vne147
Member
All,
I’m working a project in which I will need to control the output of a DC/DC converter using a microcontroller. I’m planning on using the LM27402 DC/DC Controller IC from TI. My application circuit will be very similar to the circuit taken from page 2 of the evaluation board datasheet and shown here:
The datasheet very clearly explains how to vary the output voltage by changing the value of a single resistor identified as Rfb2. The equation that governs that relationship is in section 9.12 on page 9 of the datasheet and is:
[LATEX]R_{fb2}=\frac{R_{fb1}}{(\frac{V_{OUT}}{0.6}1)}[/LATEX]
I want to vary the output voltage from around 1V to around 10V. So, I can easily calculate the range of resistances I’ll need to accomplish that task using the equation above. However, since I’m using a microcontroller, I’m thinking I’ll have to use a digital potentiometer in place of Rfb2. This would work, but my problem is that the above equation is nonlinear.
If I used an 8 bit digital potentiometer, I would only be on step number 128 (50% of full scale) by the time I used up 90% of my output voltage range. Here is a graph to show what I mean:
Since the digital potentiometer’s resistance varies linearly with its bit setting, the voltage output of the DC/DC converter will be nonlinear with respect to the bit setting. This is unacceptable for my purposes. Basically, what I need is a way to vary the resistance nonlinearly so that the output voltage is linear with respect to the bit setting. That is the change in output voltage is the same for each bit 12, 23, 34…255256.
Is there some resistor network arrangement that someone can think of in which when I linearly vary the digital potentiometer, the resultant total resistance would be nonlinear? The general function I need the resistance to vary by is of the form:
[LATEX]R_{fb2}=\frac{\alpha}{(R_{POT}+\beta)}[/LATEX]
Where:
[LATEX]\alpha,\beta[/LATEX]
are constants.
Here is a graph of how I’d like it to work:
Thanks in advance for any help you can provide.
I’m working a project in which I will need to control the output of a DC/DC converter using a microcontroller. I’m planning on using the LM27402 DC/DC Controller IC from TI. My application circuit will be very similar to the circuit taken from page 2 of the evaluation board datasheet and shown here:
The datasheet very clearly explains how to vary the output voltage by changing the value of a single resistor identified as Rfb2. The equation that governs that relationship is in section 9.12 on page 9 of the datasheet and is:
[LATEX]R_{fb2}=\frac{R_{fb1}}{(\frac{V_{OUT}}{0.6}1)}[/LATEX]
I want to vary the output voltage from around 1V to around 10V. So, I can easily calculate the range of resistances I’ll need to accomplish that task using the equation above. However, since I’m using a microcontroller, I’m thinking I’ll have to use a digital potentiometer in place of Rfb2. This would work, but my problem is that the above equation is nonlinear.
If I used an 8 bit digital potentiometer, I would only be on step number 128 (50% of full scale) by the time I used up 90% of my output voltage range. Here is a graph to show what I mean:
Since the digital potentiometer’s resistance varies linearly with its bit setting, the voltage output of the DC/DC converter will be nonlinear with respect to the bit setting. This is unacceptable for my purposes. Basically, what I need is a way to vary the resistance nonlinearly so that the output voltage is linear with respect to the bit setting. That is the change in output voltage is the same for each bit 12, 23, 34…255256.
Is there some resistor network arrangement that someone can think of in which when I linearly vary the digital potentiometer, the resultant total resistance would be nonlinear? The general function I need the resistance to vary by is of the form:
[LATEX]R_{fb2}=\frac{\alpha}{(R_{POT}+\beta)}[/LATEX]
Where:
[LATEX]\alpha,\beta[/LATEX]
are constants.
Here is a graph of how I’d like it to work:
Thanks in advance for any help you can provide.
Attachments

62.9 KB Views: 416

19.9 KB Views: 599

21.5 KB Views: 278
Last edited: