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LTC2912IDDB-1 Datasheet(PDF) 8 Page - Linear Technology |
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LTC2912IDDB-1 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LTC2912 8 2912fa APPLICATIONS INFORMATION Power-Up/Power-Down As soon as VCC reaches 1V during power up, the UV output asserts low and the OV output weakly pulls to VCC. The LTC2912 is guaranteed to assert UV low, OV high (LTC2912-1, LTC2912-2) and OV low (LTC2912-3) under conditions of low VCC, down to VCC = 1V. Above VCC = 2V (2.1V maximum), the VH and VL inputs take control. Once the VH input and VCC become valid an internal timer is started. After an adjustable delay time, UV weakly pulls high. Threshold Accuracy Reset threshold accuracy is important in a supply-sensitive system. Ideally, such a system resets only if supply voltages fall outside the exact thresholds for a specified margin. Both LTC2912 inputs have a relative threshold accuracy of ±1.5% over the full operating temperature range. For example, when the LTC2912 is programmed to moni- tor a 5V input with a 10% tolerance, the desired UV trip point is 4.5V. Because of the ±1.5% relative accuracy of the LTC2912, the UV trip point can be anywhere between 4.433V and 4.567V which is 4.5V ±1.5%. Likewise, the accuracy of the resistances chosen for RA, RB and RC can affect the UV and OV trip points as well. Using the example just given, if the resistances used to set the UV trip point have 1% accuracy, the UV trip range is between 4.354V and 4.650V. This is illustrated in the following calculations. The UV trip point is given as: VUV = 0.5V 1+ RC RA + RB The two extreme conditions, with a relative accuracy of 1.5% and resistance accuracy of 1%, result in: VUV(MIN) = 0.5V • 0.985 • 1+ RC • 0.99 RA + RB () •1.01 and VUV(MAX) = 0.5V • 1.015 • 1+ RC •1.01 RA + RB () •0.99 For a desired trip point of 4.5V, RC RA + RB = 8 Therefore, VUV(MIN) = 0.5V • 0.985 • 1+ 8 0.99 1.01 = 4.354V and VUV(MAX) = 0.5V • 1.015 • 1+ 8 1.01 0.99 = 4.650V Glitch Immunity In any supervisory application, noise riding on the moni- tored DC voltage causes spurious resets. To solve this problem without adding hysteresis, which causes a new error term in the trip voltage, the LTC2912 lowpass filters the output of the first stage comparator at each input. This filter integrates the output of the comparator before as- serting the UV or OV logic. A transient at the input of the comparator of sufficient magnitude and duration triggers the output logic. The Typical Performance Characteristics show a graph of the Transient Duration vs Comparator Overdrive. UV/OV Timing The LTC2912 has an adjustable timeout period (tUOTO) that holds OV, OV or UV asserted after each fault has cleared. This delay assures a minimum reset pulse width allowing settling time for the monitored voltage after it has entered the “valid” region of operation. |
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