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Design >> Analog Design >> Voltage Reference Circuit vs. Voltage Regulator
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Message started by chickenvlsi1 on Jun 4^th, 2009, 3:42am

Title: Voltage Reference Circuit vs. Voltage Regulator
Post by chickenvlsi1 on Jun 4^th, 2009, 3:42am

Hi All,
Could you pleas let me know the difference between voltage regulator and voltage reference circuits?
As I understand, reference voltage circuit aims to provide a voltage which is not dependent on PVT variations. And the purpose of voltage regulator is to provide a voltage which is constant regardless of power supply variation. Thus, it seems to me that voltage regulator is just kind a subset of voltage reference circuit. Is that right ?
Why does we need a voltage regulator while we can have fixed voltage with reference circuit?
I'm looking forward to hearing from you.
Thanks.

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by aaron_do on Jun 4^th, 2009, 4:10am

Hi,

from my understanding, a voltage regulator is used for supply regulation whereby the supply voltage is independent of the current drawn by the load (i.e. the supply voltage should be the same whether the system draws 1mA or 100 mA). At the same time, a voltage regulator should operate with high efficiency. i.e. the power dissipated by the regulator should be much less than the load.

As for a voltage bias such as a bandgap reference, current is not drawn from the reference itself, and the load is for the most part unimportant.

cheers,
Aaron

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by raja.cedt on Jun 4^th, 2009, 4:32am

hi,
i think voltage reference is the one which gives constant voltage independent of PVT, but it wont drive any load (like band gap, zenar diode). Where as voltage regulator is ckt which is mainly to supply as much as possible load current by using some voltage reference.

thanks,
Rajasekhar.

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by Berti on Jun 4^th, 2009, 8:09am

My contribution to this thread:

A voltage regulator (LDO or DcDc-Converter) provides voltage conversion, while a voltage reference provides a reference voltage (as the name suggests ;-).

A voltage reference is typically used for a voltage regulator to regulate the output voltage. However, as Aaron and Rajasekhar wrote, the requirements and application of a voltage regulator (to provide a supply voltage with good regulation, efficiency etc.) and a voltage reference (e.g. bandgap providing an accurate voltage) are completely different.

Regards

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by chickenvlsi1 on Jun 4^th, 2009, 8:15am

raja.cedt wrote on Jun 4^th, 2009, 4:32am:

Hi Rajasekhar,
Thank you so much for your reply.
From your answer, I can conclude following points. Please correct me if I'm wrong. My points are as follows:
1. If I have a resistive ladder ( in DAC for instance) and this ladder is connected to a Vref voltage. Because the ladder itself consumes a large curent, Vref should be connected the output of a regulator (rather than the output of a reference circuit) so that Vref is not varied with the current variations in that resistive ladder. Am I right ?
2. If I need to bias a transistor MOSFET by a Vbias voltage which is connected to MOSFET gate terminal, then Vbias should come from the output of a reference circuit rather than a regulator. That's because there's almost zero current flowing into the gate of transistor. I am right ?
Thanks,

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by chickenvlsi1 on Jun 4^th, 2009, 8:18am

Berti wrote on Jun 4^th, 2009, 8:09am:

Thank you very much for you reply,
Is the output voltage of a typical regulator independent of process and temperature variations like references?

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by raja.cedt on Jun 4^th, 2009, 8:44am

hi chickenvlsi1,
i think you got correctly about those. But in the second point even though it is correct conceptually there your bias should vary with process, so no one use voltage reference rather people relay on replica.
For any voltage regulator i think output impedance is depends on PVT because normally it like 1/gm and if you want you can think about how to make gm constant (but i don't have complete knowledge on that )

thanks,
Rajasekhar.

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by aaron_do on Jun 4^th, 2009, 8:57am

Hi,

Quote:

If I have a resistive ladder ( in DAC for instance) and this ladder is connected to a Vref voltage. Because the ladder itself consumes a large curent, Vref should be connected the output of a regulator (rather than the output of a reference circuit) so that Vref is not varied with the current variations in that resistive ladder. Am I right ?

I don't think this is correct. A resistive ladder is a more or less constant load. You don't need a voltage regulator here. You need a voltage reference with maybe a unity gain buffer. BTW a voltage regulator usually has an extremely large off-chip inductor while a bandgap can be implemented entirely on-chip.

I believe you would need voltage regulators to separate power domains. For example, although a system may have only 1 power supply such as a battery, it may include several chips operating at different voltage levels. Furthermore, their power consumptions may vary in time depending on the functions they are performing. Voltage regulators are required to ensure that the power domains don't interact with each other, and the supply voltages are stable despite changes in loading conditions.

Quote:

Is the output voltage of a typical regulator independent of process and temperature variations like references?

It is not as stable as a voltage reference (due to the changing loading conditions), and doesn't need to be as precise either. A voltage reference should be very precise.

cheers,
Aaron

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by raja.cedt on Jun 4^th, 2009, 9:56am

hi Aaron,
ya what your is telling is same but what i feel is voltage regulator and op amp buffer or same , there is no much difference.

Thanks,
rajasekhar

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by chickenvlsi1 on Jun 4^th, 2009, 7:42pm

aaron_do wrote on Jun 4^th, 2009, 8:57am:

Hi,

Quote:

Hi Aaron,
1. You suggest using a unity-gain buffer to isolate Vref from load current. However, in that case, is the output of unity-gain buffer independent of PVT, especially temperature?

2. In the case output of unity-gain buffer does not exhibit PVT independence, I think a reference, which is independent on not only PVT but also its load current, need to be designed.
However,I haven't seen any Bandgap reference which is independent on both PVT and load current yet. Could you please recommend a reference which is both PVT and load current independent?
Thank you very much.

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by aaron_do on Jun 4^th, 2009, 11:19pm

Hi,

Quote:

ya what your is telling is same but what i feel is voltage regulator and op amp buffer or same , there is no much difference.

conceptually they have similarities, but their designs (and design requirements) are very different.

Quote:

However, in that case, is the output of unity-gain buffer independent of PVT, especially temperature?

Yes. The output would follow the bandgap voltage itself, although I guess there would be some error due to the finite DC gain, and also the offset voltage of the opamp.

This is not really my area of expertise (if I have one), so I recommend you read up on voltage regulators and bandgap references to get a better idea of the differences.

cheers,
Aaron

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by Berti on Jun 5^th, 2009, 12:18am

Hi Chickenvlsi1 (funny user name :)

I recommend you the read a few textbooks because I think that this discussion gets confusing.

A buffer and a voltage regulator are for sure not the same!!!! They use completely different principles and have different characteristics, beside a voltage regulator has typically a low bandwidth while a buffer is often designed having a high bandwidth.

Cheers

Title: Re: Voltage Reference Circuit vs. Voltage Regulator
Post by chickenvlsi1 on Jun 5^th, 2009, 1:07am

Berti wrote on Jun 5^th, 2009, 12:18am:

Thank Berti,
I really appreciate your help.
Have a nice weekend.