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LTC1726 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1726 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC2917/LTC2918 9 29178f APPLICATIONS INFORMATION Supply Monitoring The LTC2917/LTC2918 are low voltage single supply monitors with selectable thresholds. Two three-state inputs select one of nine internally programmed thresholds. For the LTC2917, a third three-state input selects the tolerance at which the supply connected to the VM pin is monitored (–5%, –10%, –15%). The tolerance for the LTC2918 is fixed at –5%. Threshold accuracy is guaranteed at ±1.5% over the entire operating temperature range. The LTC2917/LTC2918 asserts the ⎯R⎯S⎯T output low when VM is below the programmed threshold, and for a reset timeout period (tRST) after VM goes above the threshold. The reset timeout can be configured to use one of two internal timers with no external components, or an ad- justed timer programmed by placing an external capacitor from RT to ground. Glitch filtering ensures reliable reset operation without false triggering. Power-Up VCC powers the drive circuits for the ⎯R⎯S⎯T pin. Therefore, as soon as VCC reaches 0.8V during power up, the ⎯R⎯S⎯T output asserts low. Until VCC reaches the undervoltage lockout threshold (guaranteed less than 1.5V), ⎯R⎯S⎯T is held low regardless of the state of VM. Once VCC is above the undervoltage lockout threshold and VM is above the programmed threshold, the reset timer is started. After the reset timeout, the open drain pull-down releases ⎯R⎯S⎯T and the external pull-up resistor pulls high. Power-Down On power-down, once VM drops below its threshold or VCC drops below the undervoltage lockout, ⎯R⎯S⎯T asserts logic low. Monitor Threshold Control The monitor threshold on the VM pin is controlled by the SEL1, SEL2 and TOL three-state pins. The SEL1 and SEL2 pins select one of nine preset nominal voltages (including one externally adjustable threshold) as shown in Table 1. The SEL1 and SEL2 three-state input pins should be con- nected to GND, VCC or left unconnected during normal operation. Note that when left unconnected, the maximum leakage allowable from the pin to either GND or VCC is ±5μA. The tolerance at which the monitored supply is measured is set by the TOL pin (LTC2917 only) as shown in Table 2. If desired (e.g. for margining purposes), the TOL pin may be driven by a three-state buffer. That three-state buffer must have a VOL and VOH which meet the VIL and VIH of the TOL pin specified in the Electrical Characteristics, and maintain less than 5μA of leakage in the open state. Table 1. Voltage Threshold Settings NOMINAL VOLTAGE SEL1 SEL2 12V VCC VCC 5V VCC Open 3.3V VCC GND 2.5V Open VCC 1.8V Open Open *1.5V Open GND *1.2V GND VCC *1V GND Open *ADJ (0.5V) GND GND *Require a separate supply for VCC Table 2. System Voltage Tolerance Settings TOLERANCE TOL –5% VCC –10% Open –15% GND |
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