LMK03000 National Semiconductor Corporation, LMK03000 Datasheet - Page 22

• Manufacturer Part Number: LMK03000
• Manufacturer: National Semiconductor Corporation

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3.5 CURRENT CONSUMPTION / POWER DISSIPATION CALCULATIONS
Due to the myriad of possible configurations the follow 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 chip with
1 LVDS (CLKout0) & 1 LVPECL (CLKout4) output in bypass mode can be calculated by adding up the following blocks: core current,
low channel buffer, high channel 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 IC. 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 IC, the total current entering the IC is multiplied by the voltage at the IC 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 IC 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 LMK0300XX can be high enough to require attention to thermal management. For reliability and per-
formance reasons the die temperature should be limited to a maximum of 125 °C. That is, as an estimate, T
plus LMK0300XX power consumption times θ
The package of the LMK0300XX 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 channel
buffer (internal)
High channel
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 channel buffer is enabled anytime one of
CLKout0 through CLKout3 are enabled
The high channel 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
CLKout0 & CLKout4 enabled in bypass mode (no
emitter resistors placed)
JA
should not exceed 125 °C.
22
Consumption at
3.3 V (mA)
Current
161.8
144.4
86.0
14.5
17.8
17.4
5.3
8.5
5.8
9.9
40
9
9
0
A
= 25 °C.
2
Dissipated in IC
/ 120 Ω = 30 mW. When the LVPECL
Power
(mW)
283.8
476.5
29.7
29.7
47.8
58.7
38.3
17.5
28.0
19.1
32.7
474
72
0
A
(ambient temperature)
LVPECL emitter
resistors (mW)
Dissipated in
Power
19.1
60
60
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