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High-Efficiency Regulator has Low Drop-Out Voltage Using LM2

2018-01-21 15:20  
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National Semiconductor LM2931CT’s regulator, the external transistors Q1 and Q2, and a few passive components, this circuit forms a high-low regulator currently has drop-outs. For the components shown in the figure, the regulator has a drop-out from 0.7 V at 5A load current and 1.3V at 10A high level. Pin on-off of the regulator U1 provides remote control, while the transistor Q2, the maximum output current limit. The same output voltage circuit with PNP regulators U1 and can be expressed as Fout = VREF (R1 R2) / R1 where VREF equal to about 1.2V reference voltage U1. To compensate for the bias-current errors and to keep the extra rest right now due to several μA resistor networks, resistor R1 is set at 28 kO. So for the regulated 5V output voltage, R2 is of 88.7 kO. In addition, the output voltage can be adjusted between 3V and 24V by varying R2.

High Efficiency Regulator has Low Drop Out Voltage Using LM2931CT High Efficiency Regulator has Low Drop Out Voltage Using LM2931CT

Circuit that can handle a lot this time because the external PNP transistor Q1. At the current high level, the circuit’s drop-out voltage is a function of saturation voltage PNP devices. As a result, Q1 should have a low saturation level for the VCE and VBE along with high beta. In addition, the maximum current output is equal to the maximum output sink regulator U1 Q1 multiplied by the maximum beta. A germanium transistors, such as the 2N4277 to the external pass element, meet the above requirements. For the components shown, the circuit provides a very good rule at VIN = 5.7V to 5A load current, providing a drop-out rate is only 0.7 V.

U1 to at least 30 mA of bias by the resistor R3, which also serves as a bleed resistor for Q1. U1 pin on-off additional permits on-off remote control and current limiting function for the circuit. Pulling this pin to ground enables the circuit, while keeping open the circuit and disables the regulator leaves in standby mode. The ratio R5: R6 limits the maximum output current. When the load current exceeds this maximum, the output voltage begins to fall and R6 voltage decreases. It cut the low voltage of the transistor Q2, so disable the output circuit. As a result, the transistor Q1 and load gear and protected from damage.

Source: National Semiconductor Application


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