Want to know the value of resistors

[rajbuddy]

Hello guys,

Please find the attached file. It is a charge pump LTC 3240-3.3 where I am using it for the microcontroller and the ISO 7816 Sim card. I am confused in the voltage divider circuit for the resistor values as my Vin is 5V and Vout will be 3.3V.

[hubi]

I am not sure what the voltage divider is for. What you have labeled as VIN is the 3.3V output, the 4.7uF capacitor should be to GND. There is a schematic on page one of the data sheet that shows how to connect it https://www.analog.com/media/en/technical-documentation/data-sheets/3240fb.pdf.

The SHDN pin cannot be left floating, it has to be high to turn it on. Also, you should not use a polarized flying capacitor. From the data sheet: Warning: A polarized capacitor such as tantalum or aluminum should never be used for the flying capacitor since its voltage can reverse upon start-up of the LTC3240. Low ESR ceramic capacitors should always be used for the flying capacitor.

[rajbuddy]

Thanks for the info. I have made the correction. Vin is the Input of Microcontroller which I am going to use. Please do find the whole circuitry photo and let me know if there's any mistake?

[hubi]

The capacitor between C+ and C- should not be polarized.

Are you using the voltage divider to read battery voltage? I am not familiar with the STM32, is PB0 an analog input?

[rajbuddy]

Yes I have corrected the capacitor part. Yes I want to read the battery voltage for that I am using a voltage divider. Yes PB0 is an analog pin. Please tell me what would be the suitable resistor value for the voltage divider looking at this circuit

[hubi]

I don't really know enough to make a good recommendation. I assume the micro can read up to 3.3V on the analog input, so dividing the voltage in half should be ok. I guess you want to use the highest resistor values you can get away with so you don't drain the battery too much. Another option would be to use a spare I/O pin to disable the voltage divider when you are not taking a measurement.

[Terry Bites]

You've got Vcc=5V and a regulated 3.3V. The regulator looks misconnected, see attched.
This regulator is not a good way to generate a ref voltage, its noisy and not very precise.
Its not a good ref source for an ADC. You'll get a load of random errors.
You dont want to pot it down with some resistors from Vcc for the same reasons.
If that's what it's for then you want an external ref like a LM4040_xx or a TL431.
Then why not use the internal ref on this micro?

[hubi]

You've got Vcc=5V and a regulated 3.3V. The regulator looks misconnected, see attched.
This regulator is not a good way to generate a ref voltage, its noisy and not very precise.
Its not a good ref source for an ADC. You'll get a load of random errors.
You dont want to pot it down with some resistors from Vcc for the same reasons.
If that's what it's for then you want an external ref like a LM4040_xx or a TL431.
Then why not use the internal ref on this micro?

I think VREF_OUT in the schematic is not a reference voltage for the ADC, it's just used to measure the battery voltage scaled down to the range of the ADC on the microcontroller.

[rajbuddy]

hI hUBI, yES IT IS JUST AN  output pin as of 3.3 V which I am going to use in voltage divider as an output voltage. So what could be the current for 3.3V? Like that I would be able to know the value of R2 in voltage divider.

[hubi]

hI hUBI, yES IT IS JUST AN  output pin as of 3.3 V which I am going to use in voltage divider as an output voltage. So what could be the current for 3.3V? Like that I would be able to know the value of R2 in voltage divider.

I am not quite sure what you mean by current for 3.3V.

You want to divide the voltage so that it is in the measurement range of the ADC. If your maximum input voltage is 6V and the maximum voltage your ADC can read is 3.3V, then dividing by half seems like a good choice. So you can use the same resistor value for R1 and R2.

There will be some current through the voltage divider, which will drain the battery. To minimize the current, you want to use large resistor values. But there are limits to that, I doubt you could get an accurate reading if you picked 1 MΩ for the resistors. If you put a capacitor parallel to R2, you can use higher resistance, but you have to wait for the capacitor to charge, i.e., you cannot read the battery voltage as often. If you want to read the voltage every second, try 1 MΩ for the resistors and 0.1µF for the capacitor. This will draw 3µA at 6V battery voltage. If you can wait 10 seconds between readings, you could try 10 MΩ and reduce the current draw by a factor of ten. Just experiment a little to find out what works for you.

[rajbuddy]

I am so confused that what could be the output pin of my microcontroller. i have chosen PB0 as ADCIN8 for the output. is it correct or should i go for another pin?