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The voltage of the Chargepump LDO- is exceeding in IC LTC3260IDE#PBF

Category: Hardware
Product Number: LTC3260IDE#PBF, LTC3260

We are using the IC LTC3260IDE#PBF for one of our applications - we are using both the LDOs - LDO + and LDO -.

We are deriving the output of +17 and -17V from LDO+ and LDO-.

However, after designing the proper feedback resistor network in our application is - R2 and R11 combination is - 820K and 62K which will give us the +17V and -17V.

This is on the design perspective. 

Our parameters:

Vin - 24V

Rt Freq set at - 500KHz.

Expected LDO+ output - 17V

LDO- = -17V

Measured -

We are measuring LDO+ = 17.02V - which is currently okay.
 

The problem we are facing is with LDO- is that the voltage is higher than expected which is = -17.6V to -17.85V.
 

 I have attached the ref image of the application schematic which is followed exactly.

Now here are some inferences from Lab testing:

1. The frequency at C+ and C- pin in developed board is around 515KHz as opposed to 500KHz which is the setting from RT pin to GND.

2. The Voltage is +17V on LDO+ and is behaving properly with Load regulation also (Varied the load to consume until 100mA).

3. The no load and the with load conditions the LDOs are behaving same.

Can you please help us with the trouble shooting of this issue - will the frequency change the output if so how?

How can we mitigate this?

  • Hello, 

    Thank you for the info. With RT = GND the switching frequency should be between 450kHz and 550kHz, typical 500kHz. 515kHz is OK and will not have an effect on the -LDO voltage. That is only for the VOUT charge pump and affects the maximum amount of current that can be supplied from the charge pump. What is measured on the VOUT turret? 

    What does the ripple look like on the -LDO output? Is the output changing?

    If changed to the -12V setting, what does the -LDO read? If there is some leakage current on the R18 resistor for contaminates then the output could read higher in amplitude.  

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