DescriptionThe MAX941/MAX942/MAX944 single-supply comparators feature internal hysteresis, high speed, and low power. Their outputs are guaranteed to pull within 0.4V of either supply rail without external pull-up or pull-down circuitry. Rail-to-rail input voltage range and low voltage single-supply operation make these devices ideal for portable equipment. The MAX941/MAX942/MAX944 interface directly to CMOS and TTL logic.
Most high-speed comparators oscillate in the linear region because of noise or undesired parasitic feedback.
This tends to occur when the voltage on one input is at or equal to the voltage on the other input. To counter the parasitic effects and noise, the MAX941/MAX942/MAX944 have internal hysteresis. The hysteresis in a comparator creates two trip points: one for the rising input voltage and one for the falling input voltage.
The difference between the trip points is the hysteresis. When the comparator’s input voltages are equal, the hysteresis effectively causes one comparator input voltage to move quickly past the other, thus taking the input out of the region where oscillation occurs. Standard comparators require hysteresis to be added with external resistors. The MAX941/MAX942/MAX944’s fixed internal hysteresis eliminates these resistors and the equations needed to determine appropriate values.
The MAX941 includes an internal latch that allows storage of comparison results. The LATCH pin has a high input impedance. If LATCH is high, the latch is transparent (i.e., the comparator operates as though the latch is not present). The comparator's output state is stored when LATCH is pulled low. All timing constraints must be met when using the latch function.
The MAX941 shuts down when SHDN is low. When shutdown, the supply current drops to less than 30μA, and the three-state output becomes high impedance. The SHDN pin has a high input impedance. Connect SHDN to V+ for normal operation. Exit shutdown with LATCH high; otherwise, the output will be indeterminate.
Input Stage Circuitry
The MAX941/MAX942/MAX944 include internal protection circuitry that prevents damage to the precision
input stage from large differential input voltages. This protection circuitry consists of four back-to-back diodes between IN+ and IN- as well as two 2.5k½ resistors. The diodes limit the differential voltage applied to the internal circuitry of the comparators to be no more than 4VF, where VF is the forward voltage drop of the diode (about 0.7V at +25°C). For a large differential input voltage (exceeding 4VF), this protection circuitry increases the input bias current at IN+ (source) and IN- (sink).
Input Current =(IN+ - IN-) - 4VF
2 x 2.5k½
Input current with large differential input voltages should not be confused with input bias current (IB). As long as the differential input voltage is less than 4VF, this input current is equal to IB. The protection circuitry also allows for the input common-mode range of the MAX941/MAX942/MAX944 to extend beyond both power-supply rails. The output is in the correct logic state if one or both inputs are within the common-mode range.
Output Stage Circuitry
The MAX941/MAX942/MAX944 contain a current-driven output stage as shown in Figure 4. During an output transition, ISOURCE or ISINK is pushed or pulled to the output pin. The output source or sink current is high during the transition, creating a rapid slew rate. Once the output voltage reaches VOH or VOL, the source or sink current decreases to a small value, capable of maintaining the VOH or VOL static condition. This significant decrease in current conserves power after an output transition has occurred.
One consequence of a current-driven output stage is a linear dependence between the slew rate and the load capacitance. A heavy capacitive load will slow down a voltage output transition. This can be useful in noise sensitive applications where fast edges may cause interference.
- In μMAX Package: Smallest 8-Pin SO
- Optimized for 3V and 5V Applications (operation down to 2.7V)
- Fast, 80ns Propagation Delay (5mV overdrive)
- Rail-to-Rail Input Voltage Range
- Low Power:
1mW Power Dissipation per Comparator (3V)
350μA Supply Current
- Low, 1mV Offset Voltage
- Internal Hysteresis for Clean Switching
- Outputs Swing 200mV of Power Rails
- CMOS/TTL-Compatible Output
- Output Latch
- 3V/5V Systems
- Battery-Powered Systems
- Threshold Detectors/Discriminators
- Line Receivers
- Zero-Crossing Detectors
- Sampling Circuits