TDA7200 Datasheet, PDF (29/49 Pages) Infineon Technologies AG – ASK/FSK Single Conversion Receiver

TDA7200
Applications
In order to reduce the turn-on time in the presence of large values of C a precharge
circuit was included in the TDA7200 as shown in the following figure.
C18
R4+R5=600k
R4
R5
U_threshold
C13
R1
24
U2
23
Uc>Us Uc<Us ^Iload
20
Uc
+
0 / 240uA
20k
+3.1V
OTA
-
U2<2.4V : I=240uA
U2>2.4V : I=0
+2.4V
19
Data Filter ^{ASK/FSK Switch}
Us
Figure 12 Principle of the precharge circuit
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 × C2
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:
T₂
=τ₂

ln^
 ^1−
1
2.4V
3V



≈τ₂
×1.6
Data Sheet
29
V 1.0, 2007-05-02