Other Parts Discussed in Thread: LMV431A, LM5175, INA213, INA214, LMV431, TLV431
Dear Garrett,
i'm sorry for my delay, i tried to analyze the circuit you provided in https://e2e.ti.com/support/power-management/f/196/t/889467
So i set the input power limit to 24W and the input voltage goes from 9V to 27V, so i calculated the current limits that we have to program
| Pin [W] | 24 | 24 | 24 | 24 | 24 |
| Vin[V] | 27 | 24 | 18 | 12 | 9 |
| Iin [A] | 0.888889 | 1 | 1.333333 | 2 | 2.666667 |
After that i set the Rsense to 15mOhm and the gain of opmap to 100V/V and calculated the Vout of OPAMP
| Rsens[Ohm] | 0.015 | 0.015 | 0.015 | 0.015 | 0.015 |
| Vsense [V] | 0.013333 | 0.015 | 0.02 | 0.03 | 0.04 |
| Gain [V/V] | 100 | 100 | 100 | 100 | 100 |
| Vout [V] | 1.333333 | 1.5 | 2 | 3 | 4 |
According to http://www.ti.com/lit/an/slva861/slva861.pdf?ts=1588679107379 i set the RFBT= 10KOhm , RFBB= 10KOhm and Rdac to 4.99kOhm
so i get for different Vdac always 1.24V on REF pin of LMV431A for current limit.
| Vref [V] | 1.24 | 1.24 | 1.24 | 1.24 | 1.24 |
| Vdac [V] | 1.812187 | 1.72902 | 1.47952 | 0.98052 | 0.48152 |
now if the current input limit is exceeded The REF pin will go above 1.24V.
Now this REF pin is compared to internal 1.24V ref of the LMV431A
1) if the voltage on REF pin is greater than internal 1.24V ref what will then happen ?
in one of you videos (3:15)
(https://www.ti.com/power-management/voltage-reference/shunt-voltage-reference/overview.html -> Learn more about shunt voltage references -> Voltage Reference Use Cases)
it says if the sensed voltage is grater than internal then the shunt regulator pulls to cathode and if it is less than internal then pulls to GND, what does this mean?
You only said in your schematic " Pulls down to create current limit"
"When the current sens signal voltage is higher than the reference voltage the voltage reference pulls down the ISNS- pin engaging the internsl average loop current loop and limiting the input current."
I don't understand this
2) when the current limit is exceeded then the sensed REF pin is greater then internal 1.24V ref and the shunt pulls to cathode.
what is then the voltage across anode and cathode of the LMV431A , what is the current that is going through LMV431A ?
3) You said " The 20kOhm value just ensure that the Voltage reference doesn't exceed the rated voltage."
how is the 20Kohm resistor calculated? Also when it this calculated when REF>1.24 , REF=1.24V or REF<1.24V?
What is the voltage across 20kOmh when REF>1.24 , REF=1.24V or REF<1.24V?
4)You said "the 100ohm just allows for a small voltage drop across the ISNS= and ISNS- pins"
form the LM5175 DS we see that when the current sense gm amplifier outputs grater than 50mV then the Css is discharged .
so when i have 100ohm across ISNS pins this means when REF=1.24V then current through 100ohm is 50mV/100Ohm= 0.5mA if the REF>1.24V then this current through 100ohm resistor is greater and Css starts discharging.
How is this resistor calculated?
Because the ISNS+ == Vin and Vin is not always the same
R=(Vin- Visns)/(0.5mA) when REF=1.24V -> does this mean for different VIn the voltage across 20kohm or across LMV431A will not be the same?
5) how is the 10nF cap and 0R selected, simulated or calculated?
Best Regards,
David.
