TPS61032: Low Battery Indicator lights lights up before the design value

Hi Andy,

What is the LBO voltage value when VBAT=2.5V and red LED is slightly turned on?

Regards,

Bryce

Hey Bryce, 

When VBAT = 2.35V, the LBO voltage is 2.13V. 

When measuring the voltage across the LED in the forward direction, I get a -0.83V. 

If useful, I used this led for my application : KT-0603R. 

Thanks, 

Andy

Hi Andy,

I found the forward voltage of the LED is 1.9V-2.5V, how did you measure the -0.83V forward voltage?

Did you see the same phenomena with a new LED?

Regards,

Bryce

Hi Bryce, 

I measure the voltage across the led with the led still lightly turned on. 

It is the voltage from the positive side of the LED.(V_Battery) to the LBO(negative side of led).

My device is connected to a battery and I measured the voltage VBAT and LBO by  connecting the board GND. 

I don't know why this is happening, thanks for helping me. 

Thanks,
Andy

Hi Andy,

It doesn't make sense that if VBAT=2.35V, the LBO voltage is 2.13V. Since you measured the voltage drop on LED is 0.83V, why the LBO voltage doesn't equal to 2.35-0.83V=1.52V?

Regards,

Bryce

Hey Bryce, 

Yeah, I know this is weird. 
Let me try with new batteries ; 

VBAT : 2.95V

LBO : 2.47
LED: -1V (yes negative from VBAT to LBO). 

I would like it to work to the VBAT = 2V , as this 3V to 2V range of discharge voltage is the linear region of the batteries. 

Also, please advise on the connection to be made on the FB pin of the chip as Nathan as stated in the other thread. 

Hi Andy,

I still get confused about the voltage drop of the LED, theoretically it should be 2.95V-2.47V=0.48V, but you measured is 1V. 

Can you take the waveforms of Vout, LBO, VBAT when VBAT is dropping? It will be better if the voltage scale setting is small enough.

The FB pin of TPS61032 is recommended to be floated.

Regards,

Bryce

Hey Brice, 

When I changed the batteries, the problem went away. The only waveform that seemed interesting was the voltage coming from the battery (VBAT). 


I'm not sure if I should do something about it. Please advise. . 

Thanks, 

Andy

Hi Andy,

Is the LED turn on voltage of Vbat equal to theoretical value now? 

What is the frequency of the Vbat ripple? And how much is the amplitude?

Regards,

Bryce

Hey Bryce,

When I use an adjustable power supply , the leds now lights up at 2.0V as per design. I don’t have batteries with enough discharge to test. 

The amplitude of the ripple is .6V and unfortunately, I did not get the frequency. Would you recommend me a capacitor to stabilize the VBAT ? If so, please advise.

Thanks

Andy

Hi Andy,

Do you see the input voltage ripple with adjustable power supply?

What is the part number of C1? Considering the derating of ceramic capacitors, you can add another 22uF to see if the situation is better.

Regards,

Bryce

Hey Bryce, 

C1 part number is CL05A106MQ5NUNC. Where would you place the 22uF capacitor  ?

Thanks, 
Andy

Hi Andy,

It seems the affective capacitance is only 50% when Vdc=2.5V.

It's recommended that you put a 0.1uF close to VABT pin and 22uF near the 0.1uF.

Regards,

Bryce

Hey Bryce, 

Thanks for this info about affective capacitance. 

Should I place it between VBAT and GND like the schematic below ? 

Thanks, 

Andy

Hi Andy,

It's OK you put them on the marked position. And on layout, you can put them as recommended in the former post.

Regards,

Bryce