M61880FP Ver la hoja de datos (PDF) - MITSUBISHI ELECTRIC

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componentes Descripción

Fabricante

M61880FP

Laser-diode driver / controller

MITSUBISHI ELECTRIC  

MITSUBISHI < DIGITAL ASSP >

M61880FP

Laser-diode driver / controller

OPERATION

1.Laser Drive Currents Setting Method

Laser Drive Current=Isw(switching current)+IB(bias current)

at switching mode

(1)Isw(Switching Current)

First it is necessary to decide the center value(Isw0) of

maximum switching current Isw(max).Isw0 is depend on

Rs ( load for switching current setting) by the following

equation.

(a) Isw0 [mA]

= 30 X

Vref(1.5V) [V]

Rs [kΩ]

(b) When switching current is center value( Isw0) , it is

necessary to set up Rm ( load resistor for monitor of

photodiode current ) as follows:

VM(voltage across resistor Rm) = Vr (reference voltage )

at this condition CH pin voltage =2.5V.

When CH pin voltage < 2.5V , Isw <Isw0

When CH pin voltage > 2.5V , Isw >Isw0

(CH pin voltage changes from 2V to 3V at APC mode)

(c) The usable range of ISW at APC mode

That is 20 ~180% of Isw0 accurately.

(2) IB(Bias Current)

Bias current (IB) is set by RB (resistor for the setting of

bias current) and VB (voltage for the setting of bias current).

VB [V]

IB [A] = 1 X RB [Ω]

1.2V ≤ VB ≤ Vcc-2.7V

IB(max.)=40mA

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 M61880'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

order to discharge the capacitor connected to the CH pin.

4.Internal Reset Function

The M61880 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.

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 :

Vcc(min.) -2.5 [V]

RO(max.) [Ω] = Isw(max.) [mA]

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

Vcc(min.)= 4.75V and Isw(max.)= 100mA, RO(max.)=22Ω.

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

maximum Isw of 100mA, RO should be 22Ω 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 M61880.

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" and DATA= "L" (sample).

When the S/H input is "H" , the CH pin is maintained at high

impedance state(hold), irrespective of the state of Vm, Vr, and

DATA input.

2Isw0

Isw0

200%

Comparator

Vr

Vm

100%

S/H

Control

circuit

Constant

current source

for charging

SW1

SW2

Constant

current source

for discharging

Output

CH

External

capacitor

0

1.0 2.0 2.5 3.0 4.0

0%

CH pin voltage VCH (V)

ENB

Tr1

Conceptual Diagram: sample-hold circuit

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