LT1800CS8 查看數據表（PDF） - Linear Technology
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LT1800CS8 Datasheet PDF : 18 Pages
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LT1800
APPLICATIONS INFORMATION
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
crossover frequency of the amplifier is around 20MHz. A
capacitor of 5pF or higher connected across the feedback
resistor will eliminate any ringing or oscillation.
TYPICAL APPLICATIONS
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 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 LT1800 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
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 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.
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 circuit
amplifies the voltage across the 0.1Ω 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 VOS and IB is less than 4mA. The minimum output
voltage is 60mV, corresponding to 30mA. The large-signal
response of the circuit is shown in Figure 3.
IL
0A TO 1A
52.3Ω
0.1Ω
52.3Ω
3V
+
LT1800
–
1k
VOUT
0V TO 2V
1800 F02
VOUT = 2 • IL
f–3dB = 4MHz
UNCERTAINTY DUE TO VOS, IB < 4mA
Figure 2. Fast 1A Current Sense
500mV/DIV
0V
VS = 3V
50ns/DIV
1800 F03
Figure 3. Current Sense Amplifier Large-Signal Response
1800fa
14
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