LMX2320 National Semiconductor, LMX2320 Datasheet - Page 21

• Manufacturer Part Number: LMX2320
• Description: Frequency Synthesizer for RF Personal Communications
• Manufacturer: National Semiconductor

Application Information
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 14 The signals
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 in Figure 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 p and 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
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
of r’s under 1 mA loads ( p’s V
k
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
R
R
R
R
R
0 1V)
4
9
5
8
6
7
e
e
e
e
e
e
V
V
i
i
(V
(V
p max
r max
R5
R8
p
P
V
b
b
R8
b
b
0 026 1n (5 mA 1 mA)
V
V
V
R5
V
V
(
(
VOL p
VOH p
T
T
(
i
source
n
i
p
sink
n
a
(
a
1n
1n
i
p max
a
i
max
p
1) i
)
1)
)
b
a
1)
b
i
i
i
source
n max
p max
i
b
sink
sink
OL
(V
1)
(V
i
source
R5
R8
drop of p and the V
a
a
OL k
Vfp)
Vfn)
0 1V and ( r s V
(Continued)
through the Q1
p
and
OH
drop
r
OH
in
21
EXAMPLE
Typical Device Parameters
Typical System Parameters
Design Parameters
Therefore select
R
R
R
R
4
5
8
6
e
e
e
e
R
1 0 mA (50
1 0 mA (100
R
9
7
e
e
0 3V
(5V
0 3V (50
0 3V (100
b
b
0 1V)
a
0 026 1n(5 0 mA 1 0 mA)
a
FIGURE 14
1)
a
1)
1 0 mA
a
b
b
b
1)
5 mA
1)
5 0 mA
(0 3V
V
V
V
I
Vfn
I
V
V
SINK
r max
5 0 mA
P
cntl
R8
OL p
n
p
e
e
e
e
e
e
100
e
a
e
5 0V
Vfp
e
e
0 0V V
V
0 5V– 4 5V
e
0 8V)
I
I
p max
R5
SOURCE
332
V
http
315 6
e
OH p
p
e
e
0 8V
e
e
0 3V
www national com
r
50
TL W 12339 – 39
3 8 k
1 mA
e
e
e
e
5 0V
100 mV
5 0 mA
51 6