TPS61021A: TPS61021A can not supply more than 0.2A

Hi Nasib,

It's hard to find the root cause without a waveform. Please share waveform of VBAT, VCC_52 and SW pin voltage in the same screen(100us/div). Trigger Vout falling edge to catch the moment when the IC went wrong.

Also, 27pF C206 is too big. Please remove or reduce to below 10pF.

Best Regards,

Travis

Thank you Travis, here you have the results: 

C206=27pF
I0=0.14A
VCC52

VBAT

SW



I0>0.14A
VCC52

VBAT

SW

C206=12pF
I0=0.14A
VCC52

VBAT

SW


I0>0.14A
VCC52

VBAT

SW

Hi, i start with less than 0.14 and then i increase the load. The voltage is kept constant until i pass 140mA.. then it goes immediately to 300mV and it cuts the output power until i reduce the load again.. i dont understand why this is happening, regarding the simulations says that i can supply up to 1A with 1.5V Vin and 3v vout 

Any idea? 

Hi Nasib,

Can you set the trigger source at Vout falling edge and use single trigger to capture the moment when Vout fails. Please zoom in to 20us/div this time.

Best Regards,

Travis

This is the capture 

I used 500us since it is quite slow

Hi Nasib,

Can you put SW and Vout in the same screen?

If you don't have 2 probes, you can give me these SW waveform in 5us/div scale:

Also, I would like to check the layout and BOM

Best Regards,

Travis

Hi Travis.

I dont fully understand what is happening now.

I am using a 47ohm resistor as a load and the maximum current i am getting from the Vout is 40mA I can not get the 140mA of yesterday.

And once i connect the load Vbat goes from 1.5V to 0.6V even if i am using a CV power source. 

Anyway, here you have the outputs: 

Io=0mA ( No load ) 






Io=40mA (47ohm load ) 






Layout: 





L200: LQM2HPNR33MGHL
C200: GRM188Z71A106KA73D
R200: RC0402FR-07100KL 
R203: RC0402FR-07280KL
C206: GRM1555C1H270JA01J
C201: GRM188R60J226MEA0D
U201: TPS61021ADSGR

Hi Nasib,

Looks like your VBAT dropped radically as long as there's current sink into the IC. The Vout fails and stays in startup phase because your Vin fails. In this PSM skip phase, all MOSFETs are closed and the inductor current is zero. So in this period L1 should be regarded as short and the SW voltage shloud be equal to Vin.

So we can assume that your Vin looks like this: 

The IC will UVLO shut down if the Vin drops to 0.5V.

So please check by placing CH1 to VBAT(not VBAT_0) and see if the Vin drops like SW voltage. And please check whether there's something that limits input current of VBAT.

Best Regards,

Travis

These are the figures without load and with the load: 

As you mention the VBat is being droped a lot. What is strange since i am using a very stable power source RSpro RS-3005D in CV mode and with a current limit of 2A ( the vbat pin is consuming 238mA ) so there is nothing limiting the output current in the source. 

Can be the dcdc the one forcing the vbat to a lower voltage ? 

With the 47 ohm load connected: 

Iout= 39mA

Iin= 238mA 

The interesting thing is that when i set the VBat to 2.1V the system starts to work as expected ( Vout=3V, perfectly) . I attach here two images, the first one setting Vbat to 2.1V the second one to 1.7V. 


Vbat=2.1V Iout=60mA (as expected) 

Vbat=1.7V Iout=52.8mA

Hi Nasib,

Our device is not possible to forcing the vbat to a lower voltage. The only thing it can do is to sink current. So most likely your DC voltage source has bad transient performance and it takes time to increase output current when out IC started switching. You can add a bulk electrolytic capacitor in parallel with VBAT and try again. Or try using a real battery.

Best Regards,

Travis

Ok, here you have the captures with an AAA alkaline panasonic battery 

Vbat: 1.27V 

No Load: 

Yelow: Vbat 1.27V
Blue: Vout 3V 


Yellow: Vbat
Blue: SW 

 


23.5ohm Load

Yelow: Vbat 0.8V
Blue: Vout 1.6V 



Yellow: Vbat
Blue: SW 



23.5ohm Load + 33uF electrolitic capacitor between VBAT and GND 

Yelow: Vbat 0.8V
Blue: Vout 1.6V 



Yellow: SW
Blue: Vbat





It seems like even an alkaline battery can not deal with 300mA at VBAT ? 

Hi Nasib,

Did you place the battery directly on VBAT_0?

If so, please put VBAT and VBAT_0 in the same screen and see whether the anti-invert circuit limits the current.

Best Regards,

Travis

Here you have the new captures: 

I plugged the AAA battery directly to VBAT ( not VBAT_0  this time ). 

With out Load: 






With 23.5ohm load + 33uF capacitor: 






VBAT is going down from 1.2V to 0.5V 

Hi Nasib,

Looks like the battery has large internal resistance and it still takes up to 10us for the RC charge:

Please try the power source with more caps.

Best Regards,

Travis

Hi Travis, i will try. On the other hand. Is there any way to reduce the operational frequency of this dcdc to avoid all these problems ? 

Hi Nasib,

You can try TPS61299 which has single pulse PFM under light load condition:

Best Regards,

Travis

Hi Travis. I increased the capacitor to 66uF and the result is exactly the same. 

Also , after measuring the series resistance of the battery it was around 2ohms , what does not explain that huge voltage drop 

Hi Nasib,

I think that actually explains everything. The 2ohm internal resistance is out of my expectation (typical AAA battery should be around 100-300mohm) and this explains the huge voltage drop. The peak current in switching phase of PSM is around 1A under your condition and it means the average current is around 500mA. This will cause a 1V voltage drop in switching phase.

So my suggestion is:

Check whether your battery is damaged (typical AAA battery should be around 100-300mohm). 2ohm means the battery supports only up to 350mA output current: (1.2V-0.5V)/2ohm=350mA

If you have to support 2ohm resistance, you can try TPS61299. 

Best Regards,

Travis