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LTC2934CDC-2-TRPBF Datasheet(PDF) 7 Page - Linear Technology |
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LTC2934CDC-2-TRPBF Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LTC2934 7 2934f APPLICATIONS INFORMATION VOLTAGE MONITORING Unmanaged power can cause various system problems. At power-up, voltage fluctuation around critical thresholds can cause improper system or processor initialization. The LTC2934 provides power management capabilities for the system power-up phase. The supervisory device issues a system reset after the monitored voltage has stabilized. Built-in hysteresis and filtering ensures that fluctuations due to load transients or supply noise do not cause chattering of the status outputs. Comparator undervoltage glitch immunity is shown in the Typical Performance Characteristics section. The curve dem- onstrates the transient amplitude and width required to switch the comparators. Because many batteries exhibit large series resistance, load currents can cause significant voltage drops. The low DC current draw of the LTC2934 (at any input volt- age) does not add to the loading problem. When voltage is initially applied to VCC, RST and PFO pull low once there is enough voltage to turn on the pull-down devices (1V maximum). If the monitored supply voltage falls to the power-fail threshold, the built-in power-fail comparator pulls PFO low. PFO remains low until the PFI input rises above 0.4V plus 2.5% hysteresis. PFO is typically used to signal preparation for controlled shutdown. For example, the PFO output may be connected to a processor nonmaskable interrupt. Upon interrupt, the processor begins shutdown procedures such as supply sequencing and/or storage/erasure of system state in nonvolatile memory. If the monitored voltage drops below the reset threshold, RST pulls low until the ADJ input rises above 0.4V plus 5% hysteresis. This may occur through battery charging or replacement. An internal reset timer delays the return of the RST output to a high state to provide settling and initialization time. The RST output is typically connected to processor reset input. Few, if any external components are necessary for reliable operation. However, a decoupling capacitor between VCC and ground is recommended (0.01μF minimum). Threshold Configuration The LTC2934 monitors voltage applied to its inputs PFI and ADJ. A resistive divider connected between a monitored voltage and ground is used to bias the inputs. Figure 1 demonstrates how the monitor inputs can be made de- pendent upon a single voltage (V1). Only three resistors are required. To calculate their values, specify desired falling power fail (VPF) and reset voltages (VR) with VPF > VR. For example: VPF = 1.72V, VR = 1.62V Figure 1. Configuration for Single Voltage Monitoring 2934 F01 ADJ R3 PFI LTC2934 V1 R2 R1 The solution for R1, R2, and R3 provides three equations and three unknowns. Maximum resistor size is governed by maximum input leakage current. For the LTC2934, the maximum input leakage current over temperature is 1nA. For a maximum error of 1% due to both input currents, the resistor divider current should be 100 times the sum of the leakage currents, or 0.2μA. At the reset threshold, V1 = 1.62V, so RSUM = V1/0.2μA = 8.1M where: RSUM = R1 + R2 + R3 The falling monitor thresholds (VTH) are 0.4 volts, so: R VR V VM V M TH SUM PF 1 04 81 172 188 == = •. • . . . The closest 1% value is 1.87M. R2 can be determined from: R VR V R VM V M R TH SUM R 21 04 81 162 187 2 130 == − = • – .• . . . kk |
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