9

Supply Bypassing and Grounding

The resistance of bypass-capacitors and the inductance of

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 2 shows the typical connection.

Also important is circuit-board layout. At the EL6204's

operating frequencies, even the ground plane is not low-

impedance. High frequency current will create voltage drops

in the ground plane. Figure 3 shows the 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 EL6204 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:

where

The supply current of the EL6204 depends on the peak-to-

peak output current and the operating frequency which are

current can be predicted approximately by the following

equation:

The power dissipation can be calculated from the following

equation:

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

exceeds the device's power derating curve. To ensure

proper operation, it is important to observe the

recommended derating curve shown in Figures 4 and 5. A

flex circuit may have a higher

dissipation would then be required.

FIGURE 2. RECOMMENDED SUPPLY BYPASSING

+5V

L Series: 70

Ω reactance at 300MHz

0.1µF

Chip

EL6204

GND

0.1µF

Chip

FIGURE 3. OUTPUT CURRENT LOOPS

Sinking Current Loop

Sourcing Current Loop

Supply

Bypass

Laser

Diode

GND

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

=

31.25mA 1k

Ω

×

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

30mA 1k

Ω

×

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

0.6mA

++

=

×

=

FIGURE 4. PACKAGE POWER DISSIPATION vs

AMBIENT TEMPERATURE

0.6

0.5

0.4

0.3

0.2

0.1

0

0

255075

100

125

150

Ambient Temperature (°C)

85

Package Power Dissipation vs Ambient Temperature

JEDEC JESD51-3 Low Effective Thermal Conductivity Test Board

488mW

6-P

in

SO

T-2

3

θ

JA =2

56°C

/W

EL6204