ST7MC2 datasheet(177/308 Pages) STMICROELECTRONICS

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ST7MC1/ST7MC2

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MOTOR CONTROLLER (Cont’d)

The Figure 98 gives the step ratio register value

(left axis) and the number of BEMF sampling dur-

ing one electrical step with the corresponding ac-

curacy on the measure (right axis) as a function of

the mechanical frequency.

For a given prescaler value (step ratio register) the

mechanical frequency can vary between two fixed

values shown on the graph as the segment ends.

In autoswitched mode, this register is automatical-

ly incremented/decremented when the step fre-

quency goes out of this segment.

At fMTC=4MHz, the range covered by the Step Ra-

tio mechanism goes from 2.39 to 235000 (pole

pair x rpm) with a minimum accuracy of 1.2% on

the step period.

To read the number of samples for Zn within one

step (right Y axis), select the mechanical frequen-

cy on the X axis and the sampling frequency curve

used for BEMF detection (PWM frequency or

measurement window frequency). For example,

for N.Frpm = 15,000 and a sampling frequency of

15kHz, there are approximately 10 samples in one

step and there is a 10% error rate on the measure-

ment.

Figure 98. Step Ratio Bits decoding and accuracy results and BEMF Sampling Rate

Step Ratio (Decimal)

N.Frpm

avg Zn ~ 7Fh ± 0.6%

100%

BEMF

samples

∆Zn/Zn

10%

50%

200 Hz

15 kHz

avg Zn ~ FFh ± 0.4%

avg Zn ~ 7Fh ± 0.6%

avg Zn ~ FFh ± 0.4%

avg Zn ~ 55h ± 1.2%

3.Fn

Fn+1 = 2.Fn

3.Fn+1 = 6.Fn

Fstep = 6.N.Frpm = N.F / 10 ⇔ N.F = 10.Fstep

N: Pole pair number

Fstep: Electrical step frequency

ST[3:0]

avg Zn ~ 55h ± 1.2%

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