LMX2305TM [ROCHESTER]
PLL Frequency Synthesizer, BICMOS, PDSO20, 0.173 INCH, PLASTIC, TSSOP-20;
| 型号： | LMX2305TM |
| 厂家： | 
Rochester Electronics |
| 描述： | PLL Frequency Synthesizer, BICMOS, PDSO20, 0.173 INCH, PLASTIC, TSSOP-20 信息通信管理 光电二极管 |
| 文件： | 总15页 (文件大小：435K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
REI Datasheet
LMX2305
550 MHz Frequency Synthesizer for RF Personal Communications
The LMX2305 is a high performance frequency synthesizer with an integrated prescaler designed
for RF operation up to 550 MHz. The LMX2305 contains a dual modulus prescaler which can select
either a 64/65 or a 128/129 divide ratio at input frequencies of up to 550 MHz. LMX2305, which
employs the digital phase lock loop technique, combined with a high quality reference oscillator and
a loop filter, provides the tuning voltage for the voltage controlled oscillator to generate a very stable,
low noise local oscillator signal.
Quality Overview
Rochester Electronics
Manufactured Components
•
•
•
ISO-9001
AS9120 certification
Qualified Manufacturers List (QML) MIL-PRF-38535
Rochester branded components are
manufactured using either die/wafers
purchased from the original suppliers
or Rochester wafers recreated from the
original IP. All recreations are done with
the approval of the OCM.
•
•
Class Q Military
Class V Space Level
•
Qualified Suppliers List of Distributors (QSLD)
•
Rochester is a critical supplier to DLA and
meets all industry and DLA standards.
Parts are tested using original factory
test programs or Rochester developed
test solutions to guarantee product
meets or exceeds the OCM data sheet.
RochesterElectronics, LLCiscommittedtosupplying
products that satisfy customer expectations for
quality and are equal to those originally supplied by
industry manufacturers.
The original manufacturer’s datasheet accompanying this document reflects the performance
and specifications of the Rochester manufactured version of this device. Rochester Electronics
guarantees the performance of its semiconductor products to the original OEM specifications.
‘Typical’ values are for reference purposes only. Certain minimum or maximum ratings may be
based on product characterization, design, simulation, or sample testing.
© 2014 Rochester Electronics, LLC. All Rights Reserved 01162014
To learn more, please visit www.rocelec.com
PRELIMINARY
August 1996
LMX2305
PLLatinumTM 550 MHz Frequency Synthesizer
for RF Personal Communications
General Description
Features
Y
RF operation up to 550 MHz
The LMX2305 is a high performance frequency synthesizer
with an integrated prescaler designed for RF operation up to
550 MHz. It is fabricated using National’s ABiC IV BiCMOS
process.
Y
2.65V to 5.5V operation
Y
Low current consumption:
e
e
2.75V
I
4.0 mA (typ) at V
CC
CC
Y
Y
Y
The LMX2305 contains a dual modulus prescaler which can
select either a 64/65 or a 128/129 divide ratio at input fre-
quencies of up to 550 MHz. LMX2305, which employs the
digital phase lock loop technique, combined with a high
quality reference oscillator and a loop filter, provides the
tuning voltage for the voltage controlled oscillator to gener-
ate a very stable, low noise local oscillator signal.
Dual modulus prescaler: 64/65 or 128/129
Internal balanced, low leakage charge pump
Small-outline, plastic, surface mount TSSOP,
0.173 wide package
×
Applications
Y
Analog Cellular telephone systems
(AMPS, ETACS, NMT)
Serial data is transferred into the LMX2305 via a three line
MICROWIRETM interface (Data, Enable, Clock). Supply volt-
age can range from 2.65V to 5.5V. The LMX2305 features
very low current consumption, typically 4.0 mA at 2.75V.
Y
Portable wireless communications
(PCS/PCN, cordless)
Y
Wireless local area networks (WLANs)
The LMX2305 is available in a TSSOP 20-pin surface mount
plastic package.
Y
Other wireless communication systems
Y
Pagers
Block Diagram
TL/W/12459–1
TRI-STATEÉ is a registered trademark of National Semiconductor Corporation.
MICROWIRETM and PLLatinumTM are trademarks of National Semiconductor Corporation.
C
1996 National Semiconductor Corporation
TL/W/12459
RRD-B30M126/Printed in U. S. A.
http://www.national.com
Connection Diagram
LMX2305
TL/W/12459–2
20-Lead (0.173 Wide) Thin Shrink
×
Small Outline Package (TM)
Order Number LMX2305TM or LMX2305TMX
See NS Package Number MTC20
Pin Descriptions
Pin No.
Pin Name
I/O
Description
1
OSC
I
Oscillator input. A CMOS inverting gate input intended for connection to a crystal resonator for
operation as an oscillator. The input has a V /2 input threshold and can be driven from an
IN
CC
external CMOS or TTL logic gate. May also be from a reference oscillator.
Oscillator output.
3
4
5
OSC
O
OUT
t
V
V
Power supply for charge pump. Must be
V
CC
.
P
Power supply voltage input. Input may range from 2.65V to 5.5V. Bypass capacitors should be
placed as close as possible to this pin and be connected directly to the ground plane.
CC
6
D
O
O
Internal charge pump output. For connection to a loop filter for driving the input of an external
VCO.
o
7
8
GND
LD
Ground.
Lock detect. Output provided to indicate when the VCO frequency is in ‘‘lock’’. When the loop is
locked, the pin’s output is HIGH with narrow low pulses.
10
11
f
I
I
Prescaler input. Small signal input from the VCO.
IN
CLOCK
High impedance CMOS Clock input. Data is clocked in on the rising edge, into the various
counters and registers.
13
14
DATA
LE
I
I
Binary serial data input. Data entered MSB first. LSB is control bit. High impedance CMOS input.
Load enable input (with internal pull-up resistor). When LE transitions HIGH, data stored in the
shift registers is loaded into the appropriate latch (control bit dependent). Clock must be low
when LE toggles high or low. See Serial Data Input Timing Diagram.
15
16
FC
I
Phase control select (with internal pull-up resistor). When FC is LOW, the polarity of the phase
comparator and charge pump combination is reversed.
BISW
O
Analog switch output. When LE is HIGH, the analog switch is ON, routing the internal charge
pump output through BISW (as well as through D ).
o
17
18
f
O
O
Monitor pin of phase comparator input. CMOS output.
OUT
w
Output for external charge pump. w is an open drain N-channel transistor and requires a pull-up
p
p
resistor.
19
PWDN
I
Power Down (with internal pull-up resistor).
e
e
PWDN
PWDN
HIGH for normal operation.
LOW for power saving.
Power down function is gated by the return of the charge pump to a TRI-STATE condition.
20
w
O
Output for external charge pump. w is a CMOS logic output.
r
r
2,9,12
NC
No connect.
http://www.national.com
2
Functional Block Diagram
TL/W/12459–3
Note 1: The power down function is gated by the charge pump to prevent any unwanted frequency jumps. Once the power down pin is brought low the part will go
into power down mode when the charge pump reaches a TRI-STATE condition.
3
http://www.national.com
Absolute Maximum Ratings (Notes 1 and 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Recommended Operating
Conditions
Power Supply Voltage
V
CC
V
P
2.65V to 5.5V
a
to 5.5V
Power Supply Voltage
V
CC
40 C to 85 C
b
b
a
0.3V to 6.5V
a
0.3V to 6.5V
V
V
CC
P
b
a
Operating Temperature (T )
A
§
§
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to
the device may occur. Operating Ratings indicate conditions for which the
device is intended to be functional, but do not guarantee specific perform-
ance limits. For guaranteed specifications and test conditions, see the Elec-
trical Characteristics. The guaranteed specifications apply only for the test
conditions listed.
Voltage on Any Pin
e
b
a
with GND
0V (V )
I
0.3V to V
CC
0.3V
b
a
65 C to 150 C
Storage Temperature Range (T )
S
§
§
a
Lead Temperature (T ) (solder, 4 sec.)
L
260 C
§
Note 2: This device is a high performance RF integrated circuit with an ESD
k
rating
2 keV and is ESD sensitive. Handling and assembly of this device
should only be done at ESD workstations.
k
k
85 C, except as specified
e
e
b
2.75V; 40 C
Electrical Characteristics V
2.75V, V
T
A
§
§
CC
P
Symbol
Parameter
Power Supply Current
Power Down Current
RF Input Operating Frequency
Conditions
Min
Typ
4
Max
6
Units
mA
I
I
CC
30
180
550
22
mA
CC-PWDN
f
f
f
45
5
MHz
MHz
MHz
dBm
IN
Oscillator Input Operating Frequency
Phase Detector Frequency
Input Sensitivity
OSC
w
10
e
CC
b
a
6
Pf
V
2.65V to 5.5V
10
0.5
0.7 V
IN
OSC
IH
V
V
V
Oscillator Sensitivity
OSC
V
PP
IN
High-Level Input Voltage
*
*
V
CC
Low-Level Input Voltage
0.3 V
CC
V
IL
e
e
b
b
I
I
I
I
I
I
High-Level Input Current (Clock, Data)
Low-Level Input Current (Clock, Data)
Oscillator Input Current
V
IH
V
IL
V
IH
V
IL
V
IH
V
IL
V
5.5V
1.0
1.0
1.0
mA
mA
mA
mA
mA
mA
IH
CC
e
e
e
e
e
e
0V, V
CC
5.5V
1.0
IL
IH
IL
IH
IL
e
V
CC
5.5V
100
e
b
100
0V, V
5.5V
CC
e
b
1.0
High-Level Input Current (LE, FC)
Low-Level Input Current (LE, FC)
V
CC
5.5V
1.0
1.0
e
b
100
0V, V
5.5V
CC
*Except f and OSC
IN
IN
http://www.national.com
4
k
k
85 C, except as specified (Continued)
e
e
b
2.75V; 40 C
Electrical Characteristics V
2.75V, V
T
A
§
§
CC
P
Symbol
Parameter
Conditions
Min
Typ
Max
Units
mA
e
b
2.5
2.5
b
1.0
I
I
I
Charge Pump Output Current
V
V
V /2
P
D
D
D
-source
-sink
-Tri
D
o
o
o
o
o
e
s
V /2
P
1.0
mA
D
s
k
b
Charge Pump TRI-STATE Current
É
0.5V
T
V
e b
V
0.5V
D
40 C
P
T
o
§
b
5.0
5.0
nA
k
85 C
§
A
A
e b
b
b
V
OH
V
OL
V
OH
V
OL
High-Level Output Voltage
Low-Level Output Voltage
I
1.0 mA**
200 mA
V
V
0.8
0.8
V
V
OH
OL
OH
OL
CC
e
I
I
I
1.0 mA**
0.4
0.4
e b
High-Level Output Voltage (OSC
)
V
OUT
CC
e
Low-Level Output Voltage (OSC
)
200 mA
V
OUT
e
OL
I
I
Open Drain Output Current (w )
V
V
0.4V
1.0
mA
mA
ns
ns
ns
ns
ns
ns
OL
OH
p
e
OH
Open Drain Output Current (w )
2.75V
100
p
t
t
t
t
t
t
Data to Clock Set Up Time
Data to Clock Hold Time
Clock Pulse Width High
Clock Pulse Width Low
Clock to Enable Set Up Time
Enable Pulse Width
See Data Input Timing
See Data Input Timing
See Data Input Timing
See Data Input Timing
See Data Input Timing
See Data Input Timing
50
10
50
50
50
50
CS
CH
CWH
CWL
ES
EW
**Except OSC
OUT
5
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Functional Description
The simplified block diagram below shows the 19-bit data register, the 14-bit R Counter and the R15 Latch, and the 18-bit
N Counter (intermediate latches are not shown). The data stream is clocked (on the rising edge) into the DATA input, MSB first.
If the Control Bit (last bit input) is HIGH, the DATA is transferred into the R Counter (programmable reference divider) and the
S Latch (prescaler select: 64/65 or 128/129). If the Control Bit (LSB) is LOW, the DATA is transferred into the N Counter
(programmable divider).
TL/W/12459–1
PROGRAMMABLE REFERENCE DIVIDER (R COUNTER) AND PRESCALER SELECT (R15 LATCH)
If the Control Bit (last bit shifted into the Data Register) is HIGH, data is transferred from the 19-bit shift register into a 14-bit
latch (which sets the 14-bit R Counter) and the 1-bit R15 Latch, which sets the prescaler: 64/65 or 128/129. Serial data format
is shown below.
TL/W/12459–14
14-BIT PROGRAMMABLE REFERENCE DIVIDER RATIO
(R COUNTER)
1-BIT PRESCALER SELECT
(R15 LATCH)
Divide
Prescaler
R
Select
R
R
R
R
R
R
9
R
8
R
7
R
6
R
5
R
4
R
3
R
2
R
1
Ratio
R
14 13 12 11 10
15
P
3
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
1
#
1
0
#
1
0
#
128/129
64/65
0
1
4
#
16383
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Notes: Divide ratios less than 3 are prohibited.
Divide ratio: 3 to 16383
R1 to R14: These bits select the divide ratio of the programmable
reference divider.
C: Control bit (set to HIGH level to load R counter and R15 Latch)
Data is shifted in MSB first.
http://www.national.com
6
Functional Description (Continued)
PROGRAMMABLE DIVIDER (N COUNTER)
The N counter consists of the 7-bit swallow counter (A counter) and the 11-bit programmable counter (B counter). If the Control
Bit (last bit shifted into the Data Register) is LOW, data is transferred from the 19-bit shift register into a 7-bit latch, which sets
the 7-bit Swallow (A) Counter, and an 11-bit latch, which sets the 11-bit programmable (B) Counter. Serial data format is shown
below.
TL/W/12459–15
Note: S8 to S18: Programmable counter divide ratio control bits (3 to 2047)
7-BIT SWALLOW COUNTER DIVIDE RATIO
(A COUNTER)
11-BIT PROGRAMMABLE COUNTER DIVIDE RATIO
(B COUNTER)
Divide
Divide
N
7
N
6
N
5
N
4
N
3
N
2
N
1
N
N
N
N
N
N
N
N
N
N
9
N
8
Ratio
A
Ratio
B
18 17 16 15 14 13 12 11 10
0
1
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
1
#
3
4
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
0
#
0
1
#
1
0
#
1
0
#
#
#
127
1
1
1
1
1
1
1
2047
1
1
1
1
1
1
1
1
1
1
1
Note: Divide ratio: 0 to 127
Note: Divide ratio: 3 to 2047 (Divide ratios less than 3 are prohibited)
t
t
A
B
A
B
PULSE SWALLOW FUNCTION
e
c
(P B)
a
c
]
A f /R
OSC
[
f
f
VCO
: Output frequency of external voltage controlled oscil-
lator (VCO)
VCO
B:
A:
Preset divide ratio of binary 11-bit programmable
counter (3 to 2047)
Preset divide ratio of binary 7-bit swallow counter
s
s
s
127, A B)
(0
A
f
: Output frequency of the external reference frequency
oscillator
OSC
R:
P:
Preset divide ratio of binary 14-bit programmable ref-
erence counter (3 to 16383)
Preset modulus of dual modulus prescaler (64 or 128)
7
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Functional Description (Continued)
SERIAL DATA INPUT TIMING
TL/W/12459–16
Notes: Parenthesis data indicates programmable reference divider data.
Data shifted into register on clock rising edge.
Data is shifted in MSB first.
Test Conditions: The Serial Data Input Timing is tested using a symmetrical waveform around V /2. The test waveform has an edge rate of 0.6 V/ns with
CC
@
@
e
e
5.5V.
amplitudes of 2.2V
V
2.7V and 2.6V
V
CC
CC
Phase Characteristics
In normal operation, the FC pin is used to reverse the polari-
ty of the phase detector. Both the internal and any external
charge pump are affected.
VCO Characteristics
Depending upon VCO characteristics, FC pin should be set
accordingly:
When VCO characteristics are like (1), FC should be set
HIGH or OPEN CIRCUIT;
When VCO characteristics are like (2), FC should be set
LOW.
When FC is set HIGH or OPEN CIRCUIT, the monitor pin of
the phase comparator input, f , is set to the reference
out
TL/W/12459–17
divider output, f . When FC is set LOW, f
out
is set to the
r
programmable divider output, f .
p
PHASE COMPARATOR AND INTERNAL CHARGE PUMP CHARACTERISTICS
TL/W/12459–18
b
a
Notes: Phase difference detection range: 2q to 2q
The minimum width pump up and pump down current pulses occur at the D pin when the loop is locked.
o
e
FC
HIGH
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8
Analog Switch
The analog switch is useful for radio systems that utilize a frequency scanning mode and a narrow band mode. The purpose of
the analog switch is to decrease the loop filter time constant, allowing the VCO to adjust to its new frequency in a shorter
amount of time. This is achieved by adding another filter stage in parallel. The output of the charge pump is normally through the
D
pin, but when LE is set HIGH, the charge pump output also becomes available at BISW. A typical circuit is shown below. The
second filter stage (LPF-2) is effective only when the switch is closed (in the scanning mode).
o
TL/W/12459–19
Typical Crystal Oscillator Circuit
A typical circuit which can be used to implement a crystal
oscillator is shown below.
Typical Lock Detect Circuit
A lock detect circuit is needed in order to provide a steady
LOW signal when the PLL is in the locked state. A typical
circuit is shown below.
TL/W/12459–20
TL/W/12459–21
9
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Typical Application Example
Operational Notes:
TL/W/12459–22
*
VCO is assumed AC coupled.
**
R increases impedance so that VCO output power is provided to the load rather than the PLL. Typical values are 10X to 200X depending on the VCO power
IN
level. f RF impedance ranges from 40X to 100X.
IN
*** 50X termination is often used on test boards to allow use of external reference oscillator. For most typical products a CMOS clock is used and no terminating
resistor is required. OSC may be AC or DC coupled. AC coupling is recommended because the input circuit provides its own bias. (See Figure below)
IN
TL/W/12459–23
Application Hints:
Proper use of grounds and bypass capacitors is essential to achieve a high level of performance.
Crosstalk between pins can be reduced by careful board layout.
This is an electrostatic sensitive device. It should be handled only at static free work stations.
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10
Application Information
LOOP FILTER DESIGN
A block diagram of the basic phase locked loop is shown.
TL/W/12459–24
FIGURE 1. Basic Charge Pump Phase Locked Loop
An example of a passive loop filter configuration, including
the transfer function of the loop filter, is shown in Figure 2.
TL/W/12459–25
a
s (C2 R2)
#
(C1 C2 R2)
1
e
Z(s)
2
s
a
a
sC2
sC1
#
#
FIGURE 2. 2nd Order Passive Filter
TL/W/12459–26
Define the time constants which determine the pole and
zero frequencies of the filter transfer function by letting
FIGURE 3. Open Loop Transfer Function
Thus we can calculate the 3rd order PLL Open Loop Gain in
terms of frequency
e
T2
R2 C2
#
(1a)
and
b
a
j0 T2) T1
Kw
K
(1
#
#
VCO
e
0
#
G(s) H(s)
#
#
e
l
s
j
C1 C2
#
2
a
0 C1 N(1
j0 T1)
#
T2 (2)
#
e
T1
R2
#
a
C1
C2
(1b)
From equation 2 we can see that the phase term will be
dependent on the single pole and zero such that
The PLL linear model control circuit is shown along with the
open loop transfer function in Figure 3. Using the phase
detector and VCO gain constants Kw and K
loop filter transfer function Z(s) , the open loop Bode plot
can be calculated. The loop bandwidth is shown on the
Bode plot (0p) as the point of unity gain. The phase margin
is shown to be the difference between the phase at the unity
b
b
1
1
e
b
a
e
w(0)
tan
(0 T2) tan
#
(0 T1)
180 (3)
§
#
[
]
and the
VCO
By setting
[
]
dw
T2
b
2
T1
e
0
2
a
a
(0 T1)
d0
1
(0 T2)
1
(4)
#
#
we find the frequency point corresponding to the phase in-
flection point in terms of the filter time constants T1 and T2.
This relationship is given in equation 5.
b
gain point and 180 .
§
e
0
1/ T2 T1
(5)
#
For the loop to be stable the unity gain point must occur
0
p
b
before the phase reaches
180 degrees. We therefore
want the phase margin to be at a maximum when the magni-
tude of the open loop gain equals 1. Equation 2 then gives
TL/W/12459–27
a
a
Kw
K
T1 (1
#
T2 (1
j0 T2)
#
#
VCO
p
e
C1
e
/Ns
VCO
e
H(s) G(s)
Open Loop Gain
e
i /i
i
e
2
0
N
j0 T1)
#
#
#
Ó
Ó
http://www.national.com
(6)
p
p
Kw Z(s) K
e
e
i
a
[
G(s)/ 1 H(s) G(s)
]
Closed Loop Gain
i /i
o
11
Application Information (Continued)
Therefore, if we specify the loop bandwidth, 0 , and the
In choosing the loop filter components a trade off must be
made between lock time, noise, stability, and reference
spurs. The greater the loop bandwidth the faster the lock
time will be, but a large loop bandwidth could result in higher
reference spurs. Wider loop bandwidths generally improve
close in phase noise but may increase integrated phase
noise depending on the reference input, VCO and division
ratios used. The reference spurs can be reduced by reduc-
ing the loop bandwidth or by adding more low pass filter
stages but the lock time will increase and stability will de-
crease as a result.
p
phase margin, w , Equations 1 through 6 allow us to calcu-
p
late the two time constants, T1 and T2, as shown in equa-
tions 7 and 8. A common rule of thumb is to begin your
design with a 45 phase margin.
§
b
secw
tanw
p
p
e
T1
0
(7)
(8)
p
1
e
T2
2
0
T1
#
p
From the time constants T1, and T2, and the loop band-
width, 0 , the values for C1, R2, and C2 are obtained in
THIRD ORDER FILTER
p
equations 9 to 11.
A low pass filter section may be needed for some applica-
tions that require additional rejection of the reference side-
bands, or spurs. This configuration is given in Figure 4. In
order to compensate for the added low pass section, the
component values are recalculated using the new open
loop unity gain frequency. The degradation of phase margin
caused by the added low pass is then mitigated by slightly
increasing C1 and C2 while slightly decreasing R2.
2
a
a
T1 Kw
K
1
1
(0 T2)
#
#
VCO
p
e
C1
#
2
2
T2
0
N
(0 T1)
(9)
(10)
(11)
#
#
0
T2
T1
p
p
e
b
1
C2
C1
R2
#
# J
T2
e
C2
K
(MHz/V)
Voltage Controlled Oscillator (VCO)
Tuning Voltage constant. The fre-
quency vs voltage tuning ratio.
The added attenuation from the low pass filter is:
2
VCO
e
a
]
1
[
ATTEN
20 log (2qf
R3 C3)
(12)
(13)
#
#
Defining the additional time constant as
ref
Kw (mA)
Phase detector/charge pump gain
constant. The ratio of the current out-
put to the input phase differential.
e
T3
R3 C3
#
Then in terms of the attenuation of the reference spurs add-
ed by the low pass pole we have
N
Main divider ratio. Equal to RF /f
opt ref
ATTEN/20
b
2
10
1
RF (MHz)
opt
Radio Frequency output of the VCO at
which the loop filter is optimized.
e
T3
(14)
0
(2q
f
ref
)
#
We then use the calculated value for loop bandwidth 0 in
c
equation 11, to determine the loop filter component values
f
ref
(kHz)
Frequency of the phase detector in-
puts. Usually equivalent to the RF
channel spacing.
in equations 15–17. 0 is slightly less than 0 , therefore
c
the frequency jump lock time will increase.
p
1
e
e
e
T2
2
a
0
(T1
T3)
(15)
(16)
(17)
#
c
2
a
a
a
tanw (T1
T3)
(T1
T3)
a
tanw (T1
T1 T3
#
T3)
#
T3)
a
b
1
0
1
#
c
2
2
a
a
[
]
[
]
(T1
T1 T3
#
#
Ð0
(
2
#
c
2
T2 )
(/2
a
T1 Kw
K
(1
2
0
#
VCO
C1
#
#
2
2
2
2
T3 )
a
a
0
T2
0
N
(1
0
T1 ) (1
#
#
#
c
Ð
(
c
c
http://www.national.com
12
Application Information (Continued)
EXTERNAL CHARGE PUMP
EXAMPLE
e
e
e
50
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.
Typical Device Parameters
Typical System Parameters
b
100, b
n
p
V
V
V
I
5.0V;
P
e
b
4.5V;
0.5V
cntl
wp
e
e
e
0.0V; V
5.0V
wr
e
Design Parameters
I
5.0 mA;
SINK
SOURCE
e
e
V
fn
V
fp
0.8V
e
e
I
I
1 mA
rmax
pmax
e
e
0.3V
V
V
V
R5
R8
OLwp
e
e
100 mV
V
OHwr
One possible architecture for an external charge pump cur-
rent source is shown in Figure 9. The signals w and w in
p
r
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 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
k
258 mV, or 42 mV R8, 5, due to the current density differ-
Ó
TL/W/12459–28
À
ences 0.026*1n (5 mA/1 mA) through the Q1, Q2/Q3, Q4
pairs.
FIGURE 9
In order to calculate the value of R7 it is necessary to first
estimate the forward base to emitter voltage drop (Vfn,p) of
Therefore select
b
0.3V 0.026 1n(5.0 mA/1.0 mA)
#
the transistors used, the V drop of wp, and the V
OL OH
drop
e
e
e
e
e
R
9
e
51.6X
R
R
R
R
4
5
8
6
k
of wr’s under 1 mA loads. (wp’s V 0.1V and wr’s
5 mA
OL
k
V
0.1V.)
OH
0.3V
e
e
300X
300X
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.
1.0 mA
0.3V
1.0 mA
i
source
b
b
(5V 0.1V) (0.3V
a
0.8V)
b
V
V
ln
#
T
R5
e
e
3.8 kX
R
7
i
# J
p max
e
1.0 mA
R
4
i
source
i
sink
b
V
R8
V
T
ln
#
i
# J
n max
e
e
R
R
9
5
i
sink
V
R5
i
i
p max
V
R8
e
e
R
R
R
8
6
r max
b
b
b
a
a
(V
V
V
)
(V
R5
Vfp)
Vfn)
p
VOLwp
i
p max
b
(V
)
(V
R8
P
VOHwr
e
7
i
max
13
http://www.national.com
Physical Dimensions inches (millimeters) unless otherwise noted
NS Package Number MTC20
20-Lead (0.173 Wide) Thin Shrink Small Outline Package (TM)
×
Order Number LMX2305TM
For Tape and Reel Order Number LMX2305TMX (2500 Units per Reel)
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DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL
SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and whose
failure to perform, when properly used in accordance
with instructions for use provided in the labeling, can
be reasonably expected to result in a significant injury
to the user.
2. A critical component is any component of a life
support device or system whose failure to perform can
be reasonably expected to cause the failure of the life
support device or system, or to affect its safety or
effectiveness.
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Corporation
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Europe
National Semiconductor
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Japan Ltd.
a
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