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LMC6762BIMX 데이터시트(PDF) 9 Page - National Semiconductor (TI) |
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LMC6762BIMX 데이터시트(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Application Hints (Continued) From Figure 3 and Figure 4 the shoot-through current for the LMC6762 can be approximated to be 0.2 mA (200 mV/1 k Ω). The duration of the transient is measured as 1 µs. The val- ues needed for the local bypass capacitors can be calcu- lated as follows: Area of ∆ = 1⁄2 (1 µs x 200 µA) = 100 pC If the local bypass capacitor has to provide this charge of 100 pC, the minimum value of the local capacitor to prevent local degradation of V CC can be calculated. Suppose that the maximum voltage droop that the system can tolerate is 100mV, ∆Q = C * (∆V) →C = (∆Q/∆V) = 100 pC/100 mV = 0.001 µF The low internal feedthrough current of the LMC6762 thus requires lower values for the local bypass capacitors. In ap- plications where precision is not critical, this is a significant advantage, as lower values of capacitors result in savings of board space, and cost. It is worth noting here that the delta shift of the power supply voltage due to the transient currents causes a threshold shift of the comparator. This threshold shift is reduced by the high PSRR of the comparator. However, the value of the PSRR applicable in this instance is the transient PSRR and not the DC PSRR. The transient PSRR is significantly lower than the DC PSRR. Generally, it is a good goal to reduce the delta voltage on the power supply to a value equal to or less than the hysteresis of the comparator. For example, if the comparator has 50 mV of hysteresis, it would be reasonable to increase the value of the local bypass capacitor to 0.01 µF to reduce the voltage delta to 10 mV. 4.0 Output Short Circuit Current The LMC6762 has short circuit protection of 40 mA. How- ever, it is not designed to withstand continuous short circuits, transient voltage or current spikes, or shorts to any voltage beyond the supplies. A resistor is series with the output should reduce the effect of shorts. For outputs which send signals off PC boards additional protection devices, such as diodes to the supply rails, and varistors may be used. 5.0 Hysteresis If the input signal is very noisy, the comparator output might trip several times as the input signal repeatedly passes through the threshold. This problem can be addressed by making use of hysteresis as shown below. The capacitor added across the feedback resistor increases the switching speed and provides more short term hyster- esis. This can result in greater noise immunity for the circuit. 6.0 Spice Macromodel A Spice Macromodel is available for the LMC6762. The model includes a simulation of: • Input common-mode voltage range • Quiescent and dynamic supply current • Input overdrive characteristics and many more characteristics as listed on the macromodel disk. Contact the National Semiconductor Customer Response Center at 1-800-272-9959 to obtain an operational amplifier spice model library disk. Typical Applications One-Shot Multivibrator DS012320-8 FIGURE 4. Measurement of the Shoot-Through Current DS012320-9 DS012320-10 FIGURE 5. Canceling the Effect of Input Capacitance DS012320-14 FIGURE 6. One-Shot Multivibrator www.national.com 9 |
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