TL431BILPE3 datasheet(21/83 Pages) TI

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9.3 Feature Description

TL43xx consists of an internal reference and amplifier that outputs a sink current base on the difference between

the reference pin and the virtual internal pin. The sink current is produced by the internal Darlington pair, shown

in the above schematic (Figure 9-2). A Darlington pair is used in order for this device to be able to sink a

maximum current of 100 mA.

When operated with enough voltage headroom (≥ 2.5 V) and cathode current (IKA), TL431 forces the reference

pin to 2.5 V. However, the reference pin can not be left floating, as it needs IREF ≥ 4 µA (please see Electrical

Characteristics, TL431C, TL432C). This is because the reference pin is driven into an npn, which needs base

current in order operate properly.

When feedback is applied from the Cathode and Reference pins, TL43xx behaves as a Zener diode, regulating

to a constant voltage dependent on current being supplied into the cathode. This is due to the internal amplifier

and reference entering the proper operating regions. The same amount of current needed in the above feedback

situation must be applied to this device in open loop, servo or error amplifying implementations in order for it to

be in the proper linear region giving TL43xx enough gain.

Unlike many linear regulators, TL43xx is internally compensated to be stable without an output capacitor

between the cathode and anode. However, if it is desired to use an output capacitor Figure 9-2 can be used as a

guide to assist in choosing the correct capacitor to maintain stability.

9.4 Device Functional Modes

9.4.1 Open Loop (Comparator)

When the cathode/output voltage or current of TL43xx is not being fed back to the reference/input pin in any

form, this device is operating in open loop. With proper cathode current (Ika) applied to this device, TL43xx

will have the characteristics shown in Figure 9-1. With such high gain in this configuration, TL43xx is typically

used as a comparator. With the reference integrated makes TL43xx the prefered choice when users are trying to

monitor a certain level of a single signal.

9.4.2 Closed Loop

When the cathode/output voltage or current of TL43xx is being fed back to the reference/input pin in any form,

this device is operating in closed loop. The majority of applications involving TL43xx use it in this manner to

regulate a fixed voltage or current. The feedback enables this device to behave as an error amplifier, computing

a portion of the output voltage and adjusting it to maintain the desired regulation. This is done by relating the

output voltage back to the reference pin in a manner to make it equal to the internal reference voltage, which can

be accomplished via resistive or direct feedback.

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TL431, TL432

SLVS543Q – AUGUST 2004 – REVISED JULY 2022

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