lmk03000c National Semiconductor Corporation, lmk03000c Datasheet - Page 22

• Manufacturer Part Number: lmk03000c
• Description: Precision Clock Conditioner With Integrated Vco
• Manufacturer: National Semiconductor Corporation

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3.5 CURRENT CONSUMPTION / POWER DISSIPATION CALCULATIONS
Due to the myriad of possible configurations the following table serves to provide enough information to allow the user to calculate
estimated current consumption of the device. Unless otherwise noted Vcc = 3.3 V, T
From Table 3.5 the current consumption can be calculated in any configuration. For example, the current for the entire device with
1 LVDS (CLKout0) & 1 LVPECL (CLKout4) output in bypass mode can be calculated by adding up the following blocks: core current,
low clock buffer, high clock buffer, one LVDS output buffer current, and one LVPECL output buffer current. There will also be one
LVPECL output drawing 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 1 LVDS (CLKout0) & 1 LVPECL (CLKout4) operating at 3.3 volts,
we calculate 3.3 V × (86 + 9 + 9 + 17.8 + 40) mA = 3.3 V × 161.8 mA = 533.9 mW. Because the LVPECL output (CLKout4) has
the emitter resistors hooked up and the power dissipated by these resistors is 60 mW, the total device power dissipation is 533.9
mW - 60 mW = 473.9 mW.
When the LVPECL output is active, ~1.9 V is the average voltage on each output as calculated from the LVPECL Voh & Vol typical
specification. Therefore the power dissipated in each emitter resistor is approximately (1.9 V)
output is disabled, the emitter resistor voltage is ~1.07 V. Therefore the power dissipated in each emitter resistor is approximately
(1.07 V)
3.6 THERMAL MANAGEMENT
Power consumption of the LMK03000/LMK03000C/LMK03001/LMK03001C devices 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
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 removal of heat from the package a thermal land pattern including 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 conduction out of the package. A recommended land and via pattern is shown in Figure 3. More information
on soldering LLP packages can be obtained at www.national.com.
Table 3.5 - Block Current Consumption
Block
Entire device,
core current
Low clock buffer
(internal)
High clock buffer
(internal)
Output buffers
Divide circuitry
per output
Delay circuitry
per output
Entire device
2
/ 120 Ω = 9.5 mW.
Condition
All outputs off; No LVPECL emitter resistors connected
The low clock buffer is enabled anytime one of
CLKout0 through CLKout3 are enabled
The high clock buffer is enabled anytime one of the
CLKout4 through CLKout7 are enabled
Fout buffer, EN_Fout = 1
LVDS output, bypass mode
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 & CLKout4 enabled in bypass mode
A
(ambient temperature) plus device power consumption times θ
22
Consumption at
3.3 V (mA)
Current
161.8
86.0
14.5
17.8
17.4
5.3
8.5
5.8
9.9
40
9
9
0
A
JA
= 25 °C.
should not exceed 125 °C.
2
/ 120 Ω = 30 mW. When the LVPECL
Dissipated in
device (mW)
Power
283.8
29.7
29.7
47.8
58.7
38.3
17.5
28.0
19.1
32.7
474
72
0
LVPECL emitter
resistors (mW)
Dissipated in
Power
19.1
60
60
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