LT1805CS datasheet(15/20 Pages) LINER

15 / 20 page

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 10

Ω 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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