ST7FMC1K2B3 STMICROELECTRONICS [STMicroelectronics], ST7FMC1K2B3 Datasheet - Page 178

• Manufacturer Part Number: ST7FMC1K2B3
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

ST7MC1xx/ST7MC2xx
MOTOR CONTROLLER (Cont’d)
The
(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.
Figure 98. Step Ratio Bits decoding and accuracy results and BEMF Sampling Rate
178/309
1
10
11
12
13
14
15
0
1
2
3
4
5
6
7
8
9
ST[3:0]
Step Ratio (Decimal)
Figure 98
avg Zn ~ FFh ± 0.4%
avg Zn ~ 7Fh ± 0.6%
gives the step ratio register value
avg Zn ~ 55h ± 1.2%
F
step
Fn
Fn+1 = 2.Fn
= 6.N.F
3.Fn
3.Fn+1 = 6.Fn
F
N: Pole pair number
step
rpm
: Electrical step frequency
= N.F / 10 ⇔ N.F = 10.F
200 Hz
At f
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.
MTC
avg Zn ~ FFh ± 0.4%
avg Zn ~ 7Fh ± 0.6%
=4MHz, the range covered by the Step Ra-
step
avg Zn ~ 55h ± 1.2%
15 kHz
samples
BEMF
N.Frpm
10
4
2
1
ΔZn/Zn
100%
50%
10%
0%