IC-TW28 Datasheet, PDF (72/80 Pages) IC-Haus GmbH – 10-BIT SIN/COS INTERPOLATOR

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Rev D2, Page 72/80
LED INTENSITY CONTROL
In serial configuration mode, the calculated sin/cos am-
plitude value, SC_AMP, can also be used to drive the
LED output to control the intensity of an optical sensor
LED. This maintains the sin/cos signals at their cali-
brated amplitude in the presence of LED ageing and
varying application conditions.
To use LED intensity control, enable the LED output by
setting LED_CFG.en = 1 and enable LED intensity con-
trol by setting LED_CFG.auto = 1. Set LED_START to
provice a nominal LED current at startup as explained
following. Configure the amplitude monitor as explained
in SIN/COS AMPLITUDE MONITOR on page 62 to
set the desired amplitude limits for LED control. Set
LED_CFG.buffer as required to provide the desired
LED intensity hysteresis as explained following.
In operation, the LED intensity control increases the
duty cycle of the LED PWM signal whenever
SC_AMP < (SC_AMP_LOW + LED_CFG.buffer)
and decreases it whenever
For example, if SC_AMP_LOW = 135 and
SC_AMP_HIGH = 165, a CFG_LED.buffer value of
0 provides LED intensity hysteresis of
165 − 135 − 2 · 8 = 14
In this case, LED intensity is controlled so that
165 + 135 14
SC_AMP =
± = 150 ± 7.
2
2
Note that the calculated LED intensity hysteresis must
be a positive value, otherwise the buffer zones overlap
resulting in undefined operation of the LED intensity
control. Thus, for proper operation,
SC_AMP_HIGH − SC_AMP_LOW
LED_CFG.buffer <
2
Continuing the example above, LED_CFG.buffer = 0 is
the only possible choice, since
165 − 135
2
= 15
SC_AMP > (SC_AMP_HIGH − LED_CFG.buffer)
To use a larger buffer value, SC_AMP_LOW must
be decreased, SC_AMP_HIGH must be increased, or
both.
Select an LED_CFG.buffer value to provide the desired
hysteresis for the LED intensity as shown in Figure 41.
See LED_CFG.buffer on page 39 for more information
on the available buffer values.
If maximum LED current is less than 15 mA under all
operating conditions and can be supplied by a 3.3 V
supply, connect the LED directly to the iC-TW28 as
shown in Figure 42. This is typically the case for red
LEDs.
LED Intensity
Hysteresis
iC-TW28
R
LED
C
iC-TW28
LED
3.3V
R
C
SC_AMP_LOW
SC_AMP_HIGH
Figure 41: LED Intensity Hysteresis
In operation, the LED intensity control changes the LED
PWM value by ±1 at a rate of 500 Hz as required to
maintain SC_AMP within the LED intensity hysteresis
area shown in Figure 41.
Specifically, the LED intensity hysteresis is
SC_AMP_HIGH − SC_AMP_LOW − 2 · LED_CFG.buffer
Positive Polarity
LED_CFG.pol = 0
Negative Polarity
LED_CFG.pol = 1
Figure 42: Direct LED Connection
Configure the LED output as open-drain by setting
LED_CFG.odrain = 1. Configure the LED polarity to_3.3V
match the speci3f.i3cV connection as shown.
iC-TW28
iC-TW28
R
If maxLiEmDum LED c^Rurrent is more thLaEnD 15 mA or th_Ce
maximum LED curre_Cnt cannot be supplied by a 3.3 V
supply, use an external FET to connect the LED to the
iC-TW^N2^e8^gata^ivs^{e P}s^oh^lao^rw^ityn in Figure 43. This^Pi^os^sit^ty^ivp^e i^Pc^oa^lall^ry^itythe
case f^Lo^Er^Db^_l^Cu^Fe^GL^.pE^olD⁼s¹ due to their higher V^LE_f.^{D_CFG.pol = 0}