L99ASC03 datasheet(23/73 Pages) STMICROELECTRONICS

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L99ASC03

Device description

In order to allow the SHx pins to go below GND, the current sink has a diode in series and

the sink current will disappear below 0.8V. Therefore, when using the test currents, the

drain-source voltage threshold should be programmed to a value greater than 0.8V.

2.13

Current-sense amplifier

The current-sense amplifier (CSA) is designed for low-side current measurement in

automotive motor control applications. The CSA differential input stage measures the

voltage generated by the motor current over an external shunt resistor. The input common-

mode range allows the CSA input pins to go below GND, as typically required in PWM motor

control applications due to switching transients. The CSA gain can be programmed over a

wide range by setting the GCSA bits.

In case of zero differential input voltage, the output voltage is at half scale:

VCSO = 0.5 * VDD

2.14

Overcurrent detection

To protect the application from overcurrent, an overcurrent threshold can be programmed

via SPI by setting the OCTH bits. The CSA output is compared to the programmed

threshold. In case of overcurrent, the CSAOC flag is set and, depending on the DMUX bit,

the DOUT output goes high. In addition, if the OCSHUTD bit is set, the gate drivers are

disabled.

The overcurrent detection feature can be used to estimate the rotor position of the motor at

standstill without any rotation by applying voltage to the motor windings and detecting

overcurrent with respect to an appropriate threshold.

2.15

BEMF module

The programmable BEMF (back electromotive force) module integrated in the device

provides a flexible means to support those applications where the BLDC motor is driven in

sensorless mode and that are based on BEMF detection.

2.15.1

BEMF comparator

Depending on the PWM driving method used in the application, three different comparators

can be selected through the BEMFMOD bits to detect the BEMF zero-crossing point. BEMF

detection can be done during the PWM ON state or the PWM OFF state. In the former case,

the VSMS/2 comparator (i.e. internally referenced to half of the VSMS supply) can be used. In

the latter case, the GND comparator (i.e. internally referenced to GND) or the VSMS

comparator (i.e. internally referenced to the VSMS supply) can be used, depending on

whether the PWM signal is applied to the external high-side or low-side MOSFET (this

reflects the setting of the BEMFSW bit).

As some applications may require advancing the timing of a phase commutation (“pre-

commutation”), it is possible to add an offset to the internal reference voltage of the VSMS/2

comparator. The absolute offset value can be programmed through the BEMFOS bits. To

achieve pre-commutation, the offset sign (i.e. positive or negative) has to vary, depending

on whether the BEMF is rising or falling. The offset sign can be selected via SPI by

programming the BEMFSIGN bit.

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