I saw a ti's constant voltage and constant current power supply design, but didn't understand the principle. Who knows, help explain the role of LMV431A in this area?
Arie Posted on August 21, 2020
This is the internal block diagram of TL431. In fact, it is an op amp, a voltage reference, and a triode.
Replace the TL431 symbol with the internal block diagram, it is easy to understand.
Hubert Posted on August 21, 2020
LM321 op amp input is 20 milliohm current sampling resistor. When the output current of the switching power supply increases, the voltage at the output terminal of the LM321 increases.
The R terminal of TL431 is compared with the 2.5V voltage reference. If the R terminal (that is, the output of the LM321 op amp) to ground voltage is less than 2.5V, then the TL431 on-chip op amp output is low and the on-chip transistor is turned off, which has no effect. The switching power supply is in a constant voltage output state. If the voltage of the R terminal to ground is greater than 2.5V, the internal op amp output of the TL431 is high, and the on-chip transistor is turned on, causing the voltage of the ISEN pin of the LM25085 to decrease, thereby limiting the output current of the entire switching power supply and making the switching power supply enter a constant current state. In this state, the switching power supply cannot maintain a constant voltage output, that is, the part of the circuit that is divided by the two resistors and sent to the FB pin loses its function.
Arjan Posted on August 21, 2020
The reason for using TL431 is to add a 2.5V voltage reference. When the output voltage of LM321 is below 2.5V, TL431 does not work and the switching power supply is in a constant voltage output state.
Schuyler Posted on August 21, 2020
The component values in the figure may be wrong. LM321 constitutes a differential amplifier, the voltage gain is only a little larger than 1. Therefore, the output current must reach 2.5V/0.02Ω=125A to enter the constant current working state, but the chip LM25085 seems to be unable to achieve 125A current output. However, the voltage gain of LM321 can be changed as long as the value of 1 kohm and 24 kohm resistance is modified.
Greer Posted on August 21, 2020
The figure uses LMV431A, the reference voltage is 1.25V, and the LM321 voltage amplification factor is not the differential pressure multiplied by 24K? The constant current point of this design is 2.5A. I just don’t know why ISEN is pulled down, isn’t there a 5KΩ in the middle?
The purpose of my research on this circuit is to use LM5085 to make a charger with a 54V input and a floating charge of 51.6V with a charging current of 1A. However, the maximum voltage of the cathode of the LMV431 is 36V. If I replace the LMV431 with a stronger one, can it be the same? Achieve their goals?
