IC-TW11 Datasheet, PDF (21/25 Pages) IC-Haus GmbH – 10-BIT ULTRA LOW POWER MAGNETIC ABSOLUTE ROTARY ENCODER

iC-TW11 10-BIT ULTRA LOW POWER^preliminary
MAGNETIC ABSOLUTE ROTARY ENCODER
USING LOW POWER MODE
Rev B1, Page 21/25
For extremely low-power applications, low power mode
can be enabled by setting the low power mode bit (CON-
FIG.lpwr) in the CONFIG register (see page 15). Low
power mode imposes certain performance restrictions,
so its use should be carefully considered.
Low power mode operates by disabling the low-pass
filters on the three Hall sensors (see page 9). Without
having to wait for the filters to settle, sampling can oc-
cur much faster thus using less power. However, the
converted angle and other sampled values are much
noisier without the filters, limiting useable resolution
compared to normal power mode.
In low power mode, the maximum sampling rate is in-
creased from 4 k Samples per second to 20 k Samples
per second. Likewise, typical current consumption is
reduced from 2.05 µA per sample per second to 260 nA
per sample per second.
CALCULATING CURRENT CONSUMPTION
Current consumption in the iC-TW11 is directly propor-
tional to the sampling frequency, f_s. Lower sampling
frequencies use less current and higher frequencies
use more current.
For example, at a sampling frequency of 1,000 samples
per second, f_s = 1 kHz and I = 2.05 mA. At 10 samples
per second, I = 20.5 µA. In low power mode the typical
current is calculated as shown below.
In normal power mode (the default configuration) the
typical current is calculated as shown below.
I [µA] = 260 · f_s [kHz]
I [mA] = 2.05 · f_s [kHz]
For example, at a sampling frequency of 1,000 samples
per second, f_s = 1 kHz and I = 260 µA. At 10 samples
per second, I = 2.6 µA.
DETERMINING THE MAGNETIC AIRGAP
With AGC enabled (default configuration), the relative
airgap between magnet and iC-TW11 can be deter-
mined by reading the Hall array gain. This is useful
for verifying that the magnet is the right distance from
iC-TW11, or to implement a button push/knob turn appli-
cation to determine when the button is pushed. The four
most significant bits of the 5-bit gain value are available
in ANGLE register for this purpose. The full 5-bit gain
value can be read from the GAIN register (see page 16).
Because the LSB is missing, the gain value in ANGLE
Register is half the value in GAIN Register.
Airgap (mm)
iC-TW11 GAIN Register vs. Airgap with Ø4x4 Magnet
18
16
14
12
10
8
6
4
2
0
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
Airgap (mm)
For example, the typical relationship between Hall ar-
ray gain and magnetic airgap for the Ø4x4 mm magnet
used on the iC-TW11_1C demo board is shown in Fig-
ure 8.
Figure 8: Typical Hall array gain vs. airgap
Notice that the gain saturates at larger gaps before an
ADC Underflow condition (X) is indicated at a gap of
3.5 mm.