9

7374.1

August 10, 2007

noise on the power lines is common. There needs to be a

lossy bead inductance and secondary bypass on the supply

side to control signals from propagating down the wires.

Figure 20 shows the typical connection.

FIGURE 20. RECOMMENDED SUPPLY BYPASSING

Also important is circuit-board layout. At the EL6208's

operating frequencies, even the ground plane is not

low-impedance. High frequency current will create voltage

drops in the ground plane. Figure 21 shows the output

current loops.

FIGURE 21. OUTPUT CURRENT LOOPS

For the pushing current loop, the current flows through the

pin to the laser, and from the laser back to the decoupling

capacitor. This loop should be small.

pin, out of the ground pin, to the laser cathode, and from the

small.

Power Dissipation

With the high output drive capability, the EL6208 is possible

to exceed the +125°C “absolute-maximum junction

temperature” under certain conditions. Therefore, it is

important to calculate the maximum junction temperature for

the application to determine if the conditions need to be

modified for the oscillator to remain in the safe operating

area.

The maximum power dissipation allowed in a package is

determined according to Equation 1:

where:

The supply current of the EL6208 depends on the peak-to-

peak output current and the operating frequency which are

current can be predicted approximately by Equation 2:

The power dissipation can be calculated from Equation 3:

convenient way to see if the device will overheat. The

maximum safe power dissipation can be found graphically,

based on the package type and the ambient temperature. By

using the previous equation, it is a simple matter to see if PD

exceeds the device's power derating curve. To ensure

proper operation, it is important to observe the

recommended derating curve shown in Figures 22 and . A

flex circuit may have a higher

dissipation would then be required.

FIGURE 22. PACKAGE POWER DISSIPATION vs AMBIENT

TEMPERATURE

+5V

L SERIES: 70

Ω REACTANCE AT 300MHz

0.1µF

CHIP

EL6208

GND

0.1µF

CHIP

SINKING CURRENT LOOP

SOURCING CURRENT LOOP

SUPPLY

BYPASS

LASER

DIODE

GND

P

DMAX

T

Θ

JA

---------------------------------------------

=

(EQ. 1)

I

SUP

31.25mA

1k

Ω

×

R

AMP

-------------------------------------------

30mA

1k

Ω

×

R

FREQ

----------------------------------

0.6mA

++

=

(EQ. 2)

P

D

V

SUP

I

SUP

×

=

(EQ. 3)

0.6

0.5

0.4

0.3

0.2

0.1

0

0

25

50

75

100

125

150

AMBIENT TEMPERATURE (°C)

85

JEDEC JESD51-3 LOW EFFECTIVE THERMAL

CONDUCTIVITY TEST BOARD

488mW

6

Ld

SO

T-2

3

θ

JA

=

+2

56

C/W

EL6208