M61880FP

Rev.1.0, Sep.19.2003, page 3 of 20

Pin Description

Pin No.

Symbol

Name

Function

1

Rs

Switching current setting

resistance connection pin

Connects switching object current (ISW) setting resistance to GND.

2

GND1

Ground 1

Internal analog GND

3

RB

Bias current setting

resistance connection pin

Connects bias current (IB) setting resistance to GND.

Leave this pin open when IB is not used.

4

VB

Bias current setting voltage

input

Bias current value (IB) is set by applying a voltage to this pin.

Leave this pin open when IB is not used.

5

Vref

Reference voltage output

M61880 internal reference voltage (1.5 V typ.) output pin

6

Vr

Reference voltage input

Connected to non-reversed input pin of comparator in sample-and-

hold circuit. Connect this pin to Vref pin when using M61880

internal reference voltage.

7

CH

Hold capacitance

connection pin

Connects hold capacitance to GND. This pin is connected to

sample-and-hold circuit output and ISW current source input in

M61880.

8

S/H

Sample-and-hold control

input

Sampling when “H”, hold when “L”

9

Vcc1

Power supply 1

Internal analog power supply. Connected to positive power supply

(+5 V).

10

NC

NC

Not connected to internal circuitry.

11, 12

1RM

2RM

Monitoring load resistance

connection pins

Connects load resistance for converting monitor photodiode current

to voltage between 1RM and 2RM. (1RM pin is connected to GND

in IC.)

13

ENB

Laser current enable input

When “H”, LD drive current source circuit is turned off. Also, CH

pin is forcibly fixed at “L” level.

14

DATA

Switching data input

ISW+IB current flows to laser diode when “+”, and IB current when

“H”.

15

PD

Monitoring photodiode

current input

Connects monitor photodiode anode.

16

GND2

Ground 2

Internal digital GND

17

LD

Laser current output

Connects semiconductor laser diode cathode.

18

NC

NC

Not connected to internal circuitry.

19

RO

Laser current load output

Connects laser current load resistance to Vcc.

20

Vcc2

Power supply 2

Internal digital power supply. Connected to positive power supply

(+5 V).

Operation

1. Laser drive current setting method

The laser drive current consists of switching current ISW + bias current IB.

(1) Switching current ISW setting method

a. Decide the maximum current value ILD(MAX) to flow in the laser diode (LD). This is decided taking account of

the LD type, dispersion, temperature changes, secular changes, etc.

b. Find ISW (initial set value) from the following equation.

ISW (initial set value) = ILD(MAX)/1.9

c. Find switching current setting resistance RS from the following equation.

Ω]

30

In this case the LD current can be controlled in a range of 10% to 90% of ISW (initial set value).