3.6

Temperature warning and thermal shutdown

If junction temperature rises above Tj TW a temperature warning flag is set and is detectable

via the SPI. If junction temperature increases above the second threshold Tj SD, the thermal

shutdown bit will be set and power DMOS transistors of all output stages are switched off to

protect the device. In order to reactivate the output stages the junction temperature must

decrease below TjSD - TjSD HYS and the thermal shutdown bit has to be cleared by the

microcontroller.

3.7

Open-load detection

The open-load detection monitors the load current in each activated output stage. If the load

current is below the open-load detection threshold for at least 1 ms (tdOL) the corresponding

open-load bit is set in the status register. Due to mechanical/electrical inertia of typical loads

a short activation of the outputs (e.g. 3ms) can be used to test the open-load status without

changing the mechanical/electrical state of the loads.

3.8

Over load detection

In case of an over-current condition a flag is set in the status register in the same way as

open-load detection. If the over-current signal is valid for at least tISC=32µs, the over-current

flag is set and the corresponding driver is switched off to reduce the power dissipation and

to protect the integrated circuit. If the over-current recovery bit of the output is zero the

microcontroller has to clear the status bits to reactivate the corresponding driver.

3.9

Current monitor

The current monitor output sources a current image at the current monitor output which has

a fixed ratio (1/10000) of the instantaneous current of the selected high side driver. The bits

9, 10 and 11 of the input data register 0 control which of the outputs OUT1 to OUT5 will be

multiplexed to the current monitor output. The current monitor output allows a more precise

analysis of the actual state of the load rather than the detection of an open- or overload

condition. For example this can be used to detect the motor state (starting, free-running,

stalled). Moreover, it is possible to regulate the power of the defroster more precise by

measuring the monitor current.

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