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LTC2919IDDB-3.3-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC2919IDDB-3.3-PBF Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page LTC2919 8 2919f APPLICATIO S I FOR ATIO The LTC2919 is a low power, high accuracy triple/dual supply monitor with two adjustable inputs and an accurate UVLO that can monitor a third supply. 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. An individual output is released when its corresponding ADJ input is valid (above threshold if configured for positive polarity, below threshold if configured for negative polarity). Both input voltages (VADJ1 and VADJ2) must be valid and VCC above the UVLO threshold for longer than the reset timeout period before RST is released. The LTC2919 as- serts the reset output during power-up, power-down and brownout conditions on any of the voltage inputs. Power-Up The LTC2919 uses proprietary low voltage drive circuitry for the RST, OUT1 and OUT2 pins which holds them low with VCC as low as 200mV. This helps prevent indeter- minate voltages from appearing on the outputs 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 pow- ers the part. Using the same supply for both ensures that RST, OUT1 and OUT2 never float 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 VCCpassestheUVLOthreshold,polarityselectionand timer initialization will occur. If the monitored ADJ input is valid, the corresponding OUT will be released. When both ADJ1 and ADJ2 are valid, the appropriate timeout delay will begin, after which RST will be released. Power-Down On power-down, once VCCdropsbelowtheUVLOthreshold or either VADJ becomes invalid, RST asserts logic low. VCC of at least 0.5V guarantees a logic low of 0.15V at RST. Shunt Regulator The LTC2919 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 current-limiting series resistor, RCC, to the supply. This resistor should be sized according to the following equation: V S(MAX) – 6.2V 10mA ≤ RCC ≤ V S(MIN) – 6.8V 200µA + IREF where VS(MIN) and VS(MAX) are the operating minimum and maximum of the supply, and IREF 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 a nominal value sufficiently close to the shunt regulation voltage to prevent sizing of the resistor according to the above equation. For such supplies, a 470Ω series resistor may be used. Adjust Polarity Selection The external connection of the SEL pin selects the polarities of the LTC2919 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 is ±5μA. 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 5μA. |
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