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LM2575T-5.0 Datasheet(PDF) 21 Page - National Semiconductor (TI) |
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LM2575T-5.0 Datasheet(HTML) 21 Page - National Semiconductor (TI) |
21 / 27 page Additional Applications (Continued) UNDERVOLTAGE LOCKOUT In some applications it is desirable to keep the regulator off until the input voltage reaches a certain threshold. An und- ervoltage lockout circuit which accomplishes this task is shown in Figure 12, while Figure 13 shows the same circuit applied to a buck-boost configuration. These circuits keep the regulator off until the input voltage reaches a predeter- mined level. V TH ≈ V Z1 +2VBE (Q1) DELAYED STARTUP The ON /OFF pin can be used to provide a delayed startup feature as shown in Figure 14. With an input voltage of 20V and for the part values shown, the circuit provides approxi- mately 10 ms of delay time before the circuit begins switch- ing. Increasing the RC time constant can provide longer delay times. But excessively large RC time constants can cause problems with input voltages that are high in 60 Hz or 120 Hz ripple, by coupling the ripple into the ON /OFF pin. ADJUSTABLE OUTPUT, LOW-RIPPLE POWER SUPPLY A 1A power supply that features an adjustable output voltage is shown in Figure 15. An additional L-C filter that reduces the output ripple by a factor of 10 or more is included in this circuit. 01147516 Typical Load Current 200 mA for VIN = −5.2V 500 mA for VIN = −7V Note: Pin numbers are for TO-220 package. FIGURE 11. Negative Boost 01147517 Note: Complete circuit not shown. Note: Pin numbers are for the TO-220 package. FIGURE 12. Undervoltage Lockout for Buck Circuit 01147518 Note: Complete circuit not shown (see Figure 10). Note: Pin numbers are for the TO-220 package. FIGURE 13. Undervoltage Lockout for Buck-Boost Circuit 01147519 Note: Complete circuit not shown. Note: Pin numbers are for the TO-220 package. FIGURE 14. Delayed Startup www.national.com 21 |
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