U2894B Datasheet, PDF (6/16 Pages) TEMIC Semiconductors – Modulation PLL for GSM, DCS and PCS Systems

U2894B
Supply Current of the Charge Pump
I_VSP 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 I_VSP 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).
Up
Down
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 = I_CPCK is available for charging the
external capacitor. Due to the charge pump’s architecture,
the charge pump current will be 2 I = 2 I_CPCK until
the voltage on CPC has reached the reference voltage
(1.1 V). The following figures illustrate this behavior.
I_CPCK x R_CPC
V_CPC
5I
I_{VSP 3I}
V_Ref
I
2I
I_CPO
–2I
t
t
14552
Figure 3. Supply current of the charge pump = f(t)
Internal current, I, |I_CPC| and I_CPC vs. R_CPC
R_CPC
CPC open
2.2 kW
680 W
I
0.5 mA
1.0 mA
2.0 mA
(typical values)
|I_CPCO|
1 mA
2 mA
4 mA
I_CPC
0
–0.5 mA
–1.5 mA
t₁ t₀
t₂
t
I_CPC
2x I_CPCK
I
t₁
t
14561
[ Time t₁ can be calculated as t₁ (1.1 V C_CPC)/I_CPCK
³ [ m e.g., C_CPC = 1 nF, I_CPCK = 2.7 mA t₁ 0.4 s.
[ W Time t₂ can be calculated as t₂ (R_CPC/2200 ) C_CPC
W ³ [ m e.g., C_CPC = 1 nF, R_CPC = 2200
t₂ 1.1 s
Figure 4.
The behavior of |I_CPO| 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.
m^{Ramp-up time for the internal band gap reference is about}
1 s. This time has to be added to the times calculated for
the charge pump reference.
6 (16)
Rev. A4, 30-Sep-98