# How to achieve Vref changing with Vdd

1. ## How to achieve Vref changing with Vdd

In the circuit, I want to compare the voltage reference Vref and source voltage Vdd. It is necessary to make Vref change with Vdd, which means a constant voltage is supposed to connect between this two voltage. But I have no idea what structure can achieve this.

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2. ## Re: How to achieve Vref changing with Vdd

This makes no sense. First you say you want to compare Vref with Vdd. Then you say you want Vref to CHANGE with Vdd. THEN you say a constant voltage is supposed to connect between the two voltages.

These are three totally unrelated requirements. Are you, perhaps, trying to design a voltage regulator? Can you restate your question?

3. ## Re: How to achieve Vref changing with Vdd

Usually "reference" means an invariant quantity, not
"tracking".

You can build in relationships by how you propertize
the sources, or use vcvs or even resistor dividers
depending on how the vref "follows" Vdd.

If there's always a constant voltage difference then
any comparison won't be very interesting.

4. ## Re: How to achieve Vref changing with Vdd

Sorry for the unclear expression. A comparator is used to obatin a oscillator. However, I find that if Vdd change and
Vref doesn't change, the output frequency will largely change, which means stable voltage of Vref will reasult poor supply pushing. Thus, I try to make this voltage change with Vdd to reduce the influence of supply induce error. A constant voltage is supposed to connect between this two voltage, but I have no idea what structure can achieve this or other methods to improve the supply pushing of oscillator.

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5. ## Re: How to achieve Vref changing with Vdd

This STILL doesn’t make a lot of sense. If you’ve got a system which has a sensitivity to a certain parameter, in this case Vdd, you can either make Vdd more stable, or make your system less sensitive to it. It has nothing to do with comparing two voltages, or making your reference track Vdd (which defeats the point of a reference!).

Without a schematic, it’s impossible to tell why your oscillator is sensitive to Vdd, or how to fix it.

6. ## Re: How to achieve Vref changing with Vdd

We can’t help unless you provide the relationship you need between “vref” and “vdd”. If you don’t know post your circuit schematic.

Common possibilities: you need a fixed difference in which case I’d “hang” a shunt reference from Vdd. Or proportional in which case you just need to buffer a divider.

7. ## Re: How to achieve Vref changing with Vdd

If you're talking about a bang-bang, switched current source
triangle wave (or sawtooth) oscillator, I have had best luck
with (1/3) / (2/3) resistor dividers as low and high "references"
(like the old NE555). If you make the charge, discharge current
-and- the trip thresholds all supply ratiometric, that's pretty
good.

Alternatively you could make the charge / discharge current
fixed, and make the threshold points fixed as well (like, take
the oscillator output through 2 dividers, both of which are
compared against vref, yielding two different and constant-
spaced switchpoints from a single reference).

Triangle wave is easier than sawtooth as the discharge
stroke of the sawtooth oscillator will have strong temperature
and supply variation, even if the slow charge ramp is made
quasi-constant. I've made triangle wave PWM oscillators
that are near dead flat with temp and supply good past
5MHz in 0.5um CMOS, all switched-resistor and comparators
(and SRFF).

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8. ## Re: How to achieve Vref changing with Vdd

The picture shows my schematic circuit. The reference voltage is compared with the VQ to charge/discharge the capacitor. The VQ is triangle wave and output is sawtooth wave. When Vdd has small supply induce error, the time to close/open the transistor M1-2,M6-7 change which affect the final frequency. So I want this Vref change with Vdd. Then, if Vdd has small increase, the drain voltage of M1-2 and M6-7 and Vref will also increase, which may maintain stable output frequency.

9. ## Re: How to achieve Vref changing with Vdd

Hi,

I assume you don't need a "reference", you just need a voltage divider.

Klaus

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