# Efficient usage of Linear Voltage Regulators

#### Yaşar Arabacı

Joined Nov 11, 2014
49
Disclamer: I don't have formal education in Electronics, I am pursuing it as a hobby.

Hi,

Given that linear voltage regulators waste power proportional to output current, I was thinking that using them only to control voltage-controlled IC's like mosfets and op-amps would be highly efficient. As a results, I came up with configuration as seen in picture below. It is very similiar to how buck converters work. I use a op-amp in comparator configuration to monitor output voltage to lm7805 output (5V) and switch a mosfet on/off accordingly in an attempt to regulate voltage. I was wondering if this configuration would work in real life or is there anything I am missing out?

#### Yaşar Arabacı

Joined Nov 11, 2014
49
And from what I see you are trying to rediscover hysteresis buck converter.
When I came up with the idea, I thought to myself it is either a stupid idea, or it is already done. I am glad it is the latter

But what is the point of using a linear voltage regulator
I use it as a voltage reference.

#### ScottWang

Joined Aug 23, 2012
6,933
The Vin min of LM7805 is 7V, but I prefer to calculates it as 8V, so when the input is 6V then it can't be regulate a stable 5V output.

#### ian field

Joined Oct 27, 2012
6,539
Disclamer: I don't have formal education in Electronics, I am pursuing it as a hobby.

Hi,

Given that linear voltage regulators waste power proportional to output current, I was thinking that using them only to control voltage-controlled IC's like mosfets and op-amps would be highly efficient. As a results, I came up with configuration as seen in picture below. It is very similiar to how buck converters work. I use a op-amp in comparator configuration to monitor output voltage to lm7805 output (5V) and switch a mosfet on/off accordingly in an attempt to regulate voltage. I was wondering if this configuration would work in real life or is there anything I am missing out?
View attachment 176649
Some computer monitor manufacturers used them to provide bias for the common base section of casc-ode CRT cathode drivers. You need some load on the output to make the series pass element work correctly - about 4k7 is probably good enough.

#### Papabravo

Joined Feb 24, 2006
14,389
Linear Voltage Regulators can be tweaked to have a Low DropOut voltage. The lower the dropout voltage, the lower the amount of wasted power. You wouldn't normally operate an LDO regulator at the dropout point because you need some margin to ensure the device continues to regulate. You certainly don't want it bouncing in and out of regulation. With an SMPS you can control the frequency of switching. the higher the frequency, the easier it is to filter.

As a data point I regularly run sensitive receivers from an SMPS with signals as low as -130 dBm without so much as a hint of noise or interference from the power supply.

#### ian field

Joined Oct 27, 2012
6,539
Linear Voltage Regulators can be tweaked to have a Low DropOut voltage. The lower the dropout voltage, the lower the amount of wasted power. You wouldn't normally operate an LDO regulator at the dropout point because you need some margin to ensure the device continues to regulate. You certainly don't want it bouncing in and out of regulation. With an SMPS you can control the frequency of switching. the higher the frequency, the easier it is to filter.

As a data point I regularly run sensitive receivers from an SMPS with signals as low as -130 dBm without so much as a hint of noise or interference from the power supply.
The TS isn't drawing enough current for that to be an issue - less than enough for the series pass element to work properly in fact.

Joined Feb 20, 2016
3,602
It is a fairly sound and well used principle to have a switching pre-regulator to lower a higher voltage down to a bit over the dropout voltage of a linear regulator. That way you do lose some efficiency of a full switch mode reg, but gain the much better noise figure of the linear one. So, while your circuit may not be the way to do it, the idea is quite sound. At least, it is if you are after the sort of reg I describe above.

Last edited:

#### Yaşar Arabacı

Joined Nov 11, 2014
49
It is a fairly sound and well used principle to have a switching pre-regulator to lower a higher voltage down to a bit over the dropout voltage of a linear regulator.
What I was thinking was actually the exact opposite of it, but I see your point.