Is there any advantage of Zener Diode as regulator over LDO other than BOM cost?

[RoadRunner]

i am currently looking through frequency counter stuff, there is one little prescaler circuit which divides couples of Ghz down to Tens of Mhz. and in this section circuit has  simple Zener Diode with and resistor to power presclaer chip with around 5V and current less than 10mA.

i am thinking why did the person who designed it, didn't used a simple LDO. was BOM cost only concern?

is there any advantage of Zener diode and resistor based regulator vs LDO , other than cost ?

any help will be highly appreciated.

[Cliff Matthews]

Where you checking without an active input? Try measuring >1 Ghz..

[bd139]

Zener blows off more power to regulate the same load. Plus the regulation is worse even if you get the zener on the lowest part of the dV curve which usually requires a lot of current. Also if zener goes pop, the circuit will fail to the input voltage likely destroying everything after it.

[RoadRunner]

Where you checking without an active input? Try measuring >1 Ghz..

i am not doing any measurement right now. i have only schematic not the board itself.  Measurement is not my primary concern i just wondered about question why a zener not a LDO.

[David Hess]

Why make things complicated when they can be simple?

With a zener diode, there is none of that nonsense about frequency compensation.  Noise is good.  Line and load regulation is not as good at low frequencies but can be better at high frequencies.

[coppice]

One key thing a zener diode, or other type of shunt regulator, gives you that an LDO doesn't is supply clamping. If energy can get into your Vcc rail, then with an LDO there is nothing to stop the voltage rising to levels which can cause damage. With a shunt regulator the Vcc is clamped. This can be important if your product is susceptible to discharges, RF and other external sources of energy getting into the heart of your electronics. Decoupling caps are usually enough to absorb brief shots of external energy, without a dangerous rise in the Vcc voltage. To withstand a sustained attack you need to clamp Vcc.

[Zero999]

Yes, as mentioned above, clamping is about the only advantage. Quite often you'll find both a voltage regulator and a slightly higher voltage zener diode, which performs the clamping function. If a zener on its own is sufficient to regulate the voltage, then why have an additional regulator?

If there was a voltage regulator, what makes you think it would need to have a low drop-out voltage? If the input voltage is always more than a couple of volts above the output, then a standard regulator would do a better job.

[RoadRunner]

Thank you all for comments. i think it is quite clear now.

[bd139]

Why would you use a zener for clamping though? You can get LDOs with clamps and overvoltage protection built in 

[Zero999]

Why would you use a zener for clamping though? You can get LDOs with clamps and overvoltage protection built in 

Cost, stability? If a cheap zener is adequate, then a fancy IC is a waste of money.

Another thing to consider is the age of the device being discussed by the original poster. It might be from an era when IC regulators with built-in clamping weren't widely available.

[bd139]

Fair points! 

[David Hess]

Why would you use a zener for clamping though? You can get LDOs with clamps and overvoltage protection built in 

Which LDOs have clamps and overvoltage protection on their output?

[coppice]

Why would you use a zener for clamping though? You can get LDOs with clamps and overvoltage protection built in 

Can you name some of the parts you are thinking of? Most LDOs with extended features don't quite do what the headline seems to suggest, or require additional parts like a external current bypass FET.

[dcarr]

Zener regulators also provide effective protection from reversed input polarity.  A LDO might just go pop.

[C]

Most are thinking that Voltage regulation is the main point, it might not be.

People go crazy for voltage regulation, before doing this would be good to know the voltage range the load needs.

It could very well be that noise isolation was the main reason to select the cheap zener.
The isolation by the series resistor is bidirectional.
This load from main supply or
Main supply from this load.

Think most will find that a resistor has a better frequency response then a LDO.

And un like a LDO a zener is a change in load regulator.
To state it a different way if you load current varies by 10% the zener is only regulating that 10% change.

With a load change a LDO creates a load change on main supply.
With a load change a zener creates a constant load on main supply.

C