LMX2301 National Semiconductor Corporation, LMX2301 Datasheet - Page 13

• Manufacturer Part Number: LMX2301
• Description: Pllatinumtm 160 Mhz Frequency Synthesizer For Rf Personal Communications
• Manufacturer: National Semiconductor Corporation

Figure 9 to enable either charging or discharging of the
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 9 The signals w
the diagram correspond to the phase detector outputs of
the LMX2301 frequency synthesizer These logic signals are
converted into current pulses using the circuitry shown in
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 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 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
ences 0 026 1n (5 mA 1 mA) 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
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
OH k
R
R
R
R
R
R
4
9
5
8
6
7
e
e
e
e
e
e
0 1V )
V
V
i
i
(V
(V
p max
r max
V
V
R5
R8
R8
p
P
R5
b
b
b
b
V
V
V
V
VOLwp
VOHwr
T
T
i
source
i
k
sink
#
#
ln
ln
i
R8 5 due to the current density differ-
p max
i
max
#
#
)
)
i
i
b
b
source
i
n max
p max
i
sink
OL
(V
(V
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
13
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
0 3V
0 3V
9
7
e
e
0 3V
(5V
e
e
b
300X
300X
b
0 1V)
0 026
FIGURE 9
1 0 mA
b
#
5 mA
(0 3V
b
V
V
V
I
V
I
V
V
1n(5 0 mA 1 0 mA)
SINK
rmax
P
cntl
fn
R8
OLwp
n
wp
e
e
e
e
e
e
100 b
e
a
e
5 0V
V
e
0 0V V
V
0 5V
fp
0 8V)
I
I
pmax
R5
SOURCE
V
http
e
OHwr
p
e
b
0 8V
e
e
e
wr
0 3V
www national com
4 5V
50
TL W 12458 – 28
3 8 kX
e
1 mA
e
e
e
100 mV
5 0V
5 0 mA
51 6X