TDA5220 Datasheet, PDF (32/44 Pages) Infineon Technologies AG – ASK/FSK Single Conversion Receiver

confidential
TDA 5220
preliminary
Applications
This circuit charges the capacitor C with an inrush current I_load of typically
220µA for a duration of T₂ until the voltage U_c appearing on the capacitor is
equal to the voltage 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.
τ₂ is the time constant of the charging process of C which can be calculated as
τ₂ ≈=20kΩ · 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
=τ
2
ln
ç
ççè
1
−
1
2 .4V
3V
ö
≈ τ 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 a linear increase in voltage which is limited to
U_Smax = 2.5V which is also the approximate operating point of the data filter
input. The time constant appearing in this case can be denoted as T3, which
can be calculated with
T3
=
U-----_{S---m----a--x----}⋅_---C--
220µA
=
2---22---,0--5--µ^{-V---A-- ⋅ C}
Wireless Components
4 - 12
Target Specification, October 2001