LMR70503: Using an external VREF

alain leemans

Part Number: LMR70503

Hi,

For an OLED design I need to generate a negative voltage to control the luminance. As the luminance is depending on the temperature, I need to adapt this negative voltage on fly.

I plan to use the LMR70503 to generate the negative voltage and I'm playing on the VREF (not using the internal VRef) to adapt the output voltage. The Vref is generated by a follower from a PWM signal.

Do you think that the LMR can work like this ?

Thanks for your help.

over 4 years ago

0 Brigitte over 4 years ago

TI__Guru 65200 points

Hello Alain,

In general you can modify the voltage externally on any device that has a feedback divider, but I would not recommend to have by accident an open feedback, so I would recommend to still have the normal feedback divider set to a safe output voltage level to make sure that there is no time when the converter can run open loop. Then you can use another voltage to trim the output voltage similar to the descriptions in these articles:

https://e2e.ti.com/blogs_/b/powerhouse/posts/adjusting-vout-in-usb-type-c-and-wireless-charging-applications-part-1

https://e2e.ti.com/blogs_/b/powerhouse/posts/adjusting-vout-in-usb-type-c-and-wireless-charging-applications-part-2

The reason is to make sure that even if something goes wrong with your amplifier, the output voltage is still in a safe area and does not damage the device or the connected load.

Best regards,
Brigitte

0 alain leemans over 4 years ago in reply to Brigitte

Prodigy 10 points

Hi Brigitte,

Thanks for the links. One question.

In the case of the LMR70503, the VREF drives the feedback divider resistors. If I'm using a PWM signal to manage Vout (as figure 4 - A two-stage RC filter to reduce the delay time - from adjusting-vout-in-usb-type-c-and-wireless-charging-applications-part-2).What will be the Vout equation ?

Regards.

0 Brigitte over 4 years ago in reply to alain leemans

TI__Guru 65200 points

Hello Alain,

The FB voltage is at ground potential. I did not take care on this with the given articles. Do you have negative voltage on the output of your amplifier?

Best regards,
Brigitte

0 alain leemans over 4 years ago in reply to Brigitte

Prodigy 10 points

Hi Brigitte,

No the amplifier gives a positive voltage as the VREF provided by the LMR. But we play on a PWM signal to adapt this voltage in function of ambiant condition. If I understand well the datasheet, with a positive VREF, the LMR supplies a negative voltage based on Rb and Rt.

Regards.

0 Brigitte over 4 years ago in reply to alain leemans

TI__Guru 65200 points

Hello Alain,

VOUT is the negative voltage, VFB is a virtual GND and the reference voltage delivers the positive point for the feedback resistor divider.

As you only have a positive signal, you need to set the output voltage most positive with the resistor divider and then additional current through the additional resistor should make the output voltage more negative because you increase the amount of current going through the feedback resistor to the negative output voltage.

Best regards,
Brigitte

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LMR70503: Using an external VREF