TL431 Constant Sinking current

Question

A user already asked a question about it at TL431 Constant current source. My SPECIFIC question is about the figure 29. Sure it is not a complete circuit, so I completed it, in order to understand it, with two resistors, R3 and R4, as place holder for possible "useful circuit". And sure, AM1 shows that the current through it is roughly constant (I am using TINA(TM) de Texas Instruments). Also note that AM2 isn't: So, the net effect can be accomplished without using the transistor, with a single diode, as shown here:

And yes, not shown, but AM1 will still be constant. In fact, if we use nothing, and no connection between R3 and R4, AM1 is constant, as it should.

So my question: what is the use of that transistor since it does not help in sinking a constant current between R3 and R4 (which was, I thought, was the purpose of the building block) and since a single diode can do the job ( maybe at a higher expense of energy though ) ?

Note that I varied R3 and R4 with 3 steps each, producing 9 cases, but similar results occur if I use a larger number of steps.

Answer 1

The regulator adjusts its cathode voltage to maintain a constant voltage at the reference input. It's like a reference voltage plus an op-amp with a transistor.

The external transistor will control current depending on the base voltage. So by controlling voltage at the emitter (where the REF input is connected), the collector current (which is about 99.5% of the emitter current) is effectively controlled to within a fraction of 1%, provided you don't run out of compliance voltage. The collector current will be approximately Vref/R2, or about 2.495V/R2

There are slight errors because the transistor gain is not infinite and the REF input draws a couple microamperes, but it's a pretty good current sink. The collector can't quite get to Vref, so the minimum collector voltage is about 2.6V, meaning the voltage across a load to +5 can't be more than about 2.4V/I.

Note that the collector current is what is controlled to be constant. The circuit block is a constant current sink. If you measure the collector current and open up R4 then the collector current should not change (unless you exceed the permissible range of voltages at the collector).

In this case, the current is 2.495V/220\$\Omega\$ = 11.34mA. The maximum load resistance (collector to +5) is about 2.4V/11.34mA = 211\$\Omega\$. When you exceed that resistance you no longer have a constant current sink. So your circuit with 1K is not a useful working example.

The emitter current as a consequence is also constant but that's pretty useless as it changes with R2 so it's more of a set voltage across a resistor.

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Here is a quick LTspice simulation with a 100 ohm resistor as a load:

Ic(Q1):  0.0113147   device_current

Reduce the 100 ohm resistor (R3) to 0 ohms and we have:

Ic(Q1):  0.0113152   device_current

So it's a pretty good current sink. Output resistance (ideally it would be infinite) is about 2M\$\Omega\$.

Similarly, if I increase the supply voltage to 10V with R3 constant at 100 ohms, I get:

Ic(Q1):  0.0113529

So about +0.06% per volt line regulation. That could be improved by increasing R2. With R2 = 1K the line regulation is improved to +0.01%/volt and load regulation is slightly better too.

Answer 2

Perhaps you miss-understand the use of the TL431 and how it works. At heart it is a shunt type voltage regulator, an adjustable zener diode. You supply a safe current through a resistor from a voltage source that is within the TL431's safe limits. By attaching resistors or a trim pot wiper to the control pin, and the CW and CCW pins to cathode and anode, the cathode becomes a multiple of the ratio \$(1 + R1/R2{\cdot} 2.50)\$.

By itself it can only be an adjustable voltage reference, but a very stable one. To get a current clamp or constant current sink you need a transistor to form a feedback loop. Your pdf of this part shown them in detail. Anytime you have a resistor (Re) from the control pin to the anode the cathode remains as positive as possible until the transistor (NPN) conducts enough current so that the Vdrop across the emitter resistor (Re) equals the 2.5 volt internal ref voltage of the TL431. The current is fixed or clamped at 2.5/Re.

This same circuit can act as a current clamp because it will not allow more current to flow at the NPN collector than what creates the 2.5 volt drop on the Re resistor. The current is 'locked' even if the voltage rises up to the safe limits of the transistor, which could be a 300 volt MJE340.

The NPN could be replaced with a N-channel MOSFET with a 700 volts rating. Note that the Collector/Drain voltage can be separate and much higher than what the TL431 needs to function ok. To control a MOSFET the TL431 needs a 12 to 15 volt source so the MOSFET gate can reach the +10 V saturation voltage.

There are many examples for using the TL431 in the datasheet, often using external transistors to boost or limit the current supplied to a load, as well as controlling the supply voltage.

Precision resistors are often required not just for accuracy but because they are very temperature stable, so the voltage/current you set is the same hours later, to .1% or better. Beware of large value trim pots, as they have a temperature drift of ~200 ppm C.

Answer 3

You haven’t included anything varying in your circuit. Of course the current will be constant.

A constant current sink draws the same current as the voltage across it varies. One way to analyze this is to determine the small-signal resistance, dV/dI. For a constant current source this will be very high.