lmk02002 National Semiconductor Corporation, lmk02002 Datasheet - Page 14

• Manufacturer Part Number: lmk02002
• Description: Precision Clock Conditioner With Integrated Pll
• Manufacturer: National Semiconductor Corporation

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3.4 CURRENT CONSUMPTION / POWER DISSIPATION
CALCULATIONS
Due to the myriad of possible configurations the following ta-
ble serves to provide enough information to allow the user to
From Table 3.4 the current consumption can be calculated in
any configuration. For example, the current for the entire de-
vice with two LVPECL (CLKout0 and CLKout3) outputs in
bypass mode can be calculated by adding up the following
blocks: core current, clock buffers, and two LVPECL output
buffer currents. There will also be two LVPECL outputs draw-
ing emitter current, but some of the power from the current
draw is dissipated in the external 120 Ω resistors which
doesn't add to the power dissipation budget for the device. If
delays or divides are switched in, then the additional current
for these stages needs to be added as well.
For power dissipated by the device, the total current entering
the device is multiplied by the voltage at the device minus the
power dissipated in any emitter resistors connected to any of
the LVPECL outputs. If no emitter resistors are connected to
the LVPECL outputs, this power will be 0 watts. For example,
in the case of two LVPECL (CLKout0 and CLKout3) operating
at 3.3 volts, we calculate 3.3 V × (70 + 9 + 40 + 40) mA = 3.3
V × 159 mA = 524.7 mW. Because the LVPECL outputs have
emitter resistors hooked up and the power dissipated by these
resistors is 60 mW for each clock, the total device power dis-
sipation is 524.7 mW - 120 mW = 404.7 mW.
When an LVPECL output is active, ~1.9 V is the average volt-
age on each output as calculated from the LVPECL V
Table 3.4 - Block Current Consumption
Block
Entire device,
core current
Clock buffers
(internal)
Output buffers
Divide circuitry
per output
Delay circuitry
per output
Entire device
Condition
All outputs off; No LVPECL emitter resistors connected
The low clock buffer is enabled anytime one of
CLKout0 through CLKout3 are enabled
LVPECL output, bypass mode (includes 120 Ω emitter
resistors)
LVPECL output, disabled mode (includes 120 Ω
emitter resistors)
LVPECL output, disabled mode. No emitter resistors
placed; open outputs
Divide enabled, divide = 2
Divide enabled, divide > 2
Delay enabled, delay < 8
Delay enabled, delay > 7
CLKout0 & CLKout3 enabled in bypass mode
OH
&
14
calculate estimated current consumption of the LMK02002.
Unless otherwise noted Vcc = 3.3 V, T
V
each emitter resistor is approximately (1.9 V)
mW. When an LVPECL output is disabled, the emitter resistor
voltage is ~1.07 V. Therefore the power dissipated in each
emitter resistor is approximately (1.07 V)
3.5 THERMAL MANAGEMENT
Power consumption of the LMK02002 can be high enough to
require attention to thermal management. For reliability and
performance reasons the die temperature should be limited
to a maximum of 125 °C. That is, as an estimate, T
temperature) plus device power consumption times θ
should not exceed 125 °C.
The package of the device has an exposed pad that provides
the primary heat removal path as well as excellent electrical
grounding to the printed circuit board. To maximize the re-
moval of heat from the package a thermal land pattern in-
cluding multiple vias to a ground plane must be incorporated
on the PCB within the footprint of the package. The exposed
pad must be soldered down to ensure adequate heat con-
duction out of the package. A recommended land and via
pattern is shown in Figure 2. More information on soldering
LLP packages can be obtained at www.national.com.
OL
typical specification. Therefore the power dissipated in
Consumption at
3.3 V (mA)
Current
17.4
159
5.3
8.5
5.8
9.9
70
40
9
0
Dissipated in
device (mW)
Power
404.7
29.7
38.3
17.5
28.0
19.1
32.7
231
72
0
A
= 25 °C.
2
/ 120 Ω = 9.5 mW.
LVPECL emitter
resistors (mW)
Dissipated in
2
/ 120 Ω = 30
Power
19.1
120
60
A
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-
-
-
-
-
-
(ambient
JA