Load Step - Infineon TLE5501 Series User Manual

TLE5501
TMR-Based Angle Sensor
Connection to a micro controller
VDD
R
TMR
R
TMR
GND
Figure 10 Equivalent circuit of TMR bridge (only half bridge is shown) and SAR ADC input
3.2.1

Load step

The following consideration is made with the initial condition that the buffer capacitor C
S/H switch is open and the sample and hold capacitor C
V /2 and the voltage U
DD ADC
When the S/H switch is closed, charge is flowing from C
U , increases. In addition, charge is flowing from the supply voltage V
ADC
C .
b
The following parameters are assumed: V
The time constant τ for charging C
br
For charging C the time constant τ
SH
charging of C is much faster (~ 9x14ns = 140ns) than the recovery of the voltage at C
b
Due to this, with the assumption that τ
approximated as follows
With above parameters and U
bridge defines how long it takes until the voltage U
0.5LSB). Therefore, the sampling time (time for which S/H switch has to be closed) must be larger than
9 x τ = 9x4µs = 36µs.
br
Figure 11 shows this behavior. At t = 1µs the S/H switch is closed and remains so until t = 37µs. In the first
moment, the voltage drops by ΔU = 17mV and then increases with the time constant of the bridge τ
User's Manual
TMR bridge
C
b
= 0V.
= 5V, R
DD
via R is given by τ
b TMR
= R x C
SH SH
>> τ
br SH
(Equation (3.1)):
ΔU
=
= 2.5V, the load step is calculated to ΔU = 17.4mV. The time constant τ
0
S/H
R
is decharged. In this condition, the voltage at C
SH
to C , the voltage at C
b SH
DD
= 8kΩ, C = 1nF, R
TMR b SH
= R /2 x C = 4µs (see also
br TMR b
= 14ns. Therefore, the charging of C
SH
the voltage at C drops by a value of ΔU which can be
b
C S H
------------------------------
U 0
( )
C b + C SH
is settled with sufficient accuracy (error less than
ADC
11
ADC
SH
U
ADC
C
SH
GND
is fully charged, the
b
drops and the voltage at C
b
via the TMR resistor R
TMR
=2kΩ, and C = 7pF.
SH
Chapter 2.2).
and also the de-
SH
(~ 9x4µs = 36µs).
b
is
b
,
SH
to charge
(3.1)
of the
br
= 4µs.
br
Rev. 1.0
2019-04-29