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LTC2908ITS8-C1-TRMPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC2908ITS8-C1-TRMPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC2908 11 2908fc APPLICATIONS INFORMATION Adjustable Input The noninverting input on the VADJ comparator is set to 0.5V. And the high impedance inverting input directly ties to the VADJ pin. In a typical application, this pin connects to a tap point on an external resistive divider between the positive voltage being monitored and ground (see Figure 3). The following formula derives the value of the R1 resistor in the divider from a particular value of R2 and the desired trip voltage: R1= VTRIP 0.5V –1 R2 R2 = 100k is recommended. Table 1 shows suggested 1% resistor values for various adjustable applications and their corresponding trip thresholds. Table 1. Suggested 1% Resistor Values for the VADJ Inputs VSUPPLY (V) VTRIP (V) R1 (kΩ) R2 (kΩ) 12 11.25 2150 100 10 9.4 1780 100 8 7.5 1400 100 7.5 7 1300 100 6 5.6 1020 100 5 4.725 845 100 3.3 3.055 511 100 3 2.82 464 100 2.5 2.325 365 100 1.8 1.685 237 100 1.5 1.410 182 100 1.2 1.120 124 100 1.0 0.933 86.6 100 0.9 0.840 68.1 100 0.8 0.750 49.9 100 0.7 0.655 30.9 100 0.6 0.561 12.1 100 REGION OF POTENTIAL MALFUNCTION WITH 2.5% MONITOR MINIMUM RELIABLE SYSTEM VOLTAGE SUPPLY TOLERANCE IDEAL SUPERVISOR THRESHOLD 5.000V ±1.5% THRESHOLD BAND ±2.5% THRESHOLD BAND NOMINAL SUPPLY VOLTAGE 4.750V 4.675V 4.600V 4.500V –5.0% –6.5% –8.0% –10% 2908 F04 Figure 4. Threshold Band Diagram Figure 3. Setting the Adjustable Trip Point – + + – R1 1% R2 1% 0.5V LTC2908-A1/LTC2908-B1/LTC2908-C1 VADJ 2908 F03 VTRIP In an application with less than six supply voltages, the unused supervisor inputs should be tied to the closest higher supply voltage available. Threshold Accuracy Specifying system voltage margin for worst-case operation requires the consideration of three factors: power supply tolerance, IC supply voltage tolerance and supervisor reset threshold accuracy. Highly accurate supervisors ease the design challenge by decreasing the overall voltage margin required for reliable system operation. Consider a 5V system with a ±5% power supply tolerance band. System ICs powered by this supply must operate reliably within this band (and a little more, as explained below). The bottom of the supply tolerance band, at 4.75V (5% below 5V), is the exact voltage at which a perfectly ac- curate supervisor generates a reset (see Figure 4). Such a perfectly accurate supervisor does not exist—the actual reset threshold may vary over a specified band (±1.5% for the LTC2908 supervisors). Figure 5 shows the typical relative threshold accuracy for all six inputs over temperature. Figure 5. Typical Threshold Accuracy vs Temperature TEMPERATURE (°C) –50 –1.5 –1.0 –0.5 0 0.5 1.5 –25 025 50 2908 F05 75 100 1.0 |
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