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LT1801/LT1802

18012fb

Single Supply 1A Laser Driver Amplifier

Figure 4 shows the LT1801 used in a 1A laser driver

application. One of the reasons the LT1801 is well suited

to this control task is that its 2.3V operation ensures that

it will be awaked during power-up and operated before the

circuit can otherwise cause significant current to flow in

the 2.1V threshold laser diode. Driving the noninverting

input of the LT1801 to a voltage VIN will control the turning

on of the high current NPN transistor, FMMT619 and the

laser diode. A current equal to VIN/R1 flows through the

laser diode. The LT1801 low offset voltage and low input

TYPICAL APPLICATIO S

1/2 LT1801

VIN

DO NOT FLOAT

IR LASER

INFINEON

SFH495

ZETEX

FMMT619

330

Ω

18012 F04

39pF

Ω

Figure 4. Single Supply 1A Laser Driver Amplifier

100mA/DIV

50ns/DIV

18012 F05

Figure 5. 500mA Pulse Response

bias current allows it to control the current that flows

through the laser diode precisely. The overall circuit is a 1A

per volt V-to-I converter. Frequency compensation com-

ponents R2 and C1 are selected for fast but zero-over-

shoot time domain response to avoid overcurrent condi-

tions in the laser. The time domain response of this circuit,

measured at R1 and given a 500mV 230ns input pulse, is

shown in Figure 5. While the circuit is capable of 1A

operation, the laser diode and the transistor are thermally

limited due to power dissipation, so they must be operated

at low duty cycles.

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