EL6202 データシートの表示（PDF） - Intersil

部品番号

コンポーネント説明

メーカー

EL6202

Laser Driver Oscillator

Intersil 

EL6202

Theory of Operation

A typical semiconductor laser will emit a small amount of

incoherent light at low values of forward laser current. But

after the threshold current is reached, the laser will emit

coherent light. Further increases in the forward current will

cause rapid increases in laser output power. A typical

threshold current is 35mA and a typical slope efficiency is

0.7mW/mA.

When the laser is lasing, it will often change its mode of

operation slightly, due to changes in current, temperature, or

optical feedback into the laser. In a DVD-ROM, the optical

feedback from the moving disk forms a significant noise

factor due to feedback-induced mode hopping. In addition to

the mode hopping noise, a diode laser will roughly have a

constant noise level regardless of the power level when a

threshold current is exceeded.

The oscillator is designed to produce a low noise oscillating

current that is provided to the laser diode. The current is to

cause the laser power to change at the oscillator frequency.

This change causes the laser to go through rapid mode

hopping. The low frequency component of laser power noise

due to mode hopping is translated up to sidebands around

the oscillator frequency by this action. Since the oscillator

frequency can be filtered out of the low frequency read and

serve channels, the net result is that the laser noise seems

to be reduced. The second source of laser noise reduction is

caused by the increase in the laser power above the average

laser power during the pushing-current time. The signal-to-

noise ratio (SNR) of the output power is better at higher laser

powers because of the almost constant noise power when a

threshold current is exceeded. In addition, when the laser is

off during no output current time, the noise is also very low.

Setting the IIN Current

By looking the typical application circuit, it can be seen that

the push only oscillator is more efficient at the laser than the

conventional push-pull oscillator. The significant current from

the main board is reduced to be IIN (≤2mA), while the

oscillator takes on the role of supplying the total laser

current.

The IIN current is the previous read current (reduced in

amplitude). Thus it does not need to be set, since it is within

the control loop. The current capability of the external source

for IIN should be made large enough to power the worst,

hottest old laser.

RFREQ Pin Interfacing

Figure 8 shows an equivalent circuit of pins associated with

the RFREQ resistor. VREF is roughly 1.27V. The resistor

RFREQ should be connected to the non-load side of the

power ground to avoid noise. This resistor should also return

to the EL6202’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 8. RFREQ PIN INTERFACE

External voltage sources can be coupled to the RFREQ pin

to effect frequency modulation or adjustment. It is

recommended that a coupling resistor 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

EL6202'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 series bead inductance and secondary bypass on the

supply side to control signals from propagating down the

wires. Figure 9 shows the typical connection.

L Series: 70Ω reactance at

300MHz (see text)

VS

EL6202

GND

0.1µF

Chip

+5V

0.1µF

Chip

FIGURE 9. RECOMMENDED SUPPLY BYPASSING

Also important is circuit-board layout. At the EL6202's

operating frequencies, even the ground plane is not low-

impedance. High frequency current will create voltage drops

in the ground plane. Figure 10 shows the output current loop.

RFREQ

RAMP

SUPPLY

BYPASS

SOURCING CURRENT LOOP

GND

(8-PIN

PACKAGE)

LOAD

FIGURE 10. OUTPUT CURRENT LOOP

7

Share Link: