MAX961–MAX964/ Single/Dual/Quad, Ultra-High-Speed, +3V/+5V, MAX997/MAX999 Beyond-the-Rails Comparators VCC ISOURCE MAX961–MAX964MAX997MAX999 Q, Q ISINK GND Figure 4. Output Stage Circuitry Figure 5. MAX961 PCB Layout Output Stage CircuitryApplications Information The MAX961–MAX964/MAX997/MAX999 contain a cur- Circuit Layout and Bypassing rent-driven output stage, as shown in Figure 4. During an output transition, I The MAX961–MAX964/MAX997/MAX999’s high band- SOURCE or ISINK is pushed or pulled to the output pin. The output source or sink current is high width requires a high-speed layout. Follow these layout during the transition, creating a rapid slew rate. Once the guidelines: output voltage reaches VOH or VOL, the source or sink 1) Use a PCB with a good, unbroken, low-inductance current decreases to a small value, capable of maintain- ground plane. ing the VOH or VOL in static condition. This decrease in 2) Place a decoupling capacitor (a 0.1μF ceramic sur- current conserves power after an output transition has face-mount capacitor is a good choice) as close to occurred. VCC as possible. One consequence of a current-driven output stage is a 3) On the inputs and outputs, keep lead lengths short to linear dependence between the slew rate and the load avoid unwanted parasitic feedback around the com- capacitance. A heavy capacitive load slows down the volt- parators. Keep inputs away from outputs. Keep imped- age output transition. ance between the inputs low. Shutdown Mode 4) Solder the device directly to the printed circuit board When SHDN is high, the MAX961/MAX963/MAX964/ rather than using a socket. MAX997 shut down. When shut down, the supply current 5) Refer to Figure 5 for a recommended circuit layout. drops to 270μA per comparator, and the outputs become 6) For slow-moving input signals, take care to prevent high impedance. SHDN has a high input impedance. parasitic feedback. A small capacitor (1000pF or less) Connect SHDN to GND for normal operation. Exit shut- placed between the inputs can help eliminate oscil- down with LE low; otherwise, the output is indeterminate. lations in the transition region. This capacitor causes negligible degradation to tPD when the source imped- ance is low. www.maximintegrated.com Maxim Integrated │
