Ideal LDO Voltage Regulator
LDO = low dropout
|Ideal Low Dropout Linear Voltage Regulator 😉|
Such a popular three-pin voltage regulator as LM317 (
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. But... as the ancient wisdom says: "He does not fight, therefore he is unbeatable in the world."
Roots of the idea
The main idea of the layout discussed in this article has been inspired by one of many
|Vo ~= Vref * (1+R1/R2)|
To my humble opinion the suggested layout has no real advantages against common parts like 7805 or LM317. Minimal voltage dropout here could hardly be less than 2 Volts. Also, there's no over-current or overheating protection. 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 12.6V at 2A from the 12V 5A transformer's secondary. 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? MOSFET: modern power devices come with the RdsON of few milliOhms max. 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. The threshold voltage of power MOSFET's can be 3 to 5 Volts with "usual" devices and still above 1 Volt for those "logic" ones. This voltage would dictate the lowest possible drop across our regulator.
It would have been very interesting to try either DEPLETION MOSFET or J-FET in а similar layout. 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!)
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. Since there is virtually no current through the MOSFET's gate - the additional power supply needs to deliver only few milliAmperes of the current into the pull-up resistor.
|Extreme LDO Regulator - skeleton sch.|
When the potential on the TL431' sense pin gets lower than its threshold of 2.5 Volts due to any reason that lowers the output voltage - the chip conducts less current. Thus it "loosens" the MOSFET's gate that gets pulled up higher by the current through that pull-up resistor. 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 - TL431 control pin) gets higher than needed - the layout works similarly well. TL431 starts conducting more current, pulls the MOSFET's gate down thus reducing the current through its channel. The output gets lower.
Please note that despite some people tend to use TL431 as a comparator - it is the truly linear device.
|TL/LM431 - equivalent block-diagram|
In a real-life device I wanted to have some protection in addition to the slow-blow fuse at the transformer's primary. Thus I decided to sacrifice some 0.5 Volts dropped across the regulator under the normal operation conditions - and gain SAFETY.
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. Nevertheless should you ever need such a solution - let's have a chat 😉
|MOSFET + TL431 = high grade LDO Voltage Linear Regulator|
With the R5-R6-R7 values as drawn - the output voltage can be regulated between 9 and 16 Volts. Of course the real maximum is dictated by the transformer's secondaries.
Make sure R4 can withstand the full load current: PmaxR4 ~= 0.5 / R. 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. There is data showing that exceeding heaters specified voltage by 10% could reduce tube's life-span by ten-fold. Consider some 5% deviations allowed in the power mains voltage plus some 5% to 10% tolerances in the transformers and its output variation in particular design depending on the actual load...
By the way the very same schematic could be used for feeding 6.3V filaments. Provided the transformer puts our at least 6V AC (RMS) and R5 value is decreased to 5.6KOhm.
Consider this LDO voltage regulator is used for supplying DC to the thermionic valve's filament. In such an application soft start would be desirable. The change necessary for obtaining smooth startup curve would be extremely simple: use 1000uF in place of C4 and add 1KOhm resistance between the doubler's bridge positive output and C4 "+" terminal.
Vacuum Tube Myth Busted
As I mentioned above feeding tube heaters with DC has several benefits. 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.
Let's consider potentially the worst scenario when one wanted to "upgrade" an existing tube amplifier by installing a rectifier and a regulator for filaments.
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 1.8 times greater than the DC current at the rectifier's output. Before such an "upgrade" it most probably was Ok working at AC currents slightly below its rating. After an "upgrade" the current spikes charging the reservoir capacitor will:
- overheat the transformer;
- induce new and very unwanted noise into the high voltage supply, especially if the device was fed with one transformer for both heaters and B+.
In place of a conclusion
I would not claim a patent for such a basic schematic. Even though I came up with this idea on my own few years ago - later on I've seen similar layouts being used by several experienced designers elsewhere. By this article I simply want to share this useful design pattern with you, my friends.