Datasheet

LT1801/LT1802
17
18012fc
TYPICAL APPLICATIONS
Single Supply 1A Laser Driver Amplifi er
Figure 4 shows the LT1801 used in a 1A laser driver ap-
plication. 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 signifi cant current to fl ow in
the 2.1V threshold laser diode. Driving the noninverting
input of the LT1801 to a voltage V
IN
will control the turning
on of the high current NPN transistor, FMMT619 and the
laser diode. A current equal to V
IN
/R1 fl ows through the
laser diode. The LT1801 low offset voltage and low input
+
–
1/2 LT1801
V
IN
DO NOT FLOAT
IR LASER
INFINEON
SFH495
Q1
ZETEX
FMMT619
5V
R2
330Ω
R1
1Ω
18012 F04
C1
39pF
R3
10Ω
Figure 4. Single Supply 1A Laser Driver Amplifi er
100mA/DIV
50ns/DIV
18012 F05
Figure 5. 500mA Pulse Response
bias current allows it to control the current that fl ows
through the laser diode precisely. The overall circuit is
a 1A per volt V-to-I converter. Frequency compensation
components R2 and C1 are selected for fast but zero-
overshoot time domain response to avoid overcurrent
conditions 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.