LMK02002

SNAS418 – AUGUST 2007

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Table 2. Block Current Consumption

Power

Current

Power

Dissipated in

Block

Condition

Consumption at

Dissipated in

LVPECL emitter

3.3 V (mA)

device (mW)

resistors (mW)

Entire device,

All outputs off; No LVPECL emitter resistors connected

70

231

-

core current

Clock buffers

The low clock buffer is enabled anytime one of CLKout0

9

29.7

-

(internal)

through CLKout3 are enabled

LVPECL output, bypass mode (includes 120

Ω emitter

40

72

60

resistors)

LVPECL output, disabled mode (includes 120

Ω emitter

Output buffers

17.4

38.3

19.1

resistors)

LVPECL output, disabled mode. No emitter resistors

0

0

-

placed; open outputs

Divide enabled, divide = 2

5.3

17.5

-

Divide circuitry

per output

Divide enabled, divide > 2

8.5

28.0

-

Delay enabled, delay < 8

5.8

19.1

-

Delay circuitry per

output

Delay enabled, delay > 7

9.9

32.7

-

Entire device

CLKout0 & CLKout3 enabled in bypass mode

159

404.7

120

From Table 2 the current consumption can be calculated in any configuration. For example, the current for the

entire device 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 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 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 dissipation is 524.7 mW - 120 mW = 404.7

mW.

When an 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)

2 /

120

Ω = 30 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)2 / 120

Ω = 9.5 mW.

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, TA (ambient temperature) plus device power consumption times θJA 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 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 WQFN packages can be

obtained at www.ti.com.

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