AS8846 datasheet(28/35 Pages) AMSCO

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Data Sheet AS8446

Revision 1.8, Date 2005-03-17

Page 28 of 35

a) Motor current measurement (normal current amplification channel: ACMV_normal , programmed by AV_RANGE_norm (Register

R8<1:0>) and AV_TRIMM (Register R7<4:0>)

The nominal measurement differential voltage Vmeas_nom (corresponding to 100% of the nominal motor current)

Vmeas_nom = (VCMp – VCMn)nom = Imotor_nom * Rmeas must be fixed by the value of Rmeas . So the value of this measurement resistor is

given by Rmeas = (VCMp – VCMn)nom / Imotor_nom .

The voltage amplification of this motor current measurement unit AVmeas_normal = VA_current / (VCMp – VCMn) is programmable in the

range AV_RANGE_norm * AV_TRIMM = (1, 2, 3, 4) * (70, 71, … 89, 90)

Examples:

AV_RANGE_norm = 1:

ACMV_normal = 70, 71, 72, … , 80, … , 89, 90

AV_RANGE_norm = 4:

ACMV_normal = 280, 284, 288, …, 320, … , 360

(The programming values of AV_RANGE_norm >1 are intended for a measurement of low currents with a higher digital

resolution, but in this cases the tolerances of the amplification factor is high due to the remaining offset of the measurement

amplifier.)

This way the analog output value VA_current of the current measurement unit can be adapted to the external measurement resistor

Rmeas (trimming of this resistor in the range + 12.5%).

The nominal (respectively 100%) voltage of the measurement unit VA_current (this is the nominal input voltage of the ADC on the

µP) must be 4.0 V.

b) Over current detection and protection (over current amplification channel ACMV_over)

The over current amplification channel uses the same input voltage Vmeas = (VCMp – VCMn) as the normal current amplification

channel.

The voltage amplification of this over current measurement channel ACMV_over = VA_overc / (VCMp – VCMn) is programmable by

AV_RANGE_over (Register R10<1:0>) and AV_TRIMM (Register R7<4:0>)

in the range

ACMV_over = AV_RANGE_over * AV_TRIMM = (1/4, 2/4, 3/4, 4/4) * (70, 71, … 89, 90)

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