about 2.2V, although the comparators will continue to

operate with a total supply voltage as low as 1V. While

the MAX924 has comparators that may be used at

supply voltages below 2V, the MAX921, MAX922, and

MAX923 may not be used with supply voltages sig-

nificantly below 2.5V.

At low supply voltages, the comparators’ output drive is

reduced and the propagation delay increases (see

Typical Operating Characteristics). The useful input

voltage range extends from the negative supply to a

little under 1V below the positive supply, which is

slightly closer to the positive rail than the device

operating from higher supply voltages. Test your

prototype over the full temperature and supply-voltage

range if operation below 2.5V is anticipated.

Comparator Output

With 100mV of overdrive, propagation delay is typically

3µs. The Typical Operating Characteristics show the

propagation delay for various overdrive levels.

The MAX921 and MAX924 output swings from V+ to

GND, so TTL compatibility is assured by using a +5V

±10% supply. The negative supply does not affect the

output swing, and can range from 0V to -5V ±10%.

The MAX922 and MAX923 have no GND pin, and their

outputs swing from V+ to V-. Connect V- to ground and

V+ to a +5V supply to achieve TTL compatibility.

The MAX921–MAX924’s unique design achieves an

output source current of more than 40mA and a sink

current of over 5mA, while keeping quiescent currents in

the microampere range. The output can source 100mA

(at V+ = 5V) for short pulses, as long as the package's

maximum power dissipation is not exceeded. The

output stage does not generate crowbar switching

currents during transitions, which minimizes feedback

through the supplies and helps ensure stability without

bypassing.

Voltage Reference

The internal bandgap voltage reference has an output

of 1.182V above V-. Note that the REF voltage is

referenced to V-, not to GND. Its accuracy is ±1% in

the range 0°C to +70°C. The REF output is typically

capable of sourcing 15µA and sinking 8µA. Do not

bypass the REF output.

Noise Considerations

Although the comparators have a very high gain, useful

gain is limited by noise. This is shown in the Transfer

Function graph (see Typical Operating Characteristics).

As the input voltage approaches the comparator's

offset, the output begins to bounce back and forth; this

the graph averages out the bouncing, making the

transfer function easy to observe.) Consequently, the

comparator has an effective wideband peak-to-peak

noise of around 0.3mV. The voltage reference has

peak-to peak noise approaching 1mV. Thus, when a

Ultra Low-Power,

Single/Dual-Supply Comparators

_______________________________________________________________________________________

9

THRESHOLDS

OUT

IN-

IN+

HYSTERESIS

BAND

VREF - VHYST

Figure 2. Threshold Hysteresis Band

7

2

5

6

HYST

REF

V-

V+

R1

R2

MAX921

MAX923

2.5V TO 11V

Figure 3. Programming the HYST Pin