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MAX922CSA Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX922CSA Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page 6. Verify the threshold voltages with these formulas: Board Layout and Bypassing Power-supply bypass capacitors are not needed if the supply impedance is low, but 100nF bypass capacitors should be used when the supply impedance is high or when the supply leads are long. Minimize signal lead lengths to reduce stray capacitance between the input and output that might cause instability. Do not bypass the reference output. _______________Typical Applications Auto-Off Power Source Figure 5 shows the schematic for a 40mA power supply that has a timed auto power-off function. The comparator output is the switched power-supply output. With a 10mA load, it typically provides a voltage of (VBATT – 0.12V), but draws only 3.5µA quiescent current. This circuit takes advantage of the four key features of the MAX921: 2.5µA supply current, an internal reference, hysteresis, and high current output. Using the component values shown, the three- resistor voltage divider programs the maximum ±50mV of hysteresis and sets the IN- voltage at 100mV. This gives an IN+ trip threshold of approximately 50mV for IN+ falling. The RC time constant determines the maximum power- on time of the OUT pin before power-down occurs. This period can be approximated by: R x C x 4.6sec For example: 2M Ω x 10µF x 4.6 = 92sec. The actual time will vary with both the leakage current of the capacitor and the voltage applied to the circuit. Window Detector The MAX923 is ideal for making window detectors (undervoltage/overvoltage detectors). The schematic is shown in Figure 6, with component values selected for an 4.5V undervoltage threshold, and a 5.5V overvoltage threshold. Choose different thresholds by changing the values of R1, R2, and R3. To prevent chatter at the output when the supply voltage is close to a threshold, hysteresis has been added using R4 and R5. OUTA provides an active-low undervoltage indication, and OUTB gives an active-low overvoltage indication. ANDing the two outputs provides an active- high, power-good signal. The design procedure is as follows: 1. Choose the required hysteresis level and calculate values for R4 and R5 according to the formulas in the Hysteresis (MAX921/MAX923) section. In this example, ±5mV of hysteresis has been added at the comparator input (VH = VHB/2). This means that the hysteresis apparent at VIN will be larger because of the input resistor divider. V rising : V V R1 1 R1 1 R2 1 R3 V falling : V V R1 V R3 IN THR REF IN THF THR =× × + + =− ×+ () Ultra Low-Power, Single/Dual-Supply Comparators ______________________________________________________________________________________ 11 MAX921 OUT IN- HYST REF V- GND IN+ V+ VBATT -0.15V 10mA 21 4 5 6 8 3 100k 1.1M 47k 4.5V TO 6.0V MOMENTARY SWITCH 7 R C Figure 5. Auto-off power switch operates on 2.5µA quiescent current. |
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