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參數資料
| 型號： | LMX2305TMX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | XO, clock |
| 英文描述： | PLLatinumTM 550 MHz Frequency Synthesizer for RF Personal Communications |
| 中文描述： | PLL FREQUENCY SYNTHESIZER, 550 MHz, PDSO20 |
| 封裝： | 0.173 INCH, PLASTIC, TSSOP-20 |
| 文件頁數： | 13/14頁 |
| 文件大小： | 228K |
| 代理商： | LMX2305TMX |
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 in Figure 9. The signals
w
p
and
w
r
in
the diagram, correspond to the phase detector outputs of
the LMX2305 frequency synthesizer. These logic signals are
converted into current pulses, using the circuitry shown in
Figure 9, to enable either charging or discharging of the
loop filter components to control the output frequency of the
PLL.
Referring toFigure 9, the design goal is to generate a 5 mA
current which is relatively constant to within 0.5V 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 current
source reference being generated to be relatively repeat-
able 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
k
R8, 5, due to the current density differ-
ences
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
#
ln
i
source
i
p max
J
i
source
R
9
e
V
R8
b
V
T
#
ln
i
sink
i
n max
i
sink
R
5
e
V
R5
i
p max
R
8
e
V
R8
i
r max
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
r
)
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
b
4.5V;
V
w
p
e
0.0V; V
w
r
e
5.0V
I
SINK
e
I
SOURCE
e
5.0 mA;
V
fn
e
V
fp
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
r
e
100 mV
Typical System Parameters
Design Parameters
TL/W/12459–28
FIGURE 9
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
1.0 mA
e
300
X
R
8
e
0.3V
1.0 mA
e
300
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
13
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