TPS65950 Backup Battery Consumption > 50uA

Hi, you may have something sinking current from VBACKUP in this case; what is the level of the main battery when it this occurs (i.e. before disconnection and after reconnection)? Is there anything connected to VRTC?

Best regards, Alain.

Hi Alain,

Main battery level is between 3.7v - 4.1v. We tested with diffrent voltages. But this problem is same.

Vrtc pin is not connected to any point.

Thak you.Regards.

Sorry. 

VRTC connected to 1uF capacitor with gnd.

Ok, so at least there is no external power consumption coming from this node as it is always on. How do you measure this 50uA current: is there a serial resistor with the backup battery? If so, would it be possible to look at the current waveform?

Best regards, Alain.

Ok, so there is something sinking a steady current for about 1mn while VBAT is between 0.8V and 0.5V; at device level the only block which is seeing power is VRTC so you are certain that there is nothing else but a capacitor on this node: no pull-up resistors, no connection to anywhere else? One thing you may try is to discharge the VBAT node when battery is unplugged to see whether parasitic current is still present or not i.e. it is not linked to something else. I guess it won't change but just to be certain. Would it be possible to see your schematic around TPS65950 only?

Thanks and regards, Alain.

Hello,

This is module schematic: http://www.4shared.com/office/_3uK6QZRce/board.html

https://drive.google.com/file/d/0B6H8SnqpSzqCRTBzbm9QaldYc28/edit?usp=sharing

If i discharge the VBAT manually when battery is unplugged; backup battery current is always 2uA. This solution is ok. But i can't do it, my PCB is complete. I can't add any mosfet...

All same boards has this backup battery consumption problem. For ex:

TI Mistral AM/DM37X EVM

LOGICPD Zoom DM3730 Torpedo Development Kit

If you disable STARTON_VBAT  bit  on CFG_PX_TRANSITION registers. You can see same problem when battery is plug and unplugged state. You can check on TI's Mistral AM/DM37X EVM Board.

We use ML614 3v 3.4mA coin battery for VBACKUP. Due to this problem, after few days(2-4) the RTC is reset. (BBCHEN Enable)

Thank you.

Emre

Yes, I understand schematic cannot be changed: I just wanted to be sure the extra power consumption is due to the discharge of the main battery, nothihg else. So this is verified now, thank you!

One possibility would be that an internal block is still seeing some power and somehow continues to work, likely because LDO output capacitors are discharged slower than VBAT itself. VINTDIG.OUT, VINTANA1.OUT and VINTANA2.OUT would be good candidates for this. If you could probe them that may help I think.

Best regards, Alain.

Ok; i will check these pins.

Can I change  lower value capacitors instead of 1uf capacitors on VINT*.OUT pins?

Hello,

I measure VINTDIG.OUT, VINTANA1.OUT, VINTANA2.OUT voltages; these pins voltage is 0V when i plug and unplug main battery. Because disable STARTON_VBAT  bit  on CFG_PX_TRANSITION. 

In this case continue same problem when BACKUP to WAIT-ON state ---> WAIT-ON to BACKUP transition state.

Thanks, Regards.

Emre

OK, so it looks like only VBACKUP is sinking current in this case. There is an application note which describes BACKUP mode operations, as well as how to reduce power consumption in this mode (chapters 4 & 5):

http://www.ti.com/lit/pdf/swca025

For LDO's, using a smaller output capacitor may cause instability so I would not recommend changing it.

Best regards, Alain.

Hello,

We already set  CK32K_LOWPWR_EN for reduce power consumption in BACKUP mode. I know http://www.ti.com/lit/pdf/swca025 chapters 4 & 5.

If we don't set CK32K_LOWPWR_EN bit BACKUP Power consumption is always >10uA and main battery replug state >50uA for a minute

But set to CK32K_LOWPWR_EN bit BACKUP, Power consumption is always 2uA and main battery replug state >50uA for a minute

I dont know what can i do...

Ok, so in this case I don't see what else can be done at device level to prevent this leakage to happen as there is no other control bit which affects backup mode power consumption. I'm afraid the only possible solution is to be done at HW level in this case.

BEst regards, Alain.

Ok, what is your suggestion for minimum hardware modification?

Thanks & regards.

Emre

Well, this is a tricky question as since there is no supply anymore it is difficult to add any comparator or something similar to discharge the main battery capacitors. One possibility would be to add a serial resistor to the backup battery, not too big to prevent voltage dropping too much but enough to possibly reduce the 70uA leakage. Something like few Kohms should do so I think. Is it something you could try out?

Best regards, Alain.

So you have a rechargeable Li-Mn battery on VbatBak pin which I assume you are charging at 100uA to 1ma or so when main battery connected and you are worried about 50uA consumption for 1 minute when main goes off?

Unless you are turning the device on and off every 5 minutes constantly what are you worried about? The extremely low duty cycle of On/Off (under any normal power cycling scenario I can think of) would not decrease you backup battery life to any significant degree.