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M61880FP

Laser-diode driver / controller

MITSUBISHI ELECTRIC  

MITSUBISHI < DIGITAL ASSP >

M61880FP

Laser-diode driver / controller

Explanation for setting Laser switching current

Vcc

The circuit of setting center

value (Isw0)of maximum

switching current(2X Isw0)

2XISWO

1.5V

+

AMP1

–

Vcc

RS

1:1

2XISWO

ISW2

D1

I1

V1

ISW

2.5V

ISW1

ISW2

D2

I2

Q1

Q2

V2

∆Vd

2KΩ

Id

VB

CH

LD

CURRENT SW

250µA

2XISWO

250µA

1:1

The circuit of setting switching current

Fig.1 Equivalent circuits of setting switching current

1. The circuit of setting center value(Isw0) of

maximum switching current

The setting center value(Isw0) of maximum switching

current, 2 X Isw0,set up Rs ( load for switching current

setting). Isw0 (the value that is initialized) is

Isw0[mA]=30

x

Vref(1.5V)[V]

RS[kΩ]

(1)

2. The circuit of setting switching current

When ∆V is a difference voltage of between

CHpin and 2.5V,Id is

Id =

∆V

2KΩ

(2)

So I1 and I2 is as follows

( I1=250µA-Id

I2=250µA+Id

(3)

D1,D2,Q1 and Q2 construct a Gilbert circuit.

The relation of I1,I2,Isw1, Isw2 and Isw0 is

I1 = Isw2

(4)

I2 Isw1

So Isw2 is

I1

Isw2=2 • Isw0 x I1+I2

(6)

Isw is as follows

Isw=2 • Isw0-Isw2

(7)

Next the relation between Isw and ∆V would be introduced .

Isw is able to solve with (6), (7),(3) and (2)equation.

I2

Isw=2 • Isw0(

)

I1+I2

(8)

Id

Isw=Isw0(1+

)

250µA

(9)

Isw=Isw0(1+

∆V/2KΩ

250µA

)

(10)

Isw1+Isw2=2 • Isw0

(5)

( 8 / 11 )

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