12

TL431LI

TL432LI

SLVSDQ6A – JULY 2018 – REVISED NOVEMBER 2018

www.ti.com

Product Folder Links: TL431LI TL432LI

Submit Documentation Feedback

Copyright © 2018, Texas Instruments Incorporated

9.3 Feature Description

TL43xLI consists of an internal reference and amplifier that outputs a sink current based 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 19). A Darlington pair is used in order for this device to be able to

sink a maximum current of 15 mA.

When operated with enough voltage headroom (

≥ 2.495 V) and cathode current (IKA), TL43xLI forces the

reference pin to 2.495 V. However, the reference pin can not be left floating, as it needs IREF ≥ 0.4 µA (please

see Specifications). 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, TL43xLI 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 TL43xLI enough gain.

Unlike many linear regulators, TL43xLI 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 12 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 TL43xLI 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, TL43xLI will

have the characteristics shown in Figure 18. With such high gain in this configuration, TL43xLI is typically used

as a comparator. With the reference integrated makes TL43xLI the preferred choice when users are trying to

monitor a certain level of a single signal. Look at SLVA987 for more details on open loop comparator applications

on the TL431LI.

9.4.2 Closed Loop

When the cathode/output voltage or current of TL43xLI is being fed back to the reference/input pin in any form,

this device is operating in closed loop. The majority of applications involving TL43xLI 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 through resistive or direct feedback.