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LM2596T-ADJ Datasheet(PDF) 25 Page - National Semiconductor (TI) |
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LM2596T-ADJ Datasheet(HTML) 25 Page - National Semiconductor (TI) |
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25 / 31 page  Application Information (Continued) DELAYED STARTUP The circuit in Figure 21 uses the the ON /OFF pin to provide a time delay between the time the input voltage is applied and the time the output voltage comes up (only the circuitry pertaining to the delayed start up is shown). As the input voltage rises, the charging of capacitor C1 pulls the ON /OFF pin high, keeping the regulator off. Once the input voltage reaches its final value and the capacitor stops charging, and resistor R 2 pulls the ON /OFF pin low, thus allowing the circuit to start switching. Resistor R 1 is included to limit the maximum voltage applied to the ON /OFF pin (maximum of 25V), reduces power supply noise sensitivity, and also limits the capacitor, C1, discharge current. When high input ripple voltage exists, avoid long delay time, because this ripple can be coupled into the ON /OFF pin and cause problems. This delayed startup feature is useful in situations where the input power source is limited in the amount of current it can deliver. It allows the input voltage to rise to a higher voltage before the regulator starts operating. Buck regulators require less input current at higher input voltages. UNDERVOLTAGE LOCKOUT Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage. An und- ervoltage lockout feature applied to a buck regulator is shown in Figure 22, while Figure 23 and 24 applies the same feature to an inverting circuit. The circuit in Figure 23 fea- tures a constant threshold voltage for turn on and turn off (zener voltage plus approximately one volt). If hysteresis is needed, the circuit in Figure 24 has a turn ON voltage which is different than the turn OFF voltage. The amount of hyster- esis is approximately equal to the value of the output volt- age. If zener voltages greater than 25V are used, an addi- tional 47 k Ω resistor is needed from the ON /OFF pin to the ground pin to stay within the 25V maximum limit of the ON /OFF pin. INVERTING REGULATOR The circuit in Figure 25 converts a positive input voltage to a negative output voltage with a common ground. The circuit operates by bootstrapping the regulator’s ground pin to the negative output voltage, then grounding the feedback pin, the regulator senses the inverted output voltage and regu- lates it. This example uses the LM2596-5.0 to generate a −5V out- put, but other output voltages are possible by selecting other output voltage versions, including the adjustable version. Since this regulator topology can produce an output voltage that is either greater than or less than the input voltage, the maximum output current greatly depends on both the input and output voltage. The curve shown in Figure 26 provides a guide as to the amount of output load current possible for the different input and output voltage conditions. The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and this must be limited to a maximum of 40V. For example, when convert- ing +20V to −12V, the regulator would see 32V between the input pin and ground pin. The LM2596 has a maximum input voltage spec of 40V. Additional diodes are required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or noise from coupling through the C IN capacitor to the output, under light or no load conditions. Also, this diode isolation changes the topology to closley resemble a buck configuration thus pro- viding good closed loop stability. A Schottky diode is recom- mended for low input voltages, (because of its lower voltage drop) but for higher input voltages, a fast recovery diode could be used. Without diode D3, when the input voltage is first applied, the charging current of C IN can pull the output positive by sev- eral volts for a short period of time. Adding D3 prevents the output from going positive by more than a diode voltage. 01258336 FIGURE 21. Delayed Startup 01258337 FIGURE 22. Undervoltage Lockout for Buck Regulator 01258338 This circuit has an ON/OFF threshold of approximately 13V. FIGURE 23. Undervoltage Lockout for Inverting Regulator www.national.com 25 |
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