Agere LCK4950 Low-voltage pll clock driver Datasheet

Data Sheet
November 2001
LCK4950
Low-Voltage PLL Clock Driver
Features
■
Fully integrated phase-locked loop (PLL)
■
Oscillator or crystal reference input
■
Output frequency up to 180 MHz
■
Outputs disable in high impedance
■
Compatible with PowerPC®, Intel ®, and highperformance RISC microprocessors
■
TQFP packaging
■
Output frequency configurable
■
±35 ps typical cycle-to-cycle jitter
■
Pin compatible with the Motorola® MPC950 clock
driver
Description
The LCK4950 is a PLL-based clock driver device
intended for high-performance clock tree designs.
The LCK4950 is 3.3 V compatible with output
frequencies of up to 180 MHz and output skews of
200 ps. The LCK4950 can accommodate the most
demanding tree designs by employing a fully
differential PLL design. This minimizes cycle-to-cycle
jitter, which is critical when the device is acting as the
reference clock for PLLs in today’s microprocessors
and ASICs. The device has nine low-skew
configurable outputs for support of the clocking
needs of the various high-performance
microprocessors.
To provide input reference clock flexibility, two
selectable division ratios are available on the
LCK4950. The internal VCO runs at either 2x or 4x
the high-speed output. The FBSEL pin is used to
select between a divide by 8 or a divide by 16 of the
VCO frequency to be compared with the input
reference. These selections allow the input reference
to be either one-half, one-fourth, or one-eighth of the
high-speed output.
The LCK4950 is capable of scan clock distribution or
system diagnostics due to an external test clock
input. The REF_SEL pin allows the selection
between a crystal input to an on-chip oscillator for the
reference or selection of a TTL level oscillator input
directly. Only a parallel resonant crystal is required
for the onboard crystal oscillator external
components.
The LCK4950 is fully 3.3 V compatible and requires
no external loop filter components. All inputs accept
LVCMOS or LVTTL compatible levels while the
outputs provide LVCMOS levels with the capability to
drive terminated 50 Ω transmission lines. The
LCK4950 can drive two traces, giving the device an
effective fan out of 1:18 for series-terminated 50 Ω
lines. For optimum performance and board density,
the device is packaged in a 7 mm x 7 mm 32-lead
TQFP package.
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Description (continued)
fsela
PLL_En
Tclk
Ref_Sel
PHASE
DETECTOR
xtal1
xtal2
(VCO)
200 MHz—480 MHz
÷2/÷4
Qa
÷4/÷8
Qb
÷4/÷8
Qc0
xtal
OSC
LPF
÷8/÷16
(PULL-DOWN)
FBsel
fselb
Qc1
fselc
MR/OE
POWER-ON RESET
÷4/÷8
Qd0
Qd1
fseld
Qd2
Qd3
Qd4
5-9698 (F)
Figure 1. Logic Diagram
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Agere Systems Inc.
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Qc0
VDD
Qc1
VSS
Qd0
VDD
Qd1
VSS
24
23
22
21
20
19
18
17
Pin Information
VSS
25
16
Qd2
Qb
26
15
VDD
VDD
27
14
Qd3
Qa
28
13
VSS
LCK4950
8
xtal1
9
7
32
VSS
Ref_Sel
6
MR/OE
fseld
10
5
31
fselc
PLL_En
4
VDD
fselb
11
3
30
fsela
TCLK
2
Qd4
FBsel
12
1
29
VDDA
VSS
xtal2
5-9699 (F)
Figure 2. Pin Diagram
Functional Description
Table 1. Function Tables
Ref_Sel
Function
1
TCLK
0
XTAL_OSC
PLL_En
Function
1
PLL Enabled
0
PLL Bypass
FBsel
Function
1
÷8
0
÷16
MR/OE
Function
1
Outputs Disabled
0
Outputs Enabled
fseln
Function
1
Qa = ÷4; Qb:d = ÷8
0
Qa = ÷2; Qb:d = ÷4
Agere Systems Inc.
3
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Functional Description (continued)
Table 2. Function Table
Inputs
Outputs
Totals
fsela
fselb
fselc
fseld
Qa(1)
Qb(1)
Qc(2)
Qd(5)
Total 2x
Total x
Total x/2
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
2x
2x
2x
2x
2x
2x
2x
2x
x
x
x
x
x
x
x
x
x
x
x
x
x/2
x/2
x/2
x/2
x
x
x
x
x/2
x/2
x/2
x/2
x
x
x/2
x/2
x
x
x/2
x/2
x
x
x/2
x/2
x
x
x/2
x/2
x
x/2
x
x/2
x
x/2
x
x/2
x
x/2
x
x/2
x
x/2
x
x/2
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
8
3
6
1
7
2
3
0
9
4
7
2
8
3
6
1
0
5
2
7
1
6
5
8
0
5
2
7
1
6
3
8
Note: x = fVCO/4; 200 MHz < fVCO < 480 MHz.
Table 3. PLL Input Reference Characteristics
Characteristic
TCLK Input Rise/Falls
Reference Input Frequency
Crystal Oscillator Frequency†
Reference Input Duty Cycle
Symbol
Min
Max
Unit
tr, tf
fref
fXtal
frefdc
—
—*
12.5
25
3.0
—*
25
75
ns
MHz
MHz
%
* Maximum and minimum input reference is limited by the VCO lock range and the feedback divider for the TCLK or xtal1 inputs.
† See the Applications section for more crystal information.
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Agere Systems Inc.
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Applications
Programming the LCK4950S
Several frequency relationships are configurable by the LCK4950. Frequency ratios of 1:1, 2:1, 4:1, and 4:2:1 are
possible from configuring the output dividers for the four output groups. To ensure that the output duty cycle is
always 50%, the LCK4950 uses even dividers. Table 4 illustrates output configurations of the LCK4950, describing
the outputs using the VCO frequency as a reference. For example, setting the Qa outputs to VCO/2, the Qb and Qc
to VCO/4, and the Qd to VCO/8 would provide the output frequency relationship of 4:2:1.
Table 4. Programmable Output Frequency Relationships
Inputs
Outputs
FSELA
FSELB
FSELC
FSELD
Qa
Qb
Qc
Qd
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
VCO/2
VCO/2
VCO/2
VCO/2
VCO/2
VCO/2
VCO/2
VCO/2
VCO/4
VCO/4
VCO/4
VCO/4
VCO/4
VCO/4
VCO/4
VCO/4
VCO/4
VCO/4
VCO/4
VCO/4
VCO/8
VCO/8
VCO/8
VCO/8
VCO/4
VCO/4
VCO/4
VCO/4
VCO/8
VCO/8
VCO/8
VCO/8
VCO/4
VCO/4
VCO/8
VCO/8
VCO/4
VCO/4
VCO/8
VCO/8
VCO/4
VCO/4
VCO/8
VCO/8
VCO/4
VCO/4
VCO/8
VCO/8
VCO/4
VCO/8
VCO/4
VCO/8
VCO/4
VCO/8
VCO/4
VCO/8
VCO/4
VCO/8
VCO/4
VCO/8
VCO/4
VCO/8
VCO/4
VCO/8
The division settings establish the output relationship, but one must still ensure that the VCO will be stable given the
frequency of the outputs desired. The feedback frequency should be used to situate the VCO into a frequency
range in which the PLL will be stable. The design of the PLL is such that for output frequencies between 25 MHz
and 180 MHz, the LCK4950 can generally be configured into a stable region.
Agere Systems Inc.
5
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Applications (continued)
Table 5. Input Reference vs. Output Frequency Relationships
Config
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
fsela
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
fselb
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
fselc
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
fseld
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
FB_Sel = 1
FB_Sel = 0
Qa
Qb
Qc
Qd
Qa
Qb
Qc
Qd
4x
4x
4x
4x
4x
4x
4x
4x
2x
2x
2x
2x
2x
2x
2x
2x
2x
2x
2x
2x
x
x
x
x
2x
2x
2x
2x
x
x
x
x
2x
2x
x
x
2x
2x
x
x
2x
2x
x
x
2x
2x
x
x
2x
x
2x
x
2x
x
2x
x
2x
x
2x
x
2x
x
2x
x
8x
8x
8x
8x
8x
8x
8x
8x
4x
4x
4x
4x
4x
4x
4x
4x
4x
4x
4x
4x
2x
2x
2x
2x
4x
4x
4x
4x
2x
2x
2x
2x
4x
4x
2x
2x
4x
4x
2x
2x
4x
4x
2x
2x
4x
4x
2x
2x
4x
2x
4x
2x
4x
2x
4x
2x
4x
2x
4x
2x
4x
2x
4x
2x
The relationship between the input reference and the output frequency is very flexible. Table 5 shows the
multiplication factors between the inputs and outputs for the LCK4950. Figure 3 through Figure 6 illustrate several
programming possibilities.
Note: The variations of the configurations shown are not complete, but do represent potential applications.
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Agere Systems Inc.
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Applications (continued)
1
fsela
1
fselb
1
fselc
0
fseld
0
16.66 MHz
FBsel
INPUT REF
Qa
Qb
Qc
Qd
1
66.66 MHz
1
33.33 MHz
2
33.33 MHz
5
66.66 MHz
1
fsela
1
fselb
1
fselc
0
fseld
0
FBsel
16.66 MHz
INPUT REF
Qa
Qb
Qc
Qd
1
1
2
5
66.66 MHz
33.33 MHz
33.33 MHz
33.33 MHz
5-9700 (F)
5-9702 (F)
Figure 3. Dual-Frequency Configuration
1
fsela
0
fselb
0
fselc
1
fseld
1
33.33 MHz
FBsel
INPUT REF
Qa
Qb
Qc
Qd
1
Figure 5. Dual-Frequency Configuration
66.66 MHz
1
66.66 MHz
2
66.66 MHz
5
33.33 MHz
0
fsela
0
fselb
1
fselc
1
fseld
0
FBsel
20 MHz
INPUT REF
Qa
Qb
Qc
Qd
1
1
2
5
160 MHz
80 MHz
40 MHz
40 MHz
5-9701 (F)
5-9703 (F)
Figure 4. Dual-Frequency Configuration
Figure 6. Triple-Frequency Configuration
Absolute Maximum Ratings
Stresses in excess of the absolute maximum ratings can cause permanent damage to the device. These are
absolute stress ratings only. Functional operation of the device is not implied at these or any other conditions in
excess of those given in the operational sections of the data sheet. Exposure to absolute maximum ratings for
extended periods can adversely affect device reliability.
Table 6. Absolute Maximum Ratings
Parameter
Supply Voltage
Input Voltage
Input Current
Storage Temperature Range
Agere Systems Inc.
Symbol
Min
Max
Unit
VDD, VDDA
VI
IIN
TStor
–0.3
–0.3
—
–40
4.6
VDD + 0.3
±20
125
V
V
mA
°C
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LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Electrical Characteristics
Table 7. dc Characteristics
TA = 0 °C to 70 °C, VCC = 3.3 V ± 5%.
Characteristic
Input High Voltage (LVCMOS inputs)
Input Low Voltage (LVCMOS inputs)
Output High Voltage
Output Low Voltage
Input Current
Input Capacitance
Power Dissipation Capacitance
Maximum Quiescent Supply Current
Non-PLL
Maximum PLL Supply Current
Symbol
Min
Typ
Max
Unit
Condition
VIH
VIL
VOH
VOL
IIN
CIN
Cpd
IDDQ
2.0
—
2.4
—
—
—
—
—
—
—
—
—
—
—
25
—
3.6
0.8
—
0.5
±120
4
—
1
V
V
V
V
µA
pF
pF
mA
—
—
IOH = –40 mA1
IDDPLL
—
—
55
mA
IOL = 40 mA1
—
—
Per Output
All VDD Pins
Except VDDA2
VDDA Pin Only
1. The LCK4950 outputs can drive series or parallel-terminated 50 Ω (or 50 Ω to VCC/2) transmission lines on the incident edge.
2. Total power = (IDDPLL + IDDQ) x V + (fQaCQa + fQbCQb + fQc0CQc0 + fQc1CQc1 + fQd0CQd0 + fQd1CQd1 + fQd2CQd2 + fQd3CQd3 +
fQd4CQd4) x V2; where V = VDD, CQa = load capacitance on Qa, CQb = load capacitance on Qb, etc.
Table 8. ac Characteristics
TA = 0 °C to 70 °C, VCC = 3.3 V ± 5%.
Characteristic
Output Rise/Fall Time
Output Duty Cycle
Same Frequencies
Output-to-Output Skews
Different Frequencies:
Qafmax < 150 MHz
Qafmax > 150 MHz
PLL VCO (feedback = VCO/4)
Lock (feedback = VCO/8)
Range (feedback = VCO/16)
Maximum Output Frequency:
Qa(÷2)
Qa/Qb (÷4)
Qb (÷8)
Output Disable Time
Output Enable Time
Cycle-to-Cycle Jitter (peak-to-peak)
Maximum PLL Clock Time
Symbol
Min
Typ
Max
Unit
Condition
tr, tf
tpw
tsk(0)
0.10
48.5
—
—
1.0
52.5
ns
%
0.8 V to 2.0 V
—
—
150
300
ps
—
—
—
200
200
200
200
—
—
—
—
400
400
480
480
480
ps
ps
MHz
MHz
MHz
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
±35
—
180
120
60
7
6
±50
10
MHz
MHz
MHz
ns
ns
ps
ms
—
—
—
—
—
—1
—
fVCO
fmax
tPLZ,HZ
tPZL
tjitter
tlock
1.See Applications section for more information.
8
Agere Systems Inc.
Data Sheet
November 2001
LCK4950
Low-Voltage PLL Clock Driver
Electrical Characteristics (continued)
Jitter Performance of the LCK4950S
More focus is given to clock distribution design and management today because of the continuing increase of
today’s digital system’s clock rates. System-clock jitter and its effect on overall system timing budget is at the
center of this focus. The LCK4950 is designed to utilize a differential CMOS PLL and incorporate multiple power
and ground pins in the design to minimize clock jitter. The following text provides details on the jitter performance,
illustrates measurement limitations, and provides guidelines to minimize the jitter of the LCK4950.
The most commonly specified jitter parameter is cycle-to-cycle jitter. With today’s high-performance measurement
equipment, there is no way to measure this parameter for jitter performance in the class demonstrated by the
LCK4950. As a result, different methods are used which approximate cycle-to-cycle jitter. The typical method of
measuring the jitter is to accumulate a large number of cycles, create a histogram of the edge placements, and
record peak-to-peak as well as standard deviations of the jitter. It is of great importance to measure the edge
immediately following the trigger edge. If this is not the case, the measurement inaccuracy will add significantly to
the measured jitter. The oscilloscope cannot collect adjacent pulses. It is safe to assume that collecting pulse
information in this mode will produce jitter values somewhat larger than if consecutive cycles were measured;
therefore, this measurement will represent an upper bound of cycle-to-cycle jitter. Most likely, this is a conservative
estimate of the cycle-to-cycle jitter.
There are two common sources of jitter for a PLL-based clock driver. The most common source of jitter is random
jitter. Less commonly known is the jitter produced by different frequency outputs switching synchronously. If all of
the outputs are switching at the same frequency, the PLL jitter is equal to the total jitter of the device. In the
LCK4950, where a number of the outputs can be switching synchronously but at different frequencies, a
multimodal jitter distribution can be seen on the highest frequency outputs. It is important to consider what is
happening on the other outputs because the output being monitored is affected by the activity on the other outputs.
From Figure 7, one can see that for each rising edge on the higher-frequency signal, the activity on the lowerfrequency signal is not consistent.
The placement of the edge that is being monitored is displaced in time due to the activity on the other outputs
altering the internal thresholds of the device. The relationship is periodic because the signals are synchronous.
The resulting jitter is a superposition of the PLL jitter on the displaced edges. The multimodal distribution will
appear to be a fat Gaussian distribution, or a truly multimodal distribution depending on the size of the PLL jitter
and displacement of the edges. When all the outputs are switching at the same frequency, there is no edge
displacement and the jitter is that of the PLL.
Agere Systems Inc.
9
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Electrical Characteristics (continued)
1
2
1
2
1
2
PEAK-TO-PEAK PLL JITTER
PEAK-TO-PEAK PERIOD JITTER
1
2
3
1
2
1
2
2
3
3
PEAK-TO-PEAK PLL JITTER
PEAK-TO-PEAK PERIOD JITTER
5-9704 (F)
Figure 7. PLL Jitter and Edge Displacement
10
Agere Systems Inc.
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Electrical Characteristics (continued)
Power Supply Filtering
3.3 V
RS =
5 Ω—10 Ω
PLL_VCC
0.01 µF
22 µF
LCK4950
VCC
0.01 µF
The LCK4950 exhibits some sensitivities that would not
be seen on a fully digital product because the LCK4950
is a mixed analog/digital product. Analog circuitry is
naturally sensitive to random noise, most noticeably
when the noise is in the power supply pins. The
LCK4950 provides a separate output buffer power
supply (VDD) and phase-locked loop (VDDA) power
supply pins. This design isolates the high
switching noise digital outputs from the sensitive
internal analog phase-locked loop. In a controlled
setup (i.e., an evaluation board), this amount of
isolation will suffice. In a digital system, where it is
much more difficult to minimize noise on the power
supplies, an additional level of isolation may be
required. The easiest means of accomplishing this is
by applying a power supply filter on the VDDA pin for
the LCK4950.
5-9707 (F)
Figure 8. Power Supply Filter
PLL jitter can be measured for configurations where
the outputs are switching at different frequencies by
triggering the lowest-frequency output. PLL jitter is
dependent on internal VCO frequency more so than
output configuration.
There are some general guidelines that will minimize
the output jitter of the device. First, always configure
the device so the VCO runs as fast as possible. This is
the most important aspect in minimizing jitter of the
LCK4950. Second, maintain the reference frequency at
the highest possible frequency. These more frequent
phase detector updates help to reduce jitter. There is a
trade-off between higher reference frequencies and
higher VCO frequency; always choose a higher VCO
frequency to reduce jitter. Third, and the most difficult
to follow, minimize the number of different frequencies
sourced from a single chip. The fixed edge
displacement associated with the switching noise, in
most cases, nearly doubles the effective jitter of a highspeed output.
Figure 8 illustrates a typical power supply filter scheme
for the LCK4950. The device is most greatly affected
by spectral content in the 1 kHz to 1 MHz range, and
therefore needs a filter to target this range. The most
important aspect of this final filter design is the dc
voltage drop between the VDD supply and VDDA pin.
The IDDPLL current (current forced through the VDDA
pin) is normally 45 mA (55 mA maximum), assuming
that a minimum of 3.0 V must be maintained on the
VDDA pin. Very little voltage drop can be tolerated when
a 3.3 V VDD supply is used. The resistor shown in
Figure 10 must have a resistance of 5 Ω—10 Ω to meet
the voltage drop criteria. The RC filter shown provides
a broadband filter with about 100:1 attenuation for
noise, with a spectral content above 20 kHz. As the
noise frequency crosses the series resonant point of an
individual capacitor, its overall impedance begins to
look inductive and therefore increases with increasing
frequency. The parallel capacitor circuit shown in
Figure 11 guarantees that a low- impedance path to
ground exists for frequencies exceeding the bandwidth
of the PLL. It is recommended that the user start with
an 8 Ω—10 Ω resistor to avoid potential VDD drop
problems and only use higher-value resistors when a
higher level of attenuation is needed.
The LCK4950 has several design features to minimize
the susceptibility to power supply noise (isolated power
and grounds and fully differential PLL). Still, there may
be applications in which overall performance is being
degraded due to system power supply noise. The
power supply filter schemes discussed in this section
should be adequate to eliminate power supply noiserelated problems in most designs.
Agere Systems Inc.
11
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Electrical Characteristics (continued)
Using the On-Chip Crystal Oscillator
The LCK4950 features an on-chip crystal oscillator buffer to allow for seed clock generation as well as final
distribution. The only external component required is the crystal since the on-chip oscillator buffer is completely
self-contained. The user is advised to mount the crystal as close to the LCK4950 as possible to avoid board-level
parasitics since the oscillator is, to a degree, sensitive to loading at the inputs. To facilitate collocation, surfacemount crystals are recommended, but not required.
The oscillator circuit is a parallel resonant circuit with on-chip shunt capacitors. A parallel resonant crystal is simply
a crystal that has been characterized in its parallel resonant mode. Therefore, in the majority of cases, a parallel
specified crystal or a series resonant crystal can be used with the LCK4950 with just a minor frequency error.
Typically, a series crystal used in a parallel resonant mode will exhibit an oscillatory frequency a few hundred ppm
different than the specified value. For most processor implementations, a few hundred ppm translates into kHz
inaccuracies, a small enough level not to represent a major issue.
The LCK4950 is a clock driver that was designed to generate outputs with programmable frequency relationships.
As a result, the crystal input frequency is a function of the desired output frequency. To determine the crystal
required to produce the desired output frequency for an application that utilizes internal feedback, the PLL block
diagram (Figure 9) should be used. The P and the M values for the LCK4950 are also included in Figure 9. The M
values can be found in Table 1 on page 3.
Table 9. Crystal Specifications
Parameter
Crystal Cut
Resonance
Frequency Tolerance
Frequency/Temperature Stability
Operating Range
Shunt Capacitors
Equivalent Series Resistance (ESR)
Correction Drive Level
Aging
12
Value
Fundamental AT Cut
Parallel Resonance
±75 ppm at 25 °C
±150 ppm at 0 °C to 70 °C
0 °C to 70 °C
10 pF—40 pF
50 Ω to 80 Ω Max
100 µW
5 ppm/yr (first three years)
Agere Systems Inc.
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Electrical Characteristics (continued)
fref
VCO
÷P
÷N
Qn
PHASE
DETECTOR
LPF
÷m
5-9708 (F)
Figure 9. PLL Block Diagram
Note: For computations refer to the following equations:
f VCO-, f vco = fQ
f = ------------m
fQn • N • P
m
∴f ref = ------------------------------
m = 8 (FBsel = 1), 16(FBsel = 0), P = 1
For the LCK4950 clock driver, the following will provide an example of how to determine the crystal frequency
required for a given design.
Given:
Qa = 160 MHz
Qb = 80 MHz
Qc = 40 MHz
Qd = 40 MHz
FBSel = 0
fQn • N • P
f ref = -----------------------------m
(eq. 1)
From Figure 3:
fQd = VCO/8 then N = 8 or fQa = VCO/2 then N = 2
From Figure 9:
m = 16 and P = 1
40 • 8 • 1
t ref = ------------------------ = 20 MHz
16
or
160 • 2 • 1
t ref = ---------------------------- = 20 MHz
16
Agere Systems Inc.
13
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Electrical Characteristics (continued)
Driving Transmission Lines
The LCK4950 clock driver was designed to drive high-speed signals in a terminated transmission line environment.
The output drivers were designed to exhibit the lowest impedance possible to provide the optimum flexibility to the
user. With an output impedance of less than 10 Ω, the drivers can drive either parallel-terminated or seriesterminated transmission lines.
Point-to-point distribution of signals is the method of choice in most high-performance clock networks. In a point-topoint scheme, either series-terminated or parallel-terminated transmission lines can be used. The parallel
technique terminates the signal at the end of the line with a 50 Ω resistance to VDD/2. Only a single terminated line
can be driven by each output of the LCK4950 clock driver because this technique draws a fairly high level of dc
current. For the series driven case, however, there is no dc current draw and the outputs can drive multiple seriesterminated lines. Figure 10 illustrates an output driving a single series-terminated line.
OUTPUT
BUFFER
IN
7Ω
RS = 43 Ω
ZO = 50 Ω
OutA
5-9709 (F)
Figure 10. Single Transmission Line
The situation in Figure 11 should be used to better match the impedances when driving multiple lines. In this case,
the series-terminating resistors are reduced so when the parallel combination is added to the output buffer
impedance the line impedance is perfectly matched.
RS = 36 Ω
ZO = 50 Ω
RS = 36 Ω
ZO = 50 Ω
OUTPUT
BUFFER
7Ω
5-9710 (F)
Figure 11. Optimized Dual Line Termination
14
Agere Systems Inc.
LCK4950
Low-Voltage PLL Clock Driver
Data Sheet
November 2001
Outline Diagram
Dimensions are in millimeters.
9.00 ± 0.20
7.00 ± 0.20
1.00 REF
PIN #1
IDENTIFIER ZONE
32
25
0.25
GAGE PLANE
24
1
SEATING PLANE
0.45/0.75
7.00
± 0.20
DETAIL A
9.00
± 0.20
8
17
9
16
0.09/0.200
DETAIL A
DETAIL B
0.30/0.45
1.40 ± 0.05
0.20
1.60 MAX
M
DETAIL B
SEATING PLANE
0.10
0.80 TYP
0.05/0.15
12-3076(F)
Agere Systems Inc.
15
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For additional information, contact your Agere Systems Account Manager or the following:
INTERNET:
http://www.agere.com
E-MAIL:
[email protected]
N. AMERICA: Agere Systems Inc., 555 Union Boulevard, Room 30L-15P-BA, Allentown, PA 18109-3286
1-800-372-2447, FAX 610-712-4106 (In CANADA: 1-800-553-2448, FAX 610-712-4106)
ASIA:
Agere Systems Hong Kong Ltd., Suites 3201 & 3210-12, 32/F, Tower 2, The Gateway, Harbour City, Kowloon
Tel. (852) 3129-2000, FAX (852) 3129-2020
CHINA: (86) 21-5047-1212 (Shanghai), (86) 10-6522-5566 (Beijing), (86) 755-695-7224 (Shenzhen)
JAPAN: (81) 3-5421-1600 (Tokyo), KOREA: (82) 2-767-1850 (Seoul), SINGAPORE: (65) 778-8833, TAIWAN: (886) 2-2725-5858 (Taipei)
EUROPE:
Tel. (44) 7000 624624, FAX (44) 1344 488 045
Agere Systems Inc. reserves the right to make changes to the product(s) or information contained herein without notice. No liability is assumed as a result of their use or application.
Copyright © 2001 Agere Systems Inc.
All Rights Reserved
November 2001
DS02-033HSI (Replaces DS01-118ANET)
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