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LM3101M Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM3101M Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 20 page Functional Description (Continued) Soft-Start Section Soft-Start is accomplished by gradually increasing the refer- ence voltage during start-up The gradual increase is imple- mented by charging the Soft-Start capacitor CSFST onpin 3 (the SFST pin) The charging current is set according to the following equation ISFST e 045RT A X TLH11436 – 12 FIGURE 6 Soft-Start Block Diagram Typically ISFST starts to flow when the supply voltage is raised above 3V As shown in Figure 6 at the beginning of start-up CSFST is not charged up and the SFST pin pulls down the reference voltage from its nominal value to VSFSTO e ISFST R1 V A X VSFSTO is designed to be 85% of the nominal reference voltage The reference voltage rises smoothly from VSFSTO to its nominal value as CSFST charges up When CSFST charges up completely the reference voltage is at its nomi- nal value start-up is over and steady-state operation be- gins The discharge time of CSFST is set by the RC network as tDS e 5 CSFST R2 This is the time delay required to prepare the Soft-Start cy- cle after the LM3101 and the entire regulator has been turned off R2 must be large enough so that the final value of VSFST is greater than the reference voltage VSFST e ISFST (R1 a R2) t 124V V A X Power Supply Monitor Functions The LM3101 provides two monitor functions a power-on reset flag with programmable delay and a crowbar driver output for overvoltage conditions POWER-ON RESET The power-on reset (POR) flag monitors the output voltage via the feedback pin (FB - pin 13) The POR flag will go low after the output voltage reaches 95% of its nominal value and the subsequent programmed delay has passed The POR flag pin (pin 11) is an open-collector pin which needs an external resistor to pull it up This pin is valid with supply voltages as low as 1V while sinking 16 mA To program the reset delay connect an external capacitor to the CRD pin (pin 12) The practical range of delay is from 10 ms to 5 ms and follows the equation TRD e CRD 60 103 s F For a power-on reset delay of 120 ms the reset delay ca- pacitor must be 0002 mF CROWBAR DRIVER OUTPUT The second monitor function is a crowbar driver output (CD – pin 10) If the output voltage gets higher than 120% of its nominal value the CD pin can supply more than 200 mA to an external SCR trigger input The SCR will fire shorting the regulator output and saving the load circuitry from ex- cessive supply voltage 11 |
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