TDA 5210
Applications
This circuit charges the capacitor C with an inrush current Iload of typically
220µA for a duration of T2 until the voltage Uc appearing on the capacitor is
equal to the voltage Us 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:
§
¨
¨
¨
¨
©
·
¸
¸
¸
¸
¹
1
T2 = τ 2 ln
≈ τ 2 1.6
2.4V
3V
1 −
The voltage transient during the charging of C2 is shown in the following figure:
U2
3V
2.4V
T2
2
e-fkt1.WMF
Figure 4-11 Voltage appearing on C2 during precharging process
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
USmax = 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
USmax
C
2.5V
220µA
----------------
T3 = ----------------------- =
C
220µA
Wireless Components
4 - 12
Specification, May 2001
INFINEON [ Infineon ]
