TPS562202: Any negative consequences of having input voltage lower than set output voltage (other than output sagging?)

Hi Charlie

Yes, the chip would keep the max duty cycle when Vin drops under the setting Vout. Would you please tell me the spec of the application and under what condition the input would drop below output?

Thanks.

BR

Ruby

Hi Ruby,

This regulator would be set to output 5V power to a speaker amplifier and RGB LED strip.  I have already verified these peripherals will work at 4.5V.  The load current is around 650 mA max.  The input is two LiFePO4 cells, and I plan on cutting off discharge at 5.7V.

Regards,

Charlie

Hi Charlie,

Thanks for your information! My colleague will check your information later.

BRs

Zixu

Zixu,

Has your colleague had a chance to check the information yet?  I marked this question as not resolved for now, only because it sounded like your colleague may have additional feedback.  Let me know if there are any additional concerns.

Regards,
Charlie

Ruby,

The difference between stopping discharge at 5.7V and 6V during preliminary testing appears to be a 5% battery capacity difference.  It may be okay to stop discharge at 6V, I am not sure yet.  It would be better to be able to stop discharge at 5.7V, which already leaves reserve capacity for battery storage.

I can see that the inductor DCR can affect the output voltage, but what resistors are you talking about?  The only resistors I can think of are the feedback resistors, but I thought that would affect the target output voltage when input voltage is higher than the dropout voltage.  I don't see that it would affect the actual dropout voltage.  Can you elaborate what you mean?

Regards,
Charlie

Hi Charlie

Sorry to make you confused, I mean the feedback resistors. When considering the worst duty cycle, you may get an output voltage based on one input voltage, but here, actually, the output voltage could also be affected by the feedback resistors together with the feedback reference accuracy which could affect the accuracy of the output voltage and also the DCR of the inductor together with high side FET Rdson which would affect the dropout voltage. As a result, to get a more accurate output voltage, you may consider these parameters and then get the lowest calculated output voltage to evaluate whether it could cork for the loading.

Hope this helps. 

BR

Ruby

Ruby,

I think understand how the feedback resistor inaccuracy and feedback reference inaccuracy could affect the set output voltage.  However my understanding was that this inaccuracy is separate from the inductor DCR, MOSFET Rdson and the duty cycle limitations.  That is the whole point of a feedback circuit, to sense output voltage as close to the load as possible to eliminate the effects of voltage drop due to component resistances.

So in my example where I calculated an input voltage of 5.7V and an output voltage of 4.7V, I assumed nominal output voltage set to 5.0 V.  I think the dropout voltage is the lower of:

duty cycle limitation + MOSFET Rdson + Inductor DCR OR the feedback inaccuracies

Therefore, I would expect the output voltage to remain at 4.7V in these conditions, even if the feedback inaccuracies cause the output voltage to be set at 5.0V, 4.9V, or 4.8V.  The feedback inaccuracy would only affect the output voltage if the feedback circuitry ends up targeting a value less than 4.7V.

It sounds like you were saying the total output voltage is the sum of all of these values:

duty cycle limitation + MOSFET Rdson + Inductor DCR + feedback inaccuracies.  This does not seem correct to me.

Is my understanding correct?  I may have misunderstood what you were saying.

Regards,
Charlie

Ruby,

Thanks for clearing that up!

Regards,
Charlie