MA780 – 8-BIT TO 12-BIT LOW-POWER ANGLE SENSOR

MA780 Rev. 1.2

MonolithicPower.com

20

8/30/2021

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In a side-shaft configuration (when the sensor

’s

center is located beyond the magnet

’s outer

diameter), the k ratio exceeds 1. If the value of k

is known, BCT[7:0] can be calculated with

Equation (4):









1

BCT[7 : 0] = 258 1

k

(4)

For optimal compensation, the sensitivity of the

radial axis should be reduced by setting the BCT

parameter. Figure 28 shows the optimal BCT

value for a particular k ratio.

0

50

100

150

200

1

1.5

2

2.5

3

3.5

4

4.5

5

k

Figure 28: k Ratio and the Optimum BCT to

Recover Linearity

Table 12 shows typical BCT values.

Table 12: Potential BCT Settings

E (deg)

Magnet Ratio (k)

BCT[7:0]

0

1.0

0

11.5

1.5

86

19.5

2.0

129

25.4

2.5

155

30.0

3.0

172

33.7

3.5

184

36.9

4.0

194

39.5

4.5

201

41.8

5.0

207

Determining k

The k ratio can be deduced from the error curve

obtained with the default BCT setting (BCT = 0).

Rotate the magnet more than one revolution and

record the

device’s output. Next, plot the error

curve (the output minus the real mechanical

position vs. the real mechanical position) and

extract two parameters: the maximum error (E),

and the position of this maximum with respect to

a zero crossing (aM) (see Figure 27). The k ratio

can be calculated with Equation (5):

M

M

tan(E + a )

k=

tan(a )

(5)

Alternatively, the value of k can be obtained

using Figure 29.

Figure 29: Relationship between the Error

Measured with BCT = 0 and the Magnet Ratio k

Sensor Orientation

The dot marked on the package indicates

whether the radial field is aligned with the sensor

coordinate X or Y (see Figure 30).

Figure 30: Package Top View with X- and Y-Axes

Determine which axis should be reduced based

on the qualitative field distribution around a ring

(see Figure 26 on page 20).

For example, Figure 30 shows that the field

along the sensor

’s Y direction is tangential and

weaker. This means that the X-axis should be

reduced (ETX = 1 and ETY = 0).

If both ETX and ETY are set to 1, the current bias

is reduced in both directions the same way (i.e.

without side-shaft correction) (see Table 13).

1

1.5

2

2.5

3

3.5

4

4.5

5

0

5

10

15

20

25

30

35

40

E (deg)