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LTC2909CDDB-3.3 Datasheet(PDF) 8 Page - Linear Technology |
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LTC2909CDDB-3.3 Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page LTC2909 8 2909fa APPLICATIO S I FOR ATIO The LTC2909 is a low power, high accuracy dual/triple supply monitor with two adjustable inputs and an ac- curate UVLO. Reset timeout may be selected with an external capacitor, set to an internally generated 200ms, or disabled entirely. The three-state polarity select pin (SEL) chooses one of three possible polarity combinations for the adjustable input thresholds, as described in Table 1. Both input voltages (VADJ1 and VADJ2) must be valid (above threshold if con- figured for positive polarity, below threshold if configured for negative polarity), and VCC above the UVLO threshold for the reset timeout before ⎯R⎯S⎯T is released. The LTC2909 asserts the reset output during power-up, power-down and brownout conditions on any of the voltage inputs. Power-Up The LTC2909 uses proprietary low voltage drive circuitry for the ⎯R⎯S⎯T pin which holds ⎯R⎯S⎯T low with as little as 200mV of VCC. This helps prevent an unknown voltage on the ⎯R⎯S⎯T line during power-up. In applications where the low voltage pull-down capabil- ity is important, the supply to which the external pull-up resistor connects should be the same supply which powers the part. Using the same supply for both ensures that ⎯R⎯S⎯T never floats above 200mV during power-up, as the pull- down ability of the pin will then increase as the required pull-down current to maintain a logic low increases. Once VCC passes the UVLO threshold, polarity selection and timer initialization will occur. If the monitored supplies (ADJ1 and ADJ2) are valid, the appropriate timeout delay will begin, after which ⎯R⎯S⎯T will be released. Otherwise, the part will wait until all supplies are valid (including VCC above the UVLO threshold) before beginning the timeout. Power-Down On power-down, once VCC drops below the UVLO threshold or either VADJ becomes invalid, ⎯R⎯S⎯T asserts logic low. VCC of at least 0.5V guarantees a logic low of 0.15V at ⎯R⎯S⎯T. Shunt Regulator The LTC2909 contains an internal 6.5V shunt regulator on the VCC pin to allow operation from a high voltage supply. To operate the part from a supply higher than 6V, the VCC pin must have a series resistor, RCC, to the supply. This resistor should be sized according to the following equation: VV mA R VV µA I SMAX CC SMIN VRE () ( ) –. –. 62 10 68 200 ≤≤ + FF where VS(MIN) and VS(MAX) are the operating minimum and maximum of the supply, and IVREF is the maximum current the user expects to draw from the reference output. As an example, consider operation from an automobile bat- tery which might dip as low as 10V or spike to 60V. Assume that the user will be drawing 100μA from the reference. We must then pick a resistance between 5.4k and 10.7k. When the VCC pin is connected to a low impedance supply, it is important that the supply voltage never exceed 6V, or the shunt regulator may begin to draw large currents. Some supplies may have nominal value sufficiently close to the shunt regulation voltage to prevent sizing of the resistor according to the above equation. For such sup- plies, a 470Ω series resistor may be used. Polarity Selection The external connection of the SEL pin selects the polarities of the LTC2909 adjustable inputs. SEL may be connected to GND, connected to VCC or left unconnected during normal operation. When left unconnected, the maximum leakage allowable from the pin to either GND or VCC is 10μA. Table 1 shows the three possible selections of polarity based on SEL connection. Table 1. Voltage Threshold Selection ADJ1 INPUT ADJ2 INPUT SEL Positive Polarity (+) UV or (–) OV Positive Polarity (+) UV or (–) OV VCC Positive Polarity (+) UV or (–) OV Negative Polarity (–) UV or (+) OV Open Negative Polarity (–) UV or (+) OV Negative Polarity (–) UV or (+) OV Ground Note: Open = open circuit or driven by a three-state buffer in high impedance state with leakage current less than 10μA. If the user’s application requires, the SEL pin may be driven using a three-state buffer which satisfies the VIL, VIH and leakage of the three-state pin. |
Similar Part No. - LTC2909CDDB-3.3 |
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Similar Description - LTC2909CDDB-3.3 |
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