LMX2314 National Semiconductor Corporation, LMX2314 Datasheet - Page 18

• Manufacturer Part Number: LMX2314
• Description: 1.2 Ghz Frequency Synthesizer For Rf Personal Communications
• Manufacturer: National Semiconductor Corporation

Application Information
EXTERNAL CHARGE PUMP
The LMX PLLatimum 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
the diagram correspond to the phase detector outputs of
the LMX2314 2315 frequency synthesizers 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 to Figure 9 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 establish as
large of a voltage drop across R5 R8 as possible without
saturating Q2 Q4 A voltage of approximately 300 mV pro-
vides a good compromise This allows the current source
reference being generated to be relatively repeatable in the
absence of good Q1 Q2 Q3 Q4 matching (Matched tran-
sistor pairs is recommended ) The wp and wr 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
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 wr’s under 1 mA loads (wp’s V
V
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
0 026 1n (5 mA 1 mA) through the Q1 Q2 Q3 Q4 pairs
OH k
R
R
R
R
R
R
4
9
5
8
6
7
e
e
e
e
e
e
0 1V )
k
V
V
i
i
(V
(V
p max
r max
R5
R8
p
P
V
R8 5 due to the current density differences
b
b
R8
b
b
V


V
V
R5
V
V

(b
(b
VOLwp
VOHwr
T
T
i
(b
source
n
i

p
sink


n
a
(b
a
ln
ln
i
p max
a
i
max


p
1) i
)
)
1)
i
i
b
b
1)
i
a
source
n max
p max
i
sink
sink
b
OL
(V
(V
1)
i
source
R5
R8
drop of wp and the V
J
J
a
a
Vfp)
Vfn)
OL k
(Continued)
0 1V and wr’s
p
and w
OH
drop
r
in
18
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
1 0 mA
R
9
7
e
e
0 3V
(5V
0 3V
0 3V


(50
(100
b
b


0 1V)
(50
a
(100
0 026
a
1)
FIGURE 9
a
1)
1 0 mA
b
b
b
1)
b

5 mA
1)
5 0 mA
(0 3V
b
V
V
V
I
V
I
V
V
1n(5 0 mA 1 0 mA)
SINK
rmax
5 0 mA
P
cntl
wp
fn
R8
OLwp
n
e
e
e
e
e
e
e
100 b
a
e
5 0V
V
e
e
0 0V V
V
0 5V
fp
I
e
0 8V)
I
R5
pmax
SOURCE
332X
V
e
315 6X
OHwr
p
e
b
0 8V
e
e
e
wr
0 3V
4 5V
TL W 11766 – 43
50
3 8 kX
e
1 mA
e
e
e
100 mV
5 0V
5 0 mA
51 6X