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1719

4.5ns Single/Dual Supply 3V/5V Comparator with Rail-to-Rail Output

Linear Technology 

LT1719

APPLICATIONS INFORMATION

2.7V TO 6V

VCC

+

+ VS

–

GND

VEE

5V

VCC

+

–

VEE

– 5V

3V

+ VS

GND

Single Supply

± 5VIN, 3VOUT

10V

VCC

+

–

VEE

5V

+ VS

GND

10VIN, 5VOUT

VCC

+

3V

+ VS

–

GND

VEE

– 5.2V

1719 F01

Front End Entirely Negative

Figure 1. Variety of Power Supply Configurations

responds correctly to a small differential input signal. If

one input is within the common mode limit, the other input

signal can go outside the common mode limits, up to the

absolute maximum limits, and the output will retain the

correct polarity.

When either input signal falls below the negative common

mode limit, the internal PN diode formed with the sub-

strate can turn on, resulting in significant current flow

through the die. An external Schottky clamp diode

between the input and the negative rail can speed up

recovery from negative overdrive by preventing the sub-

strate diode from turning on.

When both input signals are below the negative common

mode limit, phase reversal protection circuitry prevents

false output inversion to at least – 400mV common mode.

However, the offset and hysteresis in this mode will

increase dramatically, to as much as 15mV each. The input

bias currents will also increase.

When both input signals are above the positive common

mode limit, the input stage will get debiased and the output

polarity will be random. However, the internal hysteresis

will hold the output to a valid logic level. When at least one

of the inputs returns to within the common mode limits,

recovery from this state will take as long as 1µs.

The input stage is protected against damage from large

differential signals, up to and beyond a differential voltage

equal to the supply voltage, limited only by the absolute

maximum currents noted. External input protection cir-

cuitry is only needed if currents would otherwise exceed

these absolute maximums. The internal catch diodes can

conduct current up to these rated maximums without

latchup, even when the supply voltage is at the absolute

maximum rating.

The propagation delay does not increase significantly

when driven with large differential voltages, but with low

levels of overdrive, an apparent increase may be seen with

large source resistances due to an RC delay caused by the

2pF typical input capacitance.

Input Bias Current

Input bias current is measured with both inputs held at 1V.

As with any PNP differential input stage, the LT1719 bias

current flows out of the device. It will go to zero on the

higher of the two inputs and double on the lower of the two

inputs. With more than two diode drops of differential

input voltage, the LT1719’s input protection circuitry

activates, and current out of the lower input will increase

an additional 30% and there will be a small bias current

into the higher of the two input pins, of 4µA or less. See the

Typical Performance curve “Input Current vs Differential

Input Voltage.”

High Speed Design Considerations

Application of high speed comparators is often plagued by

oscillations. The LT1719 has 4mV of internal hysteresis,

which will prevent oscillations as long as parasitic output

to input feedback is kept below 4mV. However, with the

2V/ns slew rate of the LT1719 outputs, a 4mV step can be

created at a 100Ω input source with only 0.02pF of output

to input coupling. The LT1719’s pinout has been arranged

to minimize problems by placing the sensitive inputs away

from the outputs, shielded by the power rails. The input

and output traces of the circuit board should also be

separated, and the requisite level of isolation is readily

achieved if a topside ground plane runs between the

output and the inputs. For multilayer boards where the

ground plane is internal, a topside ground or supply trace

should be run between the inputs and the output.

7

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