LT1800 Linear Technology, LT1800 Datasheet - Page 13

• Manufacturer Part Number: LT1800
• Description: 80MHz/ 25V/ms Low Power Rail-to-Rail Input and Output Precision Op Amp
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Single Supply 1A Laser Driver Amplifier
The circuit in the front page of this data sheet shows the
LT1800 used in a 1A laser driver application. One of the
reasons the LT1800 is well suited to this control task is that
its 2.3V operation ensures that it will be awake 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 LT1800 to a
voltage V
NPN transistor, FMMT619 and the laser diode. A current
equal to V
low offset voltage and low input 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 con-
verter. 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 also shown in the
graphic on the front page. 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.
APPLICATIO S I FOR ATIO
diodes should be limited to less than 10mA when they are
active. The worst-case differential input voltage usually
occurs when the input is driven while the output is shorted
to ground in a unity gain configuration. In addition, the
amplifier is protected against ESD strikes up to 3kV on all
pins by a pair of protection diodes on each pin that are
connected to the power supplies as shown in Figure1.
Capacitive Load
The LT1800 is optimized for high bandwidth, low power
and precision applications. It can drive a capacitive load of
about 75pF in a unity gain configuration, and more for
higher gain. When driving a larger capacitive load, a
resistor of 10 to 50 should be connected between the
output and the capacitive load to avoid ringing or oscilla-
tion. The feedback should still be taken from the output so
that the resistor will isolate the capacitive load to ensure
IN
IN
/R1 flows through the laser diode. The LT1800
will control the turning on of the high current
U
U
U
U
W
W
U
U
Fast 1A Current Sense Amplifier
A simple, fast current sense amplifier in Figure 2 is suitable
for quickly responding to out-of-range currents. The cir-
cuit amplifies the voltage across the 0.1 sense resistor
by a gain of 20, resulting in a conversion gain of 2V/A. The
–3dB bandwidth of the circuit is 4MHz, and the uncertainty
due to V
voltage is 60mV, corresponding to 30mA. The large-signal
response of the circuit is shown in Figure 3.
stability. Graphs on capacitive loads indicate the transient
response of the amplifier when driving capacitive load with
a specified series resistor.
Feedback Components
When feedback resistors are used to set up gain, care must
be taken to ensure that the pole formed by the feedback
resistors and the total capacitance at the inverting input
does not degrade stability. For instance, the LT1800 in a
noninverting gain of 2, set up with two 5k resistors and a
capacitance of 5pF (part plus PC board) will probably ring
in transient response. The pole is formed at 12.7MHz that
will reduce phase margin by 32 degrees when the cross-
over frequency of the amplifier is around 20MHz. A capaci-
tor of 5pF or higher connected across the feedback resis-
tor will eliminate any ringing or oscillation.
OS
and I
0.1
I
0A TO 1A
L
Figure 2. Fast 1A Current Sense
V
f
UNCERTAINTY DUE TO V
–3dB
B
OUT
52.3
52.3
is less than 4mA. The minimum output
= 2 • I
= 4MHz
L
+
–
LT1800
OS,
3V
I
1800 F02
B
< 4mA
1k
V
0V TO 2V
OUT
LT1800
13
1800f