M61880FP

Rev.1.0, Sep.19.2003, page 16 of 20

Description of Laser Switching Current Setting Circuit

I2

V2

Q2

D2

D1

Q1

Id

2k

Ω

250µA

250µA

2.5V

I1

V1

1:1

LD

1.5V

AMP1

Switching current varying circuit

Switching current initial

setting circuit

Current SW

1:1

Vd

Figure 1 Switching Current Setting Equivalent Circuit

1. Switching current initial setting circuit

The switching current initial set value is set by switching current setting resistance Rs in a V-I conversion circuit using

an op-amp.

×

Vref (1.5V) [V]

Ω]

............................ (1)

2. Switching current varying circuit

If the potential difference between the CH pin voltage and internal reference voltage is designated

∆V (= VCH – 2.5 V),

Id flowing in a 2 k

Ω resistance due to this ∆V voltage is as follows.

Id =

∆V

2k

Ω

............................ (2)

Therefore, the I1 and I2 currents are given by the following equations.

I1 = 250

µA-Id

I2 = 250

µA-Id

............................ (3)

Next, the relationship between I1, I2, ISW1, and ISW2 due to a Gilbert circuit comprising D1, D2, Q1, and Q2, is given

by the following equation.

=

Isw1

Isw2

I1

I2

............................ (4)

Also, the relationship between ISW1, ISW2, and ISW0 is given by the following equation.

Isw1 + Isw2 = 2 • Iswo

............................ (5)

Finding ISW2 from Equations (4) and (5),

×

I1

I1 + I2

............................ (6)