LT1797IS5 데이터 시트보기 (PDF) - Linear Technology

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LT1797IS5

10MHz, Rail-to-Rail Input and Output Op Amp in SOT-23

Linear Technology 

LT1797

APPLICATIONS INFORMATION

Supply Voltage

The positive supply pin of the LT1797 should be bypassed

with a small capacitor (about 0.1μF) within an inch of the

pin. When driving heavy loads an additional 4.7μF electro-

lytic capacitor should be used. When using split supplies

the same is true for the negative supply pin.

Inputs

The LT1797 is fully functional for an input signal range

from the negative supply to the positive supply. Figure 1

shows a simplified schematic of the amplifier. The input

stage consists of two differential amplifiers, a PNP stage

Q3/Q4 and an NPN stage Q1/Q2 that are active over dif-

ferent ranges of input common mode voltage. The PNP

differential pair is active for input common mode voltages

VCM between the negative supply to approximately 1.3V

below the positive supply. As VCM moves closer toward

the positive supply, the transistor QB1 will steer the tail

current I1 to the current mirror Q5/Q6, activating the

NPN differential pair and the PNP pair becomes inactive

for the rest of the input common mode range up to the

positive supply.

The input offset voltage and the input bias current are

dependent on which input stage is active. The input offset

voltage is trimmed on a single 5V supply with the common

mode at 1/2 supply and is typically 1mV with the PNP stage

active. The input offset of the NPN stage is untrimmed and

is typically 1.5mV. The input bias current polarity depends

on the input common mode voltage. When the PNP dif-

ferential pair is active, the input bias currents flow out of

the input pins. They flow in the opposite direction when

the NPN input stage is active. The offset error due to the

input bias currents can be minimized by equalizing the

noninverting and inverting source impedance.

The input stage of the LT1797 incorporates phase reversal

protection to prevent false outputs from occurring when

the inputs are driven up to 5V beyond the rails. Protective

resistors are included in the input leads so that current

does not become excessive when the inputs are forced

beyond the supplies or when a large differential signal is

applied.

Output

The output is configured with a pair of complementary

common emitter stages Q19/Q20, which enable the output

to swing from rail-to-rail. The output voltage swing of the

LT1797 is affected by input overdrive as shown in the Typi-

cal Performance Characteristics. When monitoring input

voltages within 50mV of V+ or within 8mV of V–, some

gain should be taken to keep the output from clipping. The

output of the LT1797 can deliver large load currents; the

short-circuit current limit is typically 50mA at ±5V. Take

care to keep the junction temperature of the IC below the

absolute maximum rating of 150°C. The output of the

amplifier has reverse biased diodes to each supply. If the

output is forced beyond either supply, unlimited current

will flow through these diodes.

The LT1797 can drive capacitive loads up to 200pF on

a single 5V supply in a unity gain configuration. When

there is a need to drive larger capacitive loads, a resistor

of a couple hundred ohms should be connected between

the output and the capacitive load. The feedback should

still be taken from the output so that the resistor isolates

the capacitive load to ensure stability. The low input bias

current of the LT1797 makes it possible to use high value

feedback resistors to set the gain. However, care must

be taken to insure that the pole formed by the feedback

resistors and the total capacitance at the inverting input

does not degrade stability.

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