Ultra-Low-Power, Open-Drain, Single/Dual-Supply Comparators 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 THRESHOLDS IN+ 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 HYSTERESIS outputs sink current to V-. IN- V BAND VREF - VHYST HB 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) OUT 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 Figure 2. Threshold Hysteresis Band gain is limited by noise. This is shown in the Transfer Low-Voltage Operation: V+ = 1V Function graph (see Typical Operating Characteristics). (MAX9_4 Only) As the input voltage approaches the comparator’s The guaranteed minimum operating voltage is 2.5V (or offset, the output begins to bounce back and forth; this ±1.25V). As the total supply voltage falls below 2.5V, peaks when VIN = VOS. (The lowpass filter shown on the performance degrades and the supply current falls. The graph averages out the bouncing, making the transfer reference will not function below about 2.2V, although function easy to observe.) Consequently, the the comparators will continue to operate with a total comparator has an effective wideband peak-to-peak supply voltage as low as 1V. While the MAX9_4 has noise of around 300µV. The voltage reference has comparators that may be used at supply voltages below peak-to-peak noise approaching 1mV. Thus, when a 2V, the MAX9_1/MAX9_2/MAX9_3 may not be used with comparator is used with the reference, the combined supply voltages below 2.5V. peak-to-peak noise is about 1mV. This, of course, is At low supply voltages, the comparators’ output sink much higher than the RMS noise of the individual capability is reduced and the propagation delay components. Take care in your layout to avoid MAX971–MAX974/MAX981–MAX984 increases (see Typical Operating Characteristics). The capacitive coupling from any output to the reference useful input voltage range extends from the negative pin. Crosstalk can significantly increase the actual supply to a little under 1V below the positive supply, noise of the reference. 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. 2.5V TO 11V 7 IREF 6 Comparator Output V+ REF With 100mV of overdrive, propagation delay is typically 3µs. The Typical Operating Characteristics show the MAX9_1 R1 propagation delay for various overdrive levels. The MAX982 MAX9_3 open-drain outputs are intended for wire-ORed and 5 level-shifting applications. The maximum output voltage HYST is 11V above V-, and may be applied even when no R2 V- supply voltage is present (V+ = V-). 2 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. Figure 3. Programming the HYST Pin 10______________________________________________________________________________________
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