lt1810 Linear Technology Corporation, lt1810 Datasheet - Page 17

• Manufacturer Part Number: lt1810
• Description: Single/dual 180mhz, 350v/?s Rail-to-rail Input And Output Low Distortion Op Amps
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
beyond either supply, unlimited current will flow through
these diodes. If the current is transient and limited to
several hundred milliamps, no damage to the device will
occur.
Overdrive Protection
When the input voltage exceeds the power supplies, two
pairs of crossing diodes, D1 to D4, will prevent the output
from reversing polarity. If the input voltage exceeds either
power supply by 700mV, diodes D1/D2 or D3/D4 will turn
on, keeping the output at the proper polarity. For the
phase reversal protection to perform properly, the input
current must be limited to less than 5mA. If the amplifier
is severely overdriven, an external resistor should be used
to limit the overdrive current.
The LT1809/LT1810’s input stages are also protected
against differential input voltages of 1.4V or higher by
back-to-back diodes, D5/D8, that prevent the emitter-base
breakdown of the input transistors. The current in these
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 Figure 1.
Capacitive Load
The LT1809/LT1810 is optimized for high bandwidth and
low distortion applications. It can drive a capacitive load
about 20pF in a unity-gain configuration and more with
higher gain. When driving a larger capacitive load, a
resistor of 10 to 50 should be connected between the
U
U
W
U
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
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 LT1809 in a
noninverting gain of 2, set up with two 1K resistors and a
capacitance of 3pF (device plus PC board), will probably
ring in transient response. The pole that is formed at
106MHz will reduce phase margin by 34 degrees when the
crossover frequency of the amplifier is around 70MHz. A
capacitor of 3pF or higher connected across the feedback
resistor will eliminate any ringing or oscillation.
SHDN Pin
The LT1809 has a SHDN pin to reduce the supply current
to less than 1.25mA. When the SHDN pin is pulled low, it
will generate a signal to power down the device. If the pin
is left unconnected, an internal pull-up resistor of 10k will
keep the part fully operating as shown in Figure 1. The
output will be high impedance during shutdown, and the
turn-on and turn-off time is less than 100ns. Because the
inputs are protected by a pair of back-to-back diodes, the
input signal will feed through to the output during shut-
down mode if the amplitude of signal between the inputs
is larger than 1.4V.
LT1809/LT1810
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