TDA7210 Datasheet, PDF (32/52 Pages) Infineon Technologies AG – Wireless Components ASK/FSK Single Conversion Receiver

TDA7210
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
C13 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 R1 connected
between the SLP and SLN pins (pins 19 and 20) is limited by the 330kΩ resistor
appearing in parallel to R1 as can be seen in Figure 4-7. Apart from this a resis-
tor 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 calcu-
lated with:
τ ₁ = (R1 || 330 kΩ )× C13
In case R1 is chosen to be 100kΩ and C13 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 C13 a pre-
charge circuit was included in the TDA7210 as shown in the following figure.
C18
R4+R5=600k
R4
R5
U_threshold
24
23
C13
R1
20
19
Uc>Us Uc<Us ^Iload
Uc
U2
+
0 / 240uA
20k
+3.1V
OTA
-
U2<2.4V : I=240uA
U2>2.4V : I=0
+2.4V
Data Filter ^{ASK/FSK Switch}
Us
Figure 4-10 Principle of the precharge circuit
precharge.WMF
Wireless Components
4 - 11
Data Sheet, December 2008