AS8846 datasheet(15/35 Pages) AMSCO

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Data Sheet AS8446

Revision 1.8, Date 2005-03-17

Page 15 of 35

Remark 3: The external current measurement voltage over the measurement resistor Rmeas

(VCMP - VCMN) = Imotor * Rmeas where the internal over current detection is switched on can be

determined by: (VCMP - VCMN)overc * AV_TRIMM * AV_RANGE_over = OVERC_level

(VCMP - VCMN)overc = OVERC_level / (AV_TRIMM * AV_RANGE_over)

example: OVERC_level = 4 V, AV_TRIMM = 80, AV_RANGE_over = 1/4

à (VCMP - VCMN)overc = 200 mV

Remark 4: Example of the programming of the current measurement / over current detection unit:

Step1: Definition of the nominal voltage drop over the external current measurement resistor Rmeas

(VCMP - VCMN)nom = Imotornom * Rmeas e.g. (VCMP - VCMN)nom = 50 A * 1 m

Ω = 50 mV

Step2: Definition of the over current threshold value of (VCMP - VCMN) e.g. (VCMP - VCMN)overc = 100 mV

Step3: Determination of the voltage amplification of the normal current measurement channel so that the nominal

output voltage VA_current_nom = 4.0 V

e.g. AV_TRIMM * AV_RANGE = 80 * 1

à VA_current-nom = 50 mV * 80 = 4.0 V

Step4: Determination of the voltage amplification of the over current measurement

e.g. AV_TRIMM * AV_RANGE_over = 80 * 1/4

à VA_overc_max = 100mV * 20 = 2.0 V < 4.0 V

Step5: Determination of the programming of the internal over current threshold value (OVERC_level)

With the formula: (VCMP - VCMN)overc = OVERC_level / (AV_TRIMM * AV_RANGE_over) and all values

above:

OVERC_level = (VCMP - VCMN)overc * (AV_TRIMM * AV_RANGE_over) = 100 mV * 80 *1/4 = 2.0 V

à R9 = 4

Remark 5: The amplifications AVmeas_norm and AVmeas_overc of the current measurement unit can be adapted to the

external measurement resistor Rmeas by the programming word AV_TRIMM in steps of 1.25 % in the range

of + 12.5% (trimming of Rmeas).

Table 13

Programming of the motor current measurement an over current detection unit

AS8846 Datasheet(PDF) 15 Page - ams AG