TDA 5210
Applications
4.7 Principle of the Precharge Circuit
In case the data slicer threshold shall be generated with an external RC network
as described in Section 4.5 it is necessary to use large values for the capacitor
C attached to the SLN pin (pin 20) in order to achieve long time constants. This
results also from the fact that the choice of the value for R connected between
the SLP and SLN pins (pins 19 and 20) is limited by the 330kΩ resistor appear-
ing in parallel to R as can be seen in Figure 4-7. Apart from this a resistor value
of 100kΩ leads to a voltage offset of 1mv at the comparator input as described
in Section 4.6.1. The resulting startup time constant τ1 can be calculated with:
τ1 = (R // 330kΩ) · C
In case R is chosen to be 100kΩ and C is chosen as 47nF this leads to
τ1 = (100kΩ // 330kΩ) · 47nF = 77kΩ · 47nF = 3.6ms
When the device is turned on this time constant dominates the time necessary
for the device to be able to demodulate data properly. In the powerdown mode
the capacitor is only discharged by leakage currents.
In order to reduce the turn-on time in the presence of large values of C a pre-
charge circuit was included in the TDA5210 as shown in the following figure.
C2
R1+R2=600k
R2
R1
C
R
U
thres hold
20
19
24
23
Iload
Uc
ASK/FSK Switch
Uc>Us Uc<Us
DataFilter
-
U2
+
0 / 240uA
OTA
Us
U2<2.4V : I=240uA
U2>2.4V: I=0
-
20k
+2.4V
+3.1V
precharge.WMF
Figure 4-10 Principle of the precharge circuit
Wireless Components
4 - 11
Specification, May 2001
INFINEON [ Infineon ]
