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Número de pieza

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Fabricante

M66515

LASER-DIODE DRIVER/CONTROLLER

Mitsumi 

MITSUBISHI 〈DIGITAL ASSP〉

M66515FP

LASER-DIODE DRIVER/CONTROLLER

OPERATION

1. Laser Drive Currents

The approximate values of laser drive currents, ISW and IB,

are obtained by the following equation, in which VC is the volt-

age of holding capacitor connected to the CH pin.

(1) ISW (switching current)

ISW

[mA]

=

12

×

VC [V]

RS [kΩ]

provided that 0 ≤ VC ≤ VCC – 1.8V and ISW (max.) =120mA

where RS is the resistance of the resistor connected between

the RS pin and GND.

(2) IB (bias current)

IB

[mA]

=

10

×

VB [V]

RB [kΩ]

provided that 0 ≤ VB ≤ Vcc – 2.7V and IB (max.) = 30mA

where RB is the resistance of the resistor connected between

the RB pin and GND.

2. Switching Operation

If DATA = “L,” the LD drive current is ISW + IB; if DATA = “H,”

IB.

3. ENB Input

When the laser drive current is controlled by the DATA input,

the M66515’s internal current source is maintained turned on.

In contrast, the control by ENB is turning on and off at the cur-

rent source. If ENB = “L” the current source turns on; if ENB =

“H” off.

When ENB = “H” the CH pin is compulsorily fixed to “L” in or-

der to discharge the capacitor connected to the CH pin.

4. Internal Reset Function

The M66515 has a reset circuit built in for the protection of

laser from an excessive current flowing at the moment of

power on. The internal current source goes off in the range

Vcc < 3.5V (typ.), and the CH pin is compulsorily fixed to “L” at

the same time.

5. RO Pin

A load resistor for laser drive current is connected to the RO

pin, through which a current almost equal to ISW flows in

(when DATA = “H”). The load resistor is connected between

the RO pin and Vcc to reduce power dissipated in the IC.

Due to reasons related to the operation of circuits, the voltage

at this pin should be 2.5V or higher.

Consequently, the maximum resistance, RO(max.), of load re-

sistor RO is:

RO(max.)

[Ω]

=

Vcc(min.) –2.5 [V]

ISW(max.) [A]

where ISW(max.) is the maximum of ISW. If, for example,

Vcc(min.) = 4.75V and ISW(max.) =120mA, RO(max.) =18.8Ω.

Accordingly, if the resistance of RS is selected so as to gain

maximum ISW of 120mA, RO should be 18.8Ω at the maxi-

mum.

6. Sample-Hold Circuit

(1) Circuit Operation Overview

The following is an overview of the operation of the sample-

hold circuit contained in the M66515.

The PD current generated by LD illumination flows through

the resistor connected between 1RM and 2RM, thereby gen-

erating a potential difference (VM). VM is compared with the

voltage applied to the Vr pin. If VM < Vr, a constant current is

sourced through the CH pin so that the external capacitor is

charged. If VM > Vr, a constant current sinks through the CH

pin to discharge the external capacitor.This operation occurs

when the S/H input is “L” (sample). When the S/H input is “H,”

the CH pin is maintained at high impedance state (hold), irre-

spective of the state of VM, Vr, and DATA input.

Reference voltage input

Vr

Potential difference on resistor RM VM

Comparator

−

+

Sample-hold

control input

S/H

Control

circuit

ENB

Constant current

source for charging

SW1

Output CH

SW2

Constant current

source for discharging

Tr1

Externat capacitor

Conceptual Diagram : Sample-Hold circuit

3

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