6

FN7216.3

August 2, 2007

Typical Application Circuit

Applications Information

The EL6201 is designed to interface easily to laser diodes to

break up optical feedback resonant modes and thereby

reduce laser noise, but it is also generally useful as a

70MHz to 430MHz oscillator. The first applications section

will focus on laser systems, and subsequent sections are of

general topics.

Laser Diode Applications

The output of the EL6201 is composed of a sourcing and a

sinking current source, switched alternately at the oscillator

with about 40

Ω of series resistance. There is no severe

squarewave distortion when the output voltage approaches

the supply extremes, although the corners will be rounded.

Being a current-source output, the output bias voltage is set

by direct connection to the laser diode, which will appear as

a low AC impedance with a DC voltage from 1.6V to 2.5V.

Thus AC coupling from the EL6201 to the diode is

unnecessary. The duty cycle of the output is between 40%

and 60%, so the DC contribution from the EL6201 is only

±5% of the peak-to-peak output. This will cause little

perturbation of the diode’s DC bias current.

Although not necessary, capacitance coupling can be

employed. A series capacitive reactance of less than 30

Ω is

recommended. A 20pF capacitor is thus appropriate at

330MHz. Benefits include no DC error current into the laser

diode, and an attenuation of low-frequency noise from the

EL6201. Disadvantages include perhaps 20% output AC

current loss.

While the diode AC impedance is generally in the low ohm

range, any interconnect will create around 8nH per cm. of

series inductance. Because the EL6201’s output is an AC

current source, higher load reactance due to series

inductance will cause the EL6201’s output voltage to swing

more than what a direct connection to the diode would

and more at harmonic frequencies. The output current

TEMPERATURE

FIGURE 20. MSOP8 POWER DISSIPATION vs AMBIENT

TEMPERATURE

SEMI G42-88 SINGLE LAYER TEST BOARD

AMBIENT TEMPERATURE (°C)

025

50

75

125

100

0.10

0.00

0.20

0.50

0.40

0.30

391mW

195mW

SEMI G42-88 SINGLE LAYER TEST BOARD

AMBIENT TEMPERATURE (°C)

025

50

75

125

100

0.10

0.00

0.20

0.60

0.50

0.30

486 mW

0.40

243mW

1

2

3

5

4

+5V

+5V

C*

*Optional AC coupling

EL6201