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LM2576T-ADJ Scheda tecnica(PDF) 20 Page - ON Semiconductor |
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LM2576T-ADJ Scheda tecnica(HTML) 20 Page - ON Semiconductor |
20 / 28 page LM2576 http://onsemi.com 20 Figure 29. Inverting Buck−Boost Regulator Shutdown Circuit Using a PNP Transistor NOTE: This picture does not show the complete circuit. R2 5.6 k Q1 2N3906 LM2576−XX 1 3 5GN D ON/OFF R1 12 k −Vout +Vin Shutdown Input Off On +V 0 +Vin Cin 100 mF Negative Boost Regulator This example is a variation of the buck−boost topology and it is called negative boost regulator. This regulator experiences relatively high switch current, especially at low input voltages. The internal switch current limiting results in lower output load current capability. The circuit in Figure 30 shows the negative boost configuration. The input voltage in this application ranges from −5.0 V to −12 V and provides a regulated −12 V output. If the input voltage is greater than −12 V, the output will rise above −12 V accordingly, but will not damage the regulator. Figure 30. Negative Boost Regulator 1N5820 100 mH Output 2 4 Feedback Vout = −12 V Typical Load Current 400 mA for Vin = −5.2 V 750 mA for Vin = −7.0 V −5.0 V to −12 V Cout 2200 mF Low Esr Cin 100 mF LM2576−12 1 5 3 ON/OFF GND Vin Vin Design Recommendations: The same design rules as for the previous inverting buck−boost converter can be applied. The output capacitor Cout must be chosen larger than would be required for a what standard buck converter. Low input voltages or high output currents require a large value output capacitor (in the range of thousands of mF). The recommended range of inductor values for the negative boost regulator is the same as for inverting converter design. Another important point is that these negative boost converters cannot provide current limiting load protection in the event of a short in the output so some other means, such as a fuse, may be necessary to provide the load protection. Delayed Startup There are some applications, like the inverting regulator already mentioned above, which require a higher amount of startup current. In such cases, if the input power source is limited, this delayed startup feature becomes very useful. To provide a time delay between the time when the input voltage is applied and the time when the output voltage comes up, the circuit in Figure 31 can be used. As the input voltage is applied, the capacitor C1 charges up, and the voltage across the resistor R2 falls down. When the voltage on the ON/OFF pin falls below the threshold value 1.3 V, the regulator starts up. Resistor R1 is included to limit the maximum voltage applied to the ON/OFF pin. It reduces the power supply noise sensitivity, and also limits the capacitor C1 discharge current, but its use is not mandatory. When a high 50 Hz or 60 Hz (100 Hz or 120 Hz respectively) ripple voltage exists, a long delay time can cause some problems by coupling the ripple into the ON/OFF pin, the regulator could be switched periodically on and off with the line (or double) frequency. Figure 31. Delayed Startup Circuitry R1 47 k LM2576−XX 1 3 5GN D ON/OFF R2 47 k +Vin +Vin C1 0.1 mF Cin 100 mF NOTE: This picture does not show the complete circuit. Undervoltage Lockout Some applications require the regulator to remain off until the input voltage reaches a certain threshold level. Figure 32 shows an undervoltage lockout circuit applied to a buck regulator. A version of this circuit for buck−boost converter is shown in Figure 33. Resistor R3 pulls the ON/OFF pin high and keeps the regulator off until the input voltage reaches a predetermined threshold level with respect to the ground Pin 3, which is determined by the following expression: V th [ V Z1 ) 1.0 ) R2 R1 V BE (Q1) |
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