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LTC2903IS6-A1 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2903IS6-A1 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LTC2903-1 29031f Calculate the trip voltage from: VV R R TRIP =+ 05 1 1 2 . Table 1 contains suggested 1% resistor values for the ADJ input to obtain nominal –11.5% thresholds. Table 1. Suggested 1% Resistor Values for the ADJ Input VSUPPLY (V) VTRIP (V) R1 (k Ω) R2(k Ω) 12 10.75 2050 100 10 8.95 1690 100 8 7.15 1330 100 7.5 6.7 1240 100 6 5.38 976 100 5 4.435 787 100 3.3 2.935 487 100 3 2.66 432 100 2.5 2.2 340 100 1.8 1.605 221 100 1.5 1.325 165 100 1.2 1.065 113 100 1 0.884 76.8 100 0.9 0.795 59 100 Connect unused supervisor inputs to the highest supply voltage available (typically V1). On the LTC2903-C1, the negative V4 input must always be applied. Implications of Threshold Accuracy Specifying system voltage margin for worst-case opera- tion requires consideration of three factors: power supply tolerance, IC supply voltage tolerance and supervisor re- set 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 ±10% 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 4.5V (5V – 10%), is the exact voltage at which a perfectly accurate supervisor generates a reset. Such a perfectly accurate supervisor does not exist—the actual reset threshold may vary over a specified band ( ±1.5% for the LTC2903-1 su- pervisors). Figure 4 shows the typical relative threshold accuracy for all four inputs, over temperature. With this variation of reset threshold in mind, the nominal reset threshold of the supervisor resides below the mini- mum supply voltage; just enough so that the reset thresh- old band and the power supply tolerance bands do not overlap. If the two bands overlap, the supervisor could generate a false or nuisance reset when the power supply remains within its specified tolerance band (say, at 4.6V). Adding half of the reset threshold accuracy spread (1.5%) to the ideal 10% thresholds puts the LTC2903-1 thresh- olds at 11.5% (typ) below the nominal input voltage. For example, the 5V typical threshold is 4.425V, or 75mV below the ideal threshold of 4.500V. The guaranteed threshold lies in the band between 4.500V and 4.350V over temperature. The powered system must work reliably down to the lowest voltage in the threshold band or risk malfunction before the reset line falls. In the 5V example, using the 1.5% accurate supervisor, the system ICs must work down to 4.35V. System ICs working with a ±2.5%accurate supervisor must operate down to 4.25V, increasing the required system voltage margin and the probability of system malfunction. APPLICATIO S I FOR ATIO Figure 4. LTC2903 Typical Threshold Accuracy vs Temperature TEMPERATURE ( °C) –50 –1.5 –1.0 –0.5 0 0.5 25 75 100 29031 F04 1.0 1.5 –25 0 50 |
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