參數(shù)資料
型號(hào): TDA5221
廠商: INFINEON TECHNOLOGIES AG
英文描述: ASK/FSK Single Conversion Receiver
中文描述: 詢價(jià)/ FSK信號(hào)單一轉(zhuǎn)換接收器
文件頁(yè)數(shù): 31/44頁(yè)
文件大?。?/td> 705K
代理商: TDA5221
Applications
4 - 11
TDA 5221
preliminary
Wireless Components
Target Specification, October 2001
4.7
Principle of the Precharge Circuit
In case the data slicer threshold shall be generated with an external RC network
as described in Section 4.5 it is necessary to use large values for the capacitor
C attached to the
SLN
pin (pin 20) in order to achieve long time constants. This
results also from the fact that the choice of the value for R connected between
the
SLP
and
SLN
pins (pins 19 and 20) is limited by the 330k
resistor appear-
ing in parallel to R as can be seen in Figure 4-7. Apart from this a resistor value
of 100k
leads to a voltage offset of 1mv at the comparator input as described
in Section 4.6.1. The resulting startup time constant
τ
1
can be calculated with:
τ
1
= (R // 330k
)
·
C
In case R is chosen to be 100k
and C is chosen as 47nF this leads to
τ
1
= (100k
// 330k
)
·
47nF = 77k
·
47nF = 3.6ms
When the device is turned on this time constant dominates the time necessary
for the device to be able to demodulate data properly. In the powerdown mode
the capacitor is only discharged by leakage currents.
In order to reduce the turn-on time in the presence of large values of C a pre-
charge circuit was included in the TDA5221 as shown in the following figure.
precharge.WMF
Figure 4-10
Principle of the precharge circuit
I
load
+3.1V
20k
+
-
OTA
+2.4V
R1
R2
24
23
U
threshold
C
0 / 240uA
+
-
20
19
R
Data Filter
ASK/FSK Swtch
C2
U2
Us
Uc
Uc<Us
Uc>Us
U2<2.4V : I=240uA
U2>2.4V : I=0
R1+R2=600k
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