TDA5212_06 Datasheet, PDF (34/49 Pages) Infineon Technologies AG – ASK/FSK 915MHz Single Conversion Receiver

TDA 5212
Applications
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
T 3 = ^{U S max × C} = ^2.5V × C
240µA 240µA
Uc
Us
T3
Figure 4-12 Voltage transient on capacitor C attached to pin 20
As an example the choice of C2 = 20nF and C = 47nF yields
e-Fkt2.WMF
τ₂ = 0.4ms
T₂ = 0.64ms
T₃ = 0.49ms
This means that in this case the inrush current could flow for a duration of
0.64ms but stops already after 0.49ms when the U_Smax limit has been reached.
T3 should always be chosen to be shorter than T2.
It has to be noted finally that during the turn-on duration T2 the overall device
power consumption is increased by the 240µA needed to charge C.
The precharge circuit may be disabled if C2 is not equipped. This yields a T2
close to zero. Note that the sum of R4 and R5 has to be 600kΩ in order to pro-
duce 3V at the THRES pin as this voltage is internally used also as the refer-
ence for the FSK demodulator.
Wireless Components
4 - 13
Specification, December 2006