LM431 PDF

Устройство и принцип действия[ править править код ] Принципиальная схема. Положительное управляющее напряжение Uref прикладывается между управляющим входом и анодом, а выходным сигналом служит ток катод-анод IKA [5]. Физически обе функции плотно, неразрывно интегрированы во входных каскадах TL Дифференциальный усилитель образуют два встречно включённых источника тока на транзисторах T8 и T9: положительная разница между токами коллекторов T8 и T9, ответвляющаяся в базу T10, управляет выходным каскадом [3]. Каких-либо средств защиты от перегрева или перегрузки по току не предусмотрено [3] [8].

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Something inside me protests against losing more than 3 Volts on a stupid transistor that does only that: heats up the whole design. The popular solution to this problem - switched mode power supplies - we do not consider here because of the noise they produce. Of course there are techniques to reduce the noise.

Minimal voltage dropout here could hardly be less than 2 Volts. The only imaginable benefit versus 3-pin regulators is: the max current can be pumped up as high as one wants to.

Evolution of the idea Recently I faced the necessity to get stabilized Power-wise that was a nice fit. The only problem was that the ripple voltage along with losses on rectifiers left me say a Volt or two max for the loss across the regulator.

Which active component can act as a regulator with sub-volt dropout? With only few Amperes current - we are losing just few milliVolts across the device.

A straight replacement of the Darlington from the schematic cited above would not do any good for us. This voltage would dictate the lowest possible drop across our regulator.

Unfortunately decent power devices of these types are not available. Nothing that I know, at least. Please, please correct me and tell me that I am wrong! The Solution An additional low-current power supply comes to the rescue. It should provide the potential of few Volts above the input positive rail: that would be just enough to pull the gate of the MOSFET up to open the device. Extreme LDO Regulator - skeleton sch. The FET starts conducting more current and pulls the output higher - by doing that effectively restoring the balance.

In an opposite scenario, when for whatever reason the output and its direct derivative - TL control pin gets higher than needed - the layout works similarly well. The output gets lower. Please note that despite some people tend to use TL as a comparator - it is the truly linear device. Thus I decided to sacrifice some 0. By the way it could be much less of a dropout voltage even with the over-current protection. But such a precision protection circuitry becomes slightly more complicated.

With load currents higher than 2A power rectifiers should be attached to some heat-sink. With the R5-R6-R7 values as drawn - the output voltage can be regulated between 9 and 16 Volts. For these particular specifications I would take R4 rated at 2W. Why would you build this For example: in a vacuum tube based project in order to feed tubes filaments with the DC. Why DC for dumb heaters? Even more: such precisely stabilized direct current? Feeding heaters by DC reduces leakage of the mains frequency or its second harmonic into the signal path.

There are several ways the hum gets into the signal through the tube heaters. In fact this topic worth another thorough article We do want the voltage applied to heaters to stay within tight tolerances.

By the way the very same schematic could be used for feeding 6. In such an application soft start would be desirable. However there can also be very realistic reasons that explain why certain "gurus" do not like DC at heaters. These worth a note here, so that those willing to utilize the LDO regulator described in this article do not fall into these traps. Believe it or not, but in most cases the transformer becomes overloaded after such vivisection of the device. In fact the secondary must be rated to supply AC current of around 1.

Before such an "upgrade" it most probably was Ok working at AC currents slightly below its rating. In place of a conclusion I would not claim a patent for such a basic schematic.

By this article I simply want to share this useful design pattern with you, my friends.

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