Datasheet

15
LT1803/LT1804/LT1805
180345f
12.6V, the absolute maximum rating, no damage will
occur to the device.
Overdrive Protection
When the input voltage exceeds the power supplies, two
pairs of crossing diodes D1 through D4 will prevent the
output from reversing polarity. If the input voltage exceeds
either power supply by 700mV, diode D1/D2 or D3/D4 will
turn on to keep the output at the proper polarity. For the
phase reversal protection to perform properly, the input
current must be limited to less than 10mA. If the amplifier
is severely overdriven, an external resistor should be used
to limit the overdrive current.
The LT1803/LT1804/LT1805’s input stages are also pro-
tected against a large differential input voltage of 1.4V or
higher by a pair of back-to-back diodes D5 through D8 to
prevent the emitter-base breakdown of the input transis-
tors. The current in these diodes should be limited to less
than 10mA when they are active. The worst-case differen-
tial 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 is connected to the power supplies
as shown in Figure 1.
Capacitive Load
The LT1803/LT1804/LT1805 are optimized for wide band-
width, low power and precision applications. They can
drive a capacitive load of about 20pF in a unity-gain
configuration, and more for higher gain. When driving a
larger capacitive load, a resistor of 10Ω to 50Ω should be
connected between the output and the capacitive load to
avoid ringing or oscillation. The feedback should still be
taken from the output so that the resistor will isolate the
capacitive load to ensure stability. Graphs on capacitive
load indicate the transient response of the amplifier when
driving a capacitive load with a specified 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 LT1803/
LT1804/LT1805 in a noninverting gain of 2 setup with two
5k resistors and a capacitance of 5pF (part plus PC board)
will probably oscillate. The pole formed at 12.7MHz,
reduces phase margin by about 58 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.
APPLICATIO S I FOR ATIO
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