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參數(shù)資料
| 型號： | U2894B-AFSG3 |
| 英文描述： | Modulation PLL for GSM, DCS and PCS Systems |
| 中文描述： | 調(diào)制鎖相環(huán)用于GSM，DCS和PCS系統(tǒng) |
| 文件頁數(shù)： | 13/16頁 |
| 文件大?。?/td> | 802K |
| 代理商： | U2894B-AFSG3 |
U2894B
Rev. A4, 30-Sep-98
6 (16)
Supply Current of the Charge Pump
IVSP vs. Time
Due to the pulsed operation of the charge pump, the cur-
rent into the charge-pump supply pin VSP is not constant.
Depending on I (see figure 6) and the phase difference at
the phase detector inputs, the current IVSP over time va-
ries. Basically, the total current is the sum of the quiescent
current, the charge-/discharge current, and – after each
phase comparison cycle – a current spike (see figure 3).
Down
Up
5I
3I
I
2I
–2I
t
14552
I
VSP
I
CPO
Figure 3. Supply current of the charge pump = f(t)
Internal current, I, |ICPC| and ICPC vs. RCPC
RCPC
I
|ICPCO|
ICPC
CPC open
0.5 mA
1 mA
0
2.2 k
W
1.0 mA
2 mA
–0.5 mA
680
W
2.0 mA
4 mA
–1.5 mA
(typical values)
Initial Charge-Pump Current after
Power-Up
Due to stability reasons, the reference current generator
for the charge pump needs an external capacitor (>500 pF
from CPC to GND). After power-up, only the on-chip
generated current I = ICPCK is available for charging the
external capacitor. Due to the charge pump’s architecture,
the charge pump current will be 2
I = 2
ICPCK until
the voltage on CPC has reached the reference voltage
(1.1 V). The following figures illustrate this behavior.
t
I
CPCK x RCPC
I
t
2x I
CPCK
14561
V
CPC
V
Ref
t
0
t
1
t
2
I
CPC
t
1
Time t1 can be calculated as t1
[ (1.1 V
CCPC)/ICPCK
e.g., CCPC = 1 nF, ICPCK = 2.7 mA
t1 [ 0.4 ms.
Time t2 can be calculated as t2
[ (RCPC/2200 W)
CCPC
e.g., CCPC = 1 nF, RCPC = 2200
W t2 [ 1.1 ms
Figure 4.
The behavior of |ICPO| after power-up can be very
advantageous for a fast settling of the loop. By using
larger capacitors (>1 nF), an even longer period with
maximum charge pump current is possible.
Ramp-up time for the internal band gap reference is about
1
ms. This time has to be added to the times calculated for
the charge pump reference.
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