TDA5212 Datasheet, PDF (32/51 Pages) Infineon Technologies AG – ASK/FSK 915MHz Single Conversion Receiver

TDA 5212
preliminary
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 330kW resistor appear-
ing in parallel to R as can be seen in Figure 4-6. Apart from this a resistor value
of 100kW leads to a voltage offset of 1mv at the comparator input as described
in Section 4.6.1. The resulting startup time constant t₁ can be calculated with:
t₁ = (R // 330kW) · C
In case R is chosen to be 100kW and C is chosen as 47nF this leads to
t₁ = (100kW // 330kW) · 47nF = 77kW · 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
R1
R2
U_{thr es hold}
24
23
C
R
20
19
Uc>Us Uc<Us ^Iload
Uc
Data Filter
U2
+
0 / 240uA
Us
OTA
-
U2<2.4V : I=240uA
U2>2.4V : I=0
20k
+3.1V
+2.4V
ASK/FSK Switch
Figure 4-10 Principle of the precharge circuit
precharge.WMF
Wireless Components
4 - 11
Specification, May 2003