TDA7210 Datasheet, PDF (33/52 Pages) Infineon Technologies AG – Wireless Components ASK/FSK Single Conversion Receiver

TDA7210
Applications
This circuit charges the capacitor C13 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 C18 which can be calculated
as
τ ₂ = 20kΩ × C18
as the sum of R4 and R5 is sufficiently large and thus can be neglected. T2 can
then be calculated according to the following formula:
T_l
=τ₂
⎜⎛
ln^⎜
⎜⎜⎝ ^{1 −}
1
2.4V
3V
⎟⎞
⎟
⎟⎟⎠
≈ τ ₂ ×1.6
The voltage transient during the charging of C18 is shown in the following
figure:
U2
3V
2.4V
2
T2
Figure 4-11 Voltage appearing on C18 during precharging process
e-fkt1.WMF
The voltage appearing on the capacitor C13 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
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--2-^m--2--^a-0-^x--_μ--^â‹--A-C----1---3- = _{2---22---,0--5--μ}-V---A-- ⋅ C13
Wireless Components
4 - 12
Data Sheet, December 2008