13

LT1719

A separate output speed limit is the clamp turnaround. The

LT1719 output is optimized for fast initial response, with

some loss of turnaround speed, limiting the toggle fre-

quency. The output transistors are idled in a low power

clamp action. It is only when the output has slewed from

the old voltage to the new voltage, and the clamp circuitry

has settled, that the idle state is reached and the LT1719

is fully ready to toggle again. This is typically 8ns for each

direction, resulting in a maximum toggle frequency of

62.5MHz. With higher frequencies, dropout and runt pulses

can result. Increases in capacitive load will increase the

time needed for slewing due to the limited slew currents

and the maximum toggle frequency will decrease further.

For high toggle frequency applications, consider the

LT1394, whose linear output stage can toggle at 100MHz

typical.

The internal speed limits manifest themselves as disper-

sion. All comparators have some degree of dispersion,

defined as a change in propagation delay versus input

overdrive. The propagation delay of the LT1719 will vary

with overdrive, from a typical of 4.5ns at 20mV overdrive

to 7ns at 5mV overdrive (typical). The LT1719’s primary

source of dispersion is the hysteresis stage. As a change

of polarity arrives at the gain stage, the positive feedback

of the hysteresis stage subtracts from the overdrive avail-

able. Only when enough time has elapsed for a signal to

propagate forward through the gain stage, backwards

through the hysteresis path and forward through the gain

stage again, will the output stage receive the same level of

overdrive that it would have received in the absence of

hysteresis.

The LT1719 is several hundred picoseconds faster when

to the internal speed limit; the gain stage operates between

bias due to reduced silicon junction capacitances.

In many applications, as shown in the following examples,

there is plenty of input overdrive. Even in applications

providing low levels of overdrive, the LT1719 is fast

enough that the absolute dispersion of 2.5ns (= 7 – 4.5) is

often small enough to ignore.

The gain and hysteresis stage of the LT1719 is simple,

short and high speed to help prevent parasitic oscillations

while adding minimum dispersion. This internal “self-latch”

can be usefully exploited in many applications because it

occurs early in the signal chain, in a low power, fully

differential stage. It is therefore highly immune to distur-

bances from other parts of the circuit, such as the output,

or on the supply lines. Once a high speed signal trips the

hysteresis, the output will respond, after a fixed propaga-

tion delay, without regard to these external influences

that can cause trouble in nonhysteretic comparators.

The input trip points test circuit uses a 1kHz triangle wave

to repeatedly trip the comparator being tested. The LT1719

output is used to trigger switched capacitor sampling of

the triangle wave, with a sampler for each direction. Be-

cause the triangle wave is attenuated 1000:1 and fed to the

LT1719’s differential input, the sampled voltages are there-

fore 1000 times the input trip voltages. The hysteresis and

offset are computed from the trip points as shown.

Crystal Oscillator

A simple crystal oscillator using an LT1719 is shown on

the first page of this data sheet. The 2k-620

Ω resistor pair

set a bias point at the comparator’s noninverting input.

The 2k-1.8k-0.1

µF path sets the inverting input node at an

appropriate DC average level based on the output. The

crystal’s path provides resonant positive feedback and

stable oscillation occurs. Although the LT1719 will give

the correct logic output when one input is outside the

common mode range, additional delays may occur when

it is so operated, opening the possibility of spurious

operating modes. Therefore, the DC bias voltages at the

inputs are set near the center of the LT1719’s common

mode range and the 220

Ω resistor attenuates the feed-

back to the noninverting input. The circuit will operate with

any AT-cut crystal from 1MHz to 10MHz over a 2.7V to 6V

supply range. As the power is applied, the circuit remains

of 2V to 2.2V (25

°C), at which point the desired frequency

output is generated.