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参数资料
| 型号： | LMX2315TMX |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | XO, clock |
| 英文描述： | Frequency Synthesizer for RF Personal Communications |
| 中文描述： | PLL FREQUENCY SYNTHESIZER, 1200 MHz, PDSO20 |
| 封装： | 0.173 INCH, PLASTIC, TSSOP-20 |
| 文件页数： | 21/22页 |
| 文件大小： | 409K |
| 代理商： | LMX2315TMX |
Application Information
(Continued)
EXTERNAL CHARGE PUMP
The LMX PLLatinum series of frequency synthesizers are
equipped with an internal balanced charge pump as well as
outputs for driving an external charge pump. Although the
superior performance of NSC’s on board charge pump elim-
inates the need for an external charge pump in most appli-
cations, certain system requirements are more stringent. In
these cases, using an external charge pump allows the de-
signer to take direct control of such parameters as charge
pump voltage swing, current magnitude, TRI-STATE leak-
age, and temperature compensation.
One possible architecture for an external charge pump cur-
rent source is shown inFigure 14. The signals
w
p
and
w
r
in
the diagram, correspond to the phase detector outputs of
the 2315/20/25 frequency synthesizers. These logic sig-
nals are converted into current pulses, using the circuitry
shown inFigure 14, to enable either charging or discharging
of the loop filter components to control the output frequency
of the PLL.
Referring to Figure 14, the design goal is to generate a
5 mA current which is relatively constant to within 5V of the
power supply rail. To accomplish this, it is important to es-
tablish as large of a voltage drop across R5, R8 as possible
without saturating Q2, Q4. A voltage of approximately
300 mV provides a good compromise. This allows the cur-
rent source reference being generated to be relatively re-
peatable in the absence of good Q1, Q2/Q3, Q4 matching.
(Matched transistor pairs is recommended.) The
w
p and
w
r
outputs are rated for a maximum output load current of
1 mA while 5 mA current sources are desired. The voltages
developed across R4, 9 will consequently be approximately
258 mV, or 42 mV less than R8, 5, due to the current density
differences
à
0.026
*
1n (5 mA/1 mA)
ó
through the Q1,
Q2/Q3, Q4 pairs.
In order to calculate the value of R7 it is necessary to first
estimate the forward base to emitter voltage drop (Vfn,p) of
the transistors used, the V
OL
drop of
w
p, and the V
OH
drop
of
w
r’s under 1 mA loads. (
w
p’s V
OL
k
0.1V and (
w
r,s V
OH
k
0.1V).
Knowing these parameters along with the desired current
allow us to design a simple external charge pump. Separat-
ing the pump up and pump down circuits facilitates the no-
dal analysis and give the following equations.
R
4
e
V
R5
b
V
T
#
1n
i
source
i
p max
i
source
R
9
e
V
R8
b
V
T
#
1n
i
sink
i
n max
i
sink
R
5
e
V
R5
#
(
b
p
a
1)
i
p max
#
(
b
p
a
1)
b
i
source
R
8
e
V
R8
#
(
b
n
a
1)
i
r max
#
(
b
n
a
1) i
sink
R
6
e
(V
p
b
V
VOL
w
p
)
b
(V
R5
a
Vfp)
i
p max
R
7
e
(V
P
b
V
VOH
w
p
)
b
(V
R8
a
Vfn)
i
max
EXAMPLE
Typical Device Parameters
b
n
e
100,
b
p
e
50
V
P
e
5.0V;
V
cntl
e
0.5V–4.5V;
V
w
p
e
0.0V, V
w
r
e
5.0V
I
SINK
e
I
SOURCE
e
5.0 mA;
Vfn
e
Vfp
e
0.8V
I
rmax
e
I
pmax
e
1 mA
V
R8
e
V
R5
e
0.3V
V
OL
w
p
e
V
OH
w
p
e
100 mV
Typical System Parameters
Design Parameters
TL/W/12339–39
FIGURE 14
Therefore select
R
4
e
R
9
e
0.3V
b
0.026
#
1n(5.0 mA/1.0 mA)
5 mA
e
51.6
X
R
5
e
0.3V
#
(50
a
1)
1.0 mA
#
(50
a
1)
b
5.0 mA
e
332
X
R
8
e
0.3V
#
(100
a
1)
1.0 mA
#
(100
a
1)
b
5.0 mA
e
315.6
X
R
6
e
R
7
e
(5V
b
0.1V)
b
(0.3V
a
0.8V)
1.0 mA
e
3.8 k
X
http://www.national.com
21
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