EL6204 View Datasheet(PDF) - Renesas Electronics

Part Name

Description

Manufacturer

EL6204

Laser Driver Oscillator

Renesas Electronics 

EL6204

RAMP and RFREQ Pin Interfacing

Figure 21 on page 9 shows an equivalent circuit of pins

associated with the RAMP and RFREQ resistors. VREF is roughly

1.27V for both RAMP and RFREQ. The RAMP and RFREQ resistors

should be connected to the non-load side of the power ground to

avoid noise pickup. These resistors should also return to the

EL6204's ground very directly to prevent noise pickup. They also

should have minimal capacitance to ground. Trimmer resistors

can be used to adjust initial operating points.

+

VREF

-

PIN

FIGURE 21. RAMP AND RFREQ PIN INTERFACE

External voltage sources can be coupled to the RAMP and RFREQ

pins to effect frequency or amplitude modulation or adjustment.

It is recommended that a coupling resistor of 1k be installed in

series with the control voltage and mounted directly next to the

pin. This will keep the inevitable high-frequency noise of the

EL6204's local environment from propagating to the modulation

source, and it will keep parasitic capacitance at the pin

minimized.

Supply Bypassing and Grounding

The resistance of bypass-capacitors and the inductance of

bonding wires prevent perfect bypass action and 150mVP-P

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 22

shows the typical connection.

L SERIES: 70Ω REACTANCE AT 300MHz

VS

EL6204

GND

0.1µF

Chip

+5V

0.1µF

Chip

FIGURE 22. RECOMMENDED SUPPLY BYPASSING

Also important is the 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 23 shows the output current loops.

RFREQ

RAMP

GND

SUPPLY

BYPASS

SOURCING CURRENT

LOOP

SINKING CURRENT LOOP

LASER

DIODE

FIGURE 23. OUTPUT CURRENT LOOPS

For the pushing current loop, the current flows through the

bypass capacitor, into the EL6204 supply pin, out the IOUT pin to

the laser, and from the laser back to the decoupling capacitor.

This loop should be small.

For the pulling current loop, the current flows into the IOUT pin,

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

diode back to the IOUT pin. This loop should also be 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 Equation 1:

PDMAX

=

T----J---M-----A----X-----------T----A----M----A----X--

JA

(EQ. 1)

Where

PDMAX = Maximum power dissipation in the package

TJMAX = Maximum junction temperature

TAMAX = Maximum ambient temperature

JA = Thermal resistance of the package

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

output current and the operating frequency, which are

determined by resistors RAMP and RFREQ. The supply current can

be predicted approximately by Equation 2:

ISUP = 3----1---.--2---5--R--m--A---A-M-----P-----1----k------ + -3---0----mR----F-A---R----E---1-Q---k------- + 0.6mA

(EQ. 2)

The power dissipation can be calculated from Equation 3:

PD = VSUP  ISUP

(EQ. 3)

FN7219 Rev 3.00

October 28, 2015

Page 9 of 12

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