Basic setup to comminicate to LTC6811 or LTC6804

V+ should be at least 11V to ensure all the workings of the parts are powered. Drive and WDT can be left open, though a digital input like SWTEN should be committed to a logic level. The commands and data should all be transferred while CS is low or there is no activity by the IC. Additionally, the IC will require a correct PEC in order to respond (or the command is rejected), and must be awakened beforehand with some CS toggles (I think you may have done that, you can tell if drive goes on [~5.6V]). Given the complexity involved, it's a good idea to use the demo boards and GUI we have for these ICs (same s/w for each) so that there is a working example to work from. The GUI will also show the byte formation for a particular command including the PEC, so this can be useful in hard-coding some PECs during initial operation.

Hi,

I'm testing the LTC6811 in isoSPI mode along with LTC6820 with an STM32F3, and when I provide around 36V on V+, the drive voltage goes to ~5.6V and then turns off in few seconds. I'm assuming for the drive voltage to be on, the core module should be on and that only happens when the CS pin on LTC6820 is toggled few times? Please confirm this.

The DRIVE pin will go low after WDT expires and it is not particularly dependent on CS pin toggling. If there is a valid communication in the SPI lines the drive will stay at 5.7V. Please see CORE LTC6811 STATE DESCRIPTIONS on page20 of the datasheet it will give you more information on the all the state of operation of the IC

Thank you for your help!

I've gone through the State descriptions given and the LTC6811 is coming out of sleep mode, with the drive voltage around 5.7V. I'm using the npn pass regulator circuit to provide VREG with 5V. I'm using two LTC6811-1 in the stack with daisy chain configuration.However the current consumption of the VREG pin is extremely high, with both the VREGs together consuming around 150mA of current. There are no shorts or any other obvious issues, i'm attaching my schematic, please let me know possible reasons!

PDF

I believe this similar issue has been described in another forum?    LTC6811 High VREG current consumption  

150mA is actually a lot of current. How are you measuring it? Are you only measuring the current which is going inside the IC? because I can see that there is a blue LED connected to VREG which is not recommended in our design. we usually connect the LED indicator before the collector of NPN. So it will operate only when drive goes to 5.7V. 

In your current measurement, does it also have the current due to the LED? 

Secondly the major problem I can see that in your schematic you have only used 1 NPN to power up 2 BMS ICs. This will not work because you have lost your isolation and it is very risky to operate in this manner. So the VREG voltage that is being generated is with reference to VSS and not GND5 and hence you might see the IC not function properly.  

I will advise you to make this change as soon as possible and not do any further testing with this configuration. 

Hi, 

Yes I'm measuring only the current going to the ICs, as I'm using a DC power supply ( which indicates the supply current) to power both the LTC6811. The current measurement does not include the LED current as I've disconnected the LED in my setup and checked the current consumption.

Please check again, I've used two npn transistors, one for each ltc6811 as given in the schematic.

GND5( gnd of the top ltc6811 in the stack) is at the same potential as the 12th cell of the bottom stack, that is fine right?

Please let me know if there are any other possible issues or solutions 

Apologies, I did not check the 2nd NPN for VREG_1 connection. Your circuit looks fine to me. GND5 and CELL12/6.2B can be at the same potential. However the high VREG current doesn't look correct to me. To debug this I need some more information. 

psychendPerspective said:

I'm using a DC power supply ( which indicates the supply current) to power both the LTC6811

How is the DC power supply connected? Can you please tell me at which 2 nets in the schematic are you connecting the DC supply+ and DC supply- ? As per your statement i understood that it supplies both the ICs does it supply V+ to the IC or VREG?

Is it connected to VREG pin of the IC or is it only connected to V+ of top IC and V- of bottom IC? 

How are you simulating cell voltages? are you using resistor divider circuit or are you connecting actual cells for C1 to C20? 

I'm using a 60V DC supply. The DC supply + is connected to the V+ of the top ltc6811 (through the RC circuit from CELL24) and the DC supply- is connected to VSS (ground of bottom ltc6811)

I'm using a resistor divider circuit to simulate the cell voltages, and hence I'm providing 36V to V+ of the bottom IC and providing 24V to the V+ of the top IC(referenced to CELL12/GND5). In this manner I'm simulating 20 cells.

I've also checked by powering on only the bottom ltc6811, by providing 48V to CELL12 and VSS, and even then the current consumption of the IC was around 150mA.

Do note, I'm not considering the current consumption of the resistor divider circuit in this.

Your inputs are appreciated, do let me know your thoughts regarding this, thanks!

Alright this makes sense, so its not just the VREG which is drawing 150mA but the entire system. This again is still too much. So to further debug can you check the voltage drop across R270 and R269? Also, what is the VREG voltage you see at the emitter of the NPN? 

R270 is in the collector path which will help us identify current flowing through VREG. As per electrical characteristic of the IC this current should not be more than 12.3mA. So the current that you see at the collector must be much lesser than this. So the voltage drop you must see should be around ~1V and not more than that. 

Likewise for R269, the current going in should not be more than 0.775mA (in measure state) so the voltage drop expected here should be <77.5mV

Similarly the same exercise can be repeated for bottom IC by checking voltage drop across R57, R56 and measuring VREG voltage when there is a valid SPI communication.

If these voltages are in range then the IC is getting the correct supply voltage and current.