Ultra-Low-Power, Open-Drain,

Single/Dual-Supply Comparators

10

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Low-Voltage Operation: V+ = 1V

(MAX9_4 Only)

The guaranteed minimum operating voltage is 2.5V (or

±1.25V). As the total supply voltage falls below 2.5V,

performance degrades and the supply current falls. The

reference will not function below about 2.2V, although

the comparators will continue to operate with a total

supply voltage as low as 1V. While the MAX9_4 has

comparators that may be used at supply voltages below

2V, the MAX9_1/MAX9_2/MAX9_3 may not be used with

supply voltages below 2.5V.

At low supply voltages, the comparators’ output sink

capability 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 when the

device operates from higher supply voltages. Test your

prototype over the full temperature and supply-voltage

range if you anticipate operation below 2.5V.

Comparator Output

With 100mV of overdrive, propagation delay is typically

3µs. The Typical Operating Characteristics show the

propagation delay for various overdrive levels. The

open-drain outputs are intended for wire-ORed and

level-shifting applications. The maximum output voltage

is 11V above V-, and may be applied even when no

supply voltage is present (V+ = V-).

The MAX9_1 and MAX9_4 outputs sink current to GND,

making these devices ideal for bipolar to single-ended

conversion and level-shifting applications.

The negative supply does not affect the output sink

current. The positive supply provides gate drive for the

output N-channel MOSFET and heavily influences the

output current capability, especially at low supply

voltages (see Typical Operating Characteristics section).

The MAX9_2 and MAX9_3 have no GND pin, and their

outputs sink current to V-.

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%

(MAX971/MAX973/MAX974) or ±2% (MAX981–MAX984)

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

capable of sourcing 25µA and sinking 15µA. 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

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 300µV. The voltage reference has

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

comparator is used with the reference, the combined

peak-to-peak noise is about 1mV. This, of course, is

much higher than the RMS noise of the individual

components. Take care in your layout to avoid

capacitive coupling from any output to the reference

pin. Crosstalk can significantly increase the actual

noise of the reference.

THRESHOLDS

OUT

IN-

IN+

HYSTERESIS

BAND

Figure 2. Threshold Hysteresis Band

7

2

5

6

HYST

REF

V-

V+

R1

R2

MAX9_1

MAX982

MAX9_3

2.5V TO 11V

Figure 3. Programming the HYST Pin