The MAX40027 is a single-supply, dual high-speed comparator with a typical propagation delay of 280ps. The overdrive dispersion is extremely low (25ps, typ.), making these comparators ideal for time-of-flight distance measurement applications.
The input common-mode range of 1.5V to VCC + 0.1V is compatible with the outputs of several widely used high-speed transimpedance amplifiers, such as the MAX40658, MAX40660, and MAX40661. The output stage is LVDS, which helps to minimize power dissipation and interfaces directly with many modern FPGAs and CPUs.
The MAX40027 operates from a +2.7V to +3.6V power supply voltage, while typically consuming only 17mA quiescent current per comparator at 3.3V. The MAX40027 is available in space-saving, 12-pin TDFN package.
Each LVDS output has a switched 3.25mA current source. The outputs are differentially terminated with an external 100Ω resistor, which produces a ±350mV differential output. The power delivered to the 100Ω load resistor is only 1.1mW while enabling transmission data rates up to a few hundreds of Megabits per second. The output common-mode voltage is maintained at 1.23V on both outputs, and is independent of power supply voltage. The fully differential LVDS outputs provide high-speed, digital-signalling with reduced EMI compared to single-ended outputs.
| SYMBOL | SPECIFICATION | DESCRIPTION |
| VOD | Overdrive Voltage | Differential voltage applied across inputs during test. |
| tPDH | Propagation Delay High on OUTx+ | Propagation delay measured from the time the differential input signal changes polarity (± input VOS) to the 50% point in the output low-to-high transition on OUTx+. |
| tPDL | Propagation Delay Low on OUTx- | Propagation delay measured from the time the differential input signal changes polarity (± input VOS) to the 50% point in the output high-to-low transition on OUTx-. |
| ΔtPDHO | Propagation Delay Skew on High | Difference in propagation delay on output transition from low to high on OUTx+ to propagation delay from high to low on OUTx-. |
| ΔtPDLO | Propagation Delay Skew on Low | Difference in propagation delay on output transition from high to low on OUTx+ to propagation delay from low to high on OUTx-. |
| tR | Output Rise Time | Time taken by either OUTx+ or OUTx- to rise from 25% of final output voltage to 75% of final output voltage. |
| tF | Output Fall Time | Time taken by either OUTx+ or OUTx- to fall from 75% of final output voltage to 25% of final output voltage. |
| tPD |
Propagation Delay Low on OUT- | Propagation delay measured from the time the differential input signal changes polarity (± input VOS) to the 50% point in the output high-to-low transition on OUTx-. |
| tPD |
Propagation Delay High on OUT- | Propagation delay measured from the time the differential input signal changes polarity (± input VOS) to the 50% point in the output low-to-high transition on OUTx-. |
| tPDHD | Differential Propagation Delay High | Propagation delay measured from the time the differential input signal changes polarity (± input VOS) to the 50% point in the output differential signal across OUTx+ to OUTx- while switching low to high. |
| tPDLD | Differential Propagation Delay Low | Propagation delay measured from the time the differential input signal changes polarity (± input VOS) to the 50% point in the output differential signal across OUTx+ to OUTx- while switching high to low. |
| VOH | Output Voltage High | Comparator output high state voltage level. |
| VOL | Output Voltage Low | Comparator output low state voltage level. |
The propagation delay is defined as the delay between the differential comparator input voltage changing polarity and the output(s) reaching the midpoint of the high-to-low or low-to-high transition. The low-to-high propagation delay is tPDH on OUTx+ and tPD
Dispersion, or variation of the propagation delay under different conditions, is affected by the amount of overdrive voltage applied to the comparator inputs. As can be seen in the Typical Operating Characteristics, the dispersion is typically under 25ps for 10mV to 1V, a wide range of input overdrive values.
Dispersion is also affected by the input slew rate. As the slew rate of the input signal changes, the propagation delay also changes. The dispersion is typically under 15ps from 0.4V/μs to 1V/μs input slew rates.
The MAX40027 has fixed internal 1.5mV hysteresis, which improves its usefulness for detecting larger differential input signals in the presence of noise. This helps to avoid the external components and potential stability degradation associated with external positive feedback paths.
The MAX40027 includes internal protection circuitry that prevents damage to the precision input stage from large differential input voltages.This protection circuitry consists of two groups of two front-to-back diodes between INx+ and INx-, as well as two 50Ω resistors (Figure 6). The diodes limit the differential voltage applied to the comparator’s internal circuitry to no more than 2(VF), where VF is the diode’s forward-voltage drop (about 0.7V at +25°C).
For a large differential input voltage (exceeding 2(VF)), this protection circuitry increases the input bias current at INx+ (source) and INx- (sink).
Input currents with large differential input voltages should not be confused with input bias currents (IB). As long as the differential input voltage is less than 2(VF), this input current is less than 2IB.
The input circuitry allows the MAX40027’s input common-mode range to extend 100mV beyond the positive power-supply rail. The output remains in the correct logic state if one or both inputs are within the common-mode range. Taking either input outside the common-mode range causes the input to saturate and the propagation delay to increase.
