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LTC690 Datasheet(PDF) 11 Page - Linear Technology |
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LTC690 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LTC690/LTC691 LTC694/LTC695 Power-Fail Warning The LTC690 family generates a Power Failure Output (PFO) for early warning of failure in the microprocessor's power supply. This is accomplished by comparing the Power Failure Input (PFI) with an internal 1.3V reference. PFO goes low when the voltage at the PFI pin is less than 1.3V. Typically PFI is driven by an external voltage divider (R1 and R2 in Figures 8 and 9) which senses either an unregulated DC input or a regulated 5V output. The voltage divider ratio can be chosen such that the voltage at the PFI pin falls below 1.3V several milliseconds before the 5V supply falls below the maximum reset voltage threshold 4.75V. PFO is normally used to interrupt the microproces- sor to execute shutdown procedure between PFO and RESET or RESET. The power-fail comparator, C3, does not have hysteresis. Hysteresis can be added however, by connecting a resis- tor between the PFO output and the noninverting PFI input pin as shown in Figures 8 and 9. The upper and lower trip points in the comparator are established as follows: When PFO output is low, R3 sinks current from the summing junction at the PFI pin. When PFO output is high, the series combination of R3 and R4 source current into the PFI summing junction. Example 1: The circuit in Figure 8 demonstrates the use of the power-fail comparator to monitor the unregulated power supply input. Assuming the the rate of decay of the supply input VIN is 100mV/ms and the total time to execute a shutdown procedure is 8ms. Also the noise of VIN is 200mV. With these assumptions in mind, we can reason- ably set VL = 7.5V which 1.25V greater than the sum of maximum reset voltage threshold and the dropout voltage S APPLICATI I FOR ATIO V = 1.3V 1+ R1 R2 R1 R3 H + V 1.3V 1 R1 R2 – (5V – 1.3V)R1 1.3V(R3 R4) L =+ + Assuming R4 R3,V 5V R1 R3 HYSTERESIS << = 5V 3V 0.1 µF 10 µF VBATT VCC LTC691 LTC695 VOUT GND 690 F06 VCC RESET CE IN CE OUT RESET 0.1 µF TO µP FROM DECODER CS 20ns PROPAGATION DELAY 62512 RAM GND + Figure 6. A Typical Nonvolatile CMOS RAM Application Figure 9. Monitoring Regulated DC Supply with the LTC690's Power-Fail Comparator 10 µF 10 µF VIN VOUT ADJ 1690 F09 0.1 µF TO µP LT1086-5 VIN ≥ 6.5V R4 10k R1 27k R3 2.7M R2 8.2k 5V R5 3.3k VCC GND PFO PFI LTC690/LTC691 LTC694/LTC695 + + 10 µF 100 µF VIN VOUT ADJ 690 F08 VCC 0.1 µF TO µP PFO GND LT1086-5 VIN ≥ 7.5V R4 10k PFI LTC690/LTC691 LTC694/LTC695 R1 51k R2 10k R3 300k 5V + + Figure 8. Monitoring Unregulated DC Supply with the LTC690's Power-Fail Comparator 5V 3V 0.1 µF 10 µF VBATT VCC LTC690 LTC694 VOUT GND 690 F07 VCC RESET 0.1 µF CS 62128 RAM CS1 CS2 GND + Figure 7. Write Protect for RAM with LTC690 or LTC694 |
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