Hi there!

I need to get +15v and -15V from a 12V source. LTC3265 appeared to be a promising solution
But now during testing, I'm getting 15V on the positive side but only -1.3v on the negative output.
Please help in suggesting a trobleshooter.

Akash

 

Hello,

How much current are you drawing from the chip?
The datasheet shows a max of 50 mA:



Did you use the schematic for the typical application?



Bertus

 

Yes @bertus, the same circuit.
And regarding the current, I haven't yet connected the load. I was just testing my circuit.
But it will be used to drive a sensor, with not more than 20mA input current.

 

Hello,

Wich package of the chip do you use?



As you can see Pin 19 on the left picture and pin 21 on the right picture are the ground connections on the bottom of the chip.
Do you have that connection made?

Bertus

 

Hello,

Wich package of the chip do you use?

View attachment 125036

As you can see Pin 19 on the left picture and pin 21 on the right picture are the ground connections on the bottom of the chip.
Do you have that connection made?

Bertus

I'm using the TSSOP package, and yes the ground pins and the plane are perfectly soldered.
Also, I've used electrolytic capacitors instead of ceramic ones, does that make a difference?
Only the Inverting voltage side of the circuit is not working properly.

 

Hello,

Did you use the correct polarity for thr capacitors?



Bertus

 

Hello,

Did you use the correct polarity for thr capacitors?

View attachment 125102

Bertus

I had reversed the capacitor at Vout- first, but even after correcting the polarity(connecting as mentioned by you), still the inverting side output voltage does not go beyond -1.3V.
The rest of the capacitors are correctly connected.

 

Hello,

What is the voltage on the Vout- pin (pin 9 of the tssop)?
I would expect something like 22.5 Volts.

Bertus

 

Hello,

What is the voltage on the Vout- pin (pin 9 of the tssop)?
I would expect something like 22.5 Volts.

Bertus

Yes, I also expect the same.
But It is coming around -1.086v. While the voltage at Vout+ is 24.48V.

 

Hello,

I assume you have the connection between Vout+ and Vin-N (pins 20 and 13).
Then You should measure the +24 Volts also on pin 13.
If so check Cinv between pin 10 and 12.

Bertus

 

Hello,

I assume you have the connection between Vout+ and Vin-N (pins 20 and 13).
Then You should measure the +24 Volts also on pin 13.
If so check Cinv between pin 10 and 12.

Bertus

The voltage at pin 10 is -1.2V and at pin 12 is 10.63V(with ref to gnd).
But across the Cinv capacitor, the voltage is 13.38V. I don't understand why should there be a difference?

 

Hello,

The voltage accross the Cinv is an AC signal of about 500 kHz.
The AC is created by an oscillator and the switches in the inverter circuit:



In the first stage S5 and S6 are closed, S7 and S8 are open, the Capacitor is being charged.
Then S5 and S6 will open, S7 and S8 will be closed, so the Capacitor is connected to Vout-.
This is the way how it sould work.
When the Capacitor is bad, the inverter can not work.

Bertus

 

Hello,

The voltage accross the Cinv is an AC signal of about 500 kHz.
The AC is created by an oscillator and the switches in the inverter circuit:

View attachment 125109

In the first stage S5 and S6 are closed, S7 and S8 are open, the Capacitor is being charged.
Then S5 and S6 will open, S7 and S8 will be closed, so the Capacitor is connected to Vout-.
This is the way how it sould work.
When the Capacitor is bad, the inverter can not work.

Bertus

So it is a bad Cinv capacitor you're saying?

 

Hello,

The capacitor could be bad.
The + of the capacitor should be on pin 12 and the - of the capacitors on pin 10.

Bertus

 

Hello,

The capacitor could be bad.
The + of the capacitor should be on pin 12 and the - of the capacitors on pin 10.

Bertus

Yeah, the cap was bad. Thanks for helping in troubleshooting
Output voltage levels are now 14.8V and -13.8V.
Will have to do some tuning now. Any suggestions?

 

Hello,

The voltage dividers will adjust the voltage.
It may be that the voltage references are off, but still in spec.
Also tollerances of the resistors will come into play.

Bertus

 

Alright. Thanks again.

 

which of these capacitors do you think are mission critical?
What if I use different values of caps to fine tune?

 

Hello,

There are two LDO regulators inside the LTC3265.
One for the positive voltage and one for the negative voltage.



In the full schematic you will find the resistors in this part:



The datasheet will show the calulatons for these resistors.

Bertus