TDA5212 Datasheet, PDF (33/51 Pages) Infineon Technologies AG – ASK/FSK 915MHz Single Conversion Receiver

TDA 5212
preliminary
Applications
This circuit charges the capacitor C with an inrush current I_load of 240µA for a
duration of T₂ until the voltage U_c appearing on the capacitor is equal to the volt-
age U_s at the input of the data filter. This voltage is limited to 2.5V. As soon as
these voltages are equal or the duration T₂ is exceeded the precharge circuit is
disabled.
t₂ is the time constant of the charging process of C which can be calculated as
t₂ » 20kW · C2
as the sum of R1 and R2 is sufficiently large and thus can be neglected. T2 can
then be calculated according to the following formula:
çæ
T₂
=
t
2
ln
ç
ççè
1
-
1
2 .4V
3V
÷ö
÷
÷÷ø
» t ₂ ×1.6
The voltage transient during the charging of C2 is shown in the following figure:
U2
3V
2.4V
2
T2
Figure 4-11 Voltage appearing on C2 during precharging process
e-fkt1.WMF
The voltage appearing on the capacitor C connected to pin 20 is shown in the
following figure. It can be seen that due to the fact that it is charged by a con-
stant current source it exhibits is a linear increase in voltage which is limited to
Wireless Components
4 - 12
Specification, May 2003