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LTC2938IMS Datasheet(PDF) 10 Page - Linear Technology |
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LTC2938IMS Datasheet(HTML) 10 Page - Linear Technology |
10 / 20 page LTC2938/LTC2939 10 293839ff Supply Monitoring The LTC2938 and LTC2939 are low power, high accuracy configurable four (LTC2938) and six (LTC2939) supply monitoring circuits with reset output and watchdog func- tions. Both watchdog and reset timeouts are adjustable using external capacitors. Single-pin configuration selects one of sixteen input voltage monitor combinations. All four (LTC2938) or six (LTC2939) voltage inputs must be above predetermined thresholds for the reset not to be invoked. The LTC2938/LTC2939 assert the reset during power-up, power-down and brownout conditions on any one of the voltage inputs. Power-Up The greater of V1 or V2 serves as the internal supply voltage (VCC). On power-up, VCC powers the drive circuits for the RST output. This ensures that the RST output will be low as soon as V1 or V2 reaches 1V. The RST output remains low until the part is configured. After configura- tion, if any one of the supply monitor inputs is below its configured threshold, RST will be at logic low. Once all the monitor inputs rise above their thresholds, an internal timer is started and RST is released after the configured delay time. APPLICATIONS INFORMATION Figure 1. 1.5% Threshold Accuracy Improves System Reliability Threshold Accuracy Consider a 5V system with ±5% tolerance. The 5V supply may vary between 4.75V to 5.25V. System ICs powered by this supply must operate reliably within this band (and a little more as subsequently explained). A perfectly accurate supervisor for this supply generates a reset at exactly 4.75V. However, no supervisor is perfect. The actual reset threshold of a supervisor varies over a specified band. The LTC2938/LTC2939 varies ±1.5% around its nominal threshold voltage (see Figure 1) over temperature. The reset threshold band and the power supply tolerance bands should not overlap. This prevents false or nuisance resets when the power supply is actually within its speci- fied tolerance band. The LTC2938 and LTC2939 have ±1.5% reset threshold accuracy, so a 5% threshold is typically set to 6.5% below the nominal input voltage. Therefore, a typical 5V, 5% threshold is 4.675V. The threshold is guaranteed to lie in the band between 4.750V and 4.600V over temperature. The powered system must work reliably down to the low end of the threshold band, or risk malfunction before a reset signal is properly issued. 5V 4.75V 4.675V ±1.5% THRESHOLD BAND 4.6V NOMINAL SUPPLY VOLTAGE SUPPLY TOLERANCE MINIMUM RELIABLE SYSTEM VOLTAGE IDEAL SUPERVISOR THRESHOLD REGION OF POTENTIAL MALFUNCTION –5% –6.5% –8% 293839 F01 |
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