TLV61046A: Possible to use PWM or DAC to control output

Hi David:

May I know the DAC voltage range and Vout target you expected? 

And the PMP15037 is designed for LED application. Do you prefer to control current? What's the load of your application?

Dear Minqiu

Minqiu Xie said:

May I know the DAC voltage range and Vout target you expected? 

0 to 3.3V; Arduino's DAC can create this voltage range.

Minqiu Xie said:

And the PMP15037 is designed for LED application.

As you see the "Original question:"

I created this to control the output voltage using the DAC.

Minqiu Xie said:

What's the load of your application?

It is a LED driver + Boost converter application; I need to use a 3.7V LiPo battery to turn on multiple white LEDs.

I tested that 30 to 40mA is enough for my LEDs.

q0. the TLV61046A's FB pin creates 0.795V, right?

q1. Since it says "R2 should be kept smaller than 80 kΩ to ensure the current flowing through R2 is at least 100 times larger than the FB pin leakage current.",

using the PMP15037 seems to be the problem for me.

Can I add an additional resistor at R2 to control the output voltage?

Hi David:

What's the Vout range? Is 25V the maximum voltage?

The TLV61048 FB is 0.795V typically. But there is a 12V fixed output function for TLV61048 if the Vfb higher than 1.2V. So please take care and ensure Vfb lower than 1.2V during the operation.

Here is an application for control the output voltage with DAC. http://www.ti.com/lit/an/slva251/slva251.pdf  It's more suitable for voltage control operation than  PMP15037 . 

Minqiu Xie said:

Here is an application for control the output voltage with DAC. http://www.ti.com/lit/an/slva251/slva251.pdf 

Thanks you! That's the one I was looking for.

Minqiu Xie said:

Is 25V the maximum voltage?

Yes

Minqiu Xie said:

What's the Vout range?

My desired range is Vout = 8V to 25V (max current 0.04A) However,

"R2 should be kept smaller than 80 kΩ to ensure the current flowing through R2 is at least 100 times larger than the FB pin leakage current.", this line from the datasheet makes me feel that it cannot be achievable.

The input is www.adafruit.com/.../1578

Lithium Ion Polymer Battery - 3.7v 500mAh

Minqiu Xie said:

But there is a 12V fixed output function for TLV61048 if the Vfb higher than 1.2V.

Didn't notice that. Thanks for telling me.

I thought we were talking about TLV61046A. Do you mean I should change to TLV61048?

Minqiu Xie said:

The TLV61048 FB is 0.795V typically.

Does this mean the feedback voltage from FB pin from both TLV61046A and TLV61048 change case by case?

Hi David:

Sorry for typo. It should be TLV61046A, all I mentioned before. (TLV61048 can't support such high voltage)

"R2 should be kept smaller than 80 kΩ to ensure the current flowing through R2 is at least 100 times larger than the FB pin leakage current.", this is for normally usage, and too large R2 will influence the Vout accuracy. Little larger R2 is acceptable if there is additional needed. 

Dear Minqiu

Thanks. Let me summarize the above.

- Then regarding my requirements, where my desired range is Vout = 8V to 25V (max current 0.04A) ,

is this achievable with the Lipo input?

- "But there is a 12V fixed output function for TLV61048 if the Vfb higher than 1.2V. "

So is the VFB = 0.795V with my Vout range?

Or can VFB become 1.2V? When will that happen?

I think I should be aware of this "12V fixed output function".

Hi David:

David__ said:

- Then regarding my requirements, where my desired range is Vout = 8V to 25V (max current 0.04A) ,

is this achievable with the Lipo input?

If the minimum input vlotage is 3.7V, it's OK. 

David__ said:

- "But there is a 12V fixed output function for TLV61048 if the Vfb higher than 1.2V. "

Sorry for my typo, it's TLV61046. If the VFB reach or higher than 1.2V, it will enter into 12V fixed function. You could check it in function block diagram. It has 50% margin for normally operate, so we didn't describe the details in datasheet. 

For Vout adjust output usage, if the Vout track the expected signal smoothly, the Vfb will keep around Vref as 0.795V. It won't be an problem. If the external signal from DAC transient too fast, there is a risk that oull up the Vfb higher than 1.2V.  

Dear Minqiu

I think we are almost done. Let me double-check;

Minqiu Xie said:

If the minimum input vlotage is 3.7V, it's OK. 

Q0. How did you calculate the minimum input voltage? Since the WEBENCH allows 2.5V input, I'm confused.

Minqiu Xie said:

If the VFB reach or higher than 1.2V, it will enter into 12V fixed function.

Q1. I don't want the VFB reaching 1.2V or above. How do I prevent this?

If I follow the schematic from WEBENCH, will this happen?

Minqiu Xie said:

the Vfb will keep around Vref as 0.795V.

Or if I have the external resistor divider, will the VFB always be 0.795V?

I can only find this part useful regarding the fixed 12V output.

Minqiu Xie said:

If the external signal from DAC transient too fast, there is a risk that oull up the Vfb higher than 1.2V.  

Can I ask the numbers? I think I can't find this part from the datasheet.

For instance, I'll create a triangle wave using the DAC.

Will a triangle wave with a frequency of 4MHz be acceptable for this Boost converter?

Hi David:

Q0. Yes, the converter allows lower input voltage. But if the Vin is too small, it may don't have enough current capability. Because the converter limits the peak current of inductance, not Iout. The minimum input voltage I talked about is for details application, which is depended on the battery or your requirements. If not 3.7V, please confirm the data. 

Q1. I'm afraid the Webench can't simulate the adjustable output solution. Do you have other simulation software? Such as TINA or PSpice?

If there is no fast transient and the converter can track the reference signal smoothly, it won't be a problem.

The triangle wave you shows is 10KhZ, not 4Mhz.(The ripple could be filter with RC circuit if it's no need). What's the requirements of the dimming frequency actually? And the current will change during the dimming, right?

Minqiu Xie said:

The minimum input voltage I talked about is for details application, which is depended on the battery or your requirements. If not 3.7V, please confirm the data. 

Q0. This Lipo battery has a range of 3V to 4.2V. Will this be an issue when generating the maximum output VOUT = 25V?

Minqiu Xie said:

Such as TINA or PSpice?

I have TINA 9.

Minqiu Xie said:

The triangle wave you shows is 10KhZ, not 4Mhz

My bad. The picture was an illustration to help to explain. Both 10kHz and 4MHz were examples.

Minqiu Xie said:

What's the requirements of the dimming frequency actually?

It is 200Hz.

Q1. Speaking about TINA, is there a way to simulate the adjustable output solution?

Hi David:

I have done the calculation, and it's OK if assume the efficiency is 80% or 76% with L=10uH, Vin=3V, Vout=25V, Io=0.04A.

You could use TINA to do the simulation following the application I shared with you some days ago. If you don't have such model, I can share you in next week later.

And I'm not sure if the converter could transient with 200Hz with such wide voltage range. It's recommended to check it by simulation or bench test.