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Topic: adjustable voltage regulator arithmetic/calculations (NCP1117) (Read 1 time) previous topic - next topic

jujun

Hello,

I'am trying to "configure" an adjustable voltage regulator to make 4.3V, but I should be stupid, I can't understand how in the datasheet.


For me the problem is about Cadj, I dont understand how to deal with it.

The datasheet :
http://www.onsemi.com/pub_link/Collateral/NCP1117-D.PDF

Thanks for your help,

Julien N

ash901226

since your only asking for the value of Cadj,
what i can say is that if you really read the datasheet you will find on page 9 this info
Quote

External Capacitors
Input bypass capacitor Cin may be required for regulator
stability if the device is located more than a few inches from
the power source. This capacitor will reduce the circuit's
sensitivity when powered from a complex source impedance
and significantly enhance the output transient response. The
input bypass capacitor should be mounted with the shortest
possible track length directly across the regulator's input
and ground terminals. A 10 micro F ceramic or tantalum
capacitor should be adequate for most applications.


well basically the higher the value of the cap, the immune it is toward variation of input voltage ripple

jujun

Ok, I understand, It's because IADJ is not related to CADJ !

But, how to know IADJ ?

michinyon

Iadj  should be in the datasheet for the device.   It's a fixed amount which is very small.
The output voltage is determined by R1 and R2.

The line from the output through R1 and R2 to ground, forms a voltage divider circuit which is
measured by the bottom connection on the chip,  to regulate the output voltage.

The Iadj in the equation,  is an adjustment to the voltage divider equation,  to compensate for
the small current going into the voltage sensing connection of the regulator chip.

I am not sure what Cadj is for at all.

If you are trying to make 4.3V,   make sure you read about how much Vin  has to be, compared to Vout.
It has to be 0.7V or 1 volt more than Vout,  that will also be in the datasheet.   You might not be able
to get 4.3V  from a 5V supply.


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