BQ76952: BQ IC Shutdown requirements query

Query 3. What is the state of internal LDO Regulators REG1 and REG2 in the SHUTDOWN mode?

Hi Dhruv,

Could you measure the voltage on the REG18 pin?  If there is a voltage present at this pin, your device is in SOFT SHUTDOWN mode. 

SOFT SHUTDOWN means that the LD and TS2 pin have not met their SHUTDOWN requirements.  Section 7.5 SHUTDOWN Mode in the TRM has more information on SHUTDOWN and SOFT SHUTDOWN.  I will link it here: BQ76952 Technical Reference Manual (Rev. B) (ti.com).

If your device is in SHUTDOWN, and you want to wake the device, either the LD pin must be pulled UP, or the TS2 pin must be pulled DOWN.

Additionally, the internal LDOs will be disabled in SHUTDOWN mode.  I hope this helps!

Best,

Andria

Hi Andria,

Thanks for the reply.

I checked we were indeed in Soft Shutdown mode with REG18 showing 1.79V. The questions that I still have is:

1. For SHUTDOWN mode entry, how does the TS2 pin see ~5V . Does it come from inside of the BQ IC or do we have to add an external pull-up to it (with an existing FET to wake up). I checked with an external 10k using an external 5V supply. Our shutdown current fell from 4.4mA to ~200uA (which is what is our shutdown spec). 

2. What should be the TS2 be configured as for SHUTDOWN, as ADC input or output?

Unable to open the first link for the wake up circuit working.

Hi Andria,

Thanks for the information and the schematic. This is helpful.

Could you please comment on:

1. The last sentence threw me off. Did you mean to say, "This means that even with the 10-kΩ resistor, the voltage at TS2 will still be BELOW the wake-up voltage, IN THE EVENT OF A WAKE-UP?."  since we want V-TS2 to be < 0.7V for wakeup, Correct? R77 will create a voltage divider with internal high impedance (How much is it?) and still keep the V-TS2 below 0.7V. Correct?

2. Its slightly difficult to understand whether the TS2 pin is an input pin or an output pin. Is it safe to say that the thermistor and the SHUTDOWN/WAKE features can co-exist on the TS2 pin and we don't have to configure the pin in one particular way, if we implement the circuit you posted in the previous comment? Are we doing any temperature sensing then?

3. In the circuit you posted, is it OK to use an N-channel MOSFET without a 10k series resistor for this design? Do we still need the D15 diode then?

Sincerely,

Dhruv

Hi Andria,

Thanks for the reply.

-- It seems we should still have the N-FET in order to prevent accidental wakeup from 0V or Floating voltages on MCU pin to TS2 while the system is in Shutdown or Shipping mode. If the MCU is not up in Shipping mode, the high-Z on it can lead to floating voltage < 0.7V and hence good enough to wake up  the BQ IC. Correct?
--Could you please point me to the reference design document for BQ76952? or if you can send the print version of the internal link that you had sent earlier , it would be great. (https://e2e.ti.com/support/power-management-group/power-management---internal/f/power-management---internal-forum/1042706/bq76952-how-do-wake-up-circuit-work-on-ts2-to-wake-up-bq76952?tisearch=e2e-sitesearch&keymatch=bq76952%2525252520ts2). 

Appreciate your support here.

Sincerely,

Hi Dhruv,

Other customers have used several options for wake-up: MOSFET, manual push button, the circuit I showed in the previous example, and an optocoupler.  There are a number of options for configuring the TS2 pin as a wake pin, and all the examples I will provide should reliably wake your device and allow shutdown.

MOSFET:

You can use a MOSFET for the wake circuit, but you must ensure that it has low leakage to avoid soft shutdown or accidental wake. You must ensure that the TS2 pin does not remain pulled down because you are correct that if voltage is low enough, it could cause wake up or prevent shutdown.  Keep in mind that the BQ76952 does have the internal pull-up resistor for TS2

Manual push button:

This option is very popular with many customers.  The TIDA 010208 reference design on ti.com uses a push button.  To find the reference design, go to the Design & Development section of the BQ76952 product folder (BQ76952 data sheet, product information and support | TI.com).

Previous circuit:

This circuit can be found in Figure 7-6 in the low-side application note.  I will link it here: Using Low-Side FETs with the BQ769x2 Battery Monitor Family (Rev. A) (ti.com).  I provided you all the information from the thread about this circuit, so there is nothing more to show you pertaining to that.  If you use this circuit, you can use it exactly as shown, just configure the TS2 pin for wake functionality.

The TIDA 010208 reference design shows a low-side FET configuration and uses a circuit similar to the one I provided.  You may want to take a look, but this is a more complex circuit.

Optocoupler:

This option is less common, but it has effectively been used before.  Here is an image of a circuit that uses one:

Best,

Andria

Hi Andria, 
I implemented the wake up circuit using an n-FET only (no resistors just the WAKE coming from MCU and nFET drain connected to TS2). I am seeing a voltage level at TS2 pin that is not ~5V level in Hibernate mode. Is that expected? Please see some captures below and let me know if the BQ TS2 voltage levels makes sense for Hibernate and Wake. Thanks.

Power up
SHUTDOWN transition
Wake up from SHUTDOWN (with Power cycle)

Hi Dhruv,

The results you are seeing seems to be consistent with the effect of leakage current resulting from an oscilloscope with a 1 Mohm probe instead of 50Mohm probe being used to measure voltage at the TS2 pin.

Because the TS2 pin has a weak 4.6 MOhm pull-up resistor to 5 V, the 1Mohm resistor used in the oscilloscope to collect voltage measurements can load the source impedance of 4.6Mohm (from the internal pull-up resistor) significantly.  This is causing the lowered voltage that you are capturing. 

You are most likely dragging the TS2 pin down too low with the scope probe, so it cannot rise enough to put the device fully into SHUTDOWN mode.  Check the REG18 pin again to see if a voltage is present.  If there is, then you are in soft SHUTDOWN.  The image below shows a configuration that will allow you to gather voltage readings with the 1MOhm probe, while not affecting the TS2 pin voltage.  It would be better to use a 50 MOhm oscilloscope probe, though.

As for the periodic voltage spikes: those are most likely due to the typical ADSCAN loop that occurs when the TS2 pin is configured for a thermistor.  Table 12-1. Direct Commands Table (continued) in the Technical Reference Manual discusses TS2 pin configuration.  I will link it here: BQ76952 Technical Reference Manual (Rev. B) (ti.com).

Best,

Andria

Hi Andria,

Thanks for the reply.

There is no easy way to probe the TS2 pin for me for now. The picture you posted, did it mean to have the TS2 pulled up externally to 5V?

Regarding the configuration, I confirmed it with my firmware team that it is indeed configured as a thermistor. This gives us the specified shutdown current (~200uA). So, it's working for us. What according to you should be the ideal configuration setting for the TS2 pin if we only need it for WAKE functionality?

Sincerely

Hi Dhruv,

In the circuit I sent you, the upper left corner with the internal pull-up resistor shows what is happening inside the part.  There is a 4.6 MOhm pull-up resistor to 5 V.  Those are not external components.  Everything after the TS2 pin (to the right of the TS2 pin) is an external component.

The information I shared with you in my last post is to help you properly see the voltage level for the circuit you have.  Essentially, your circuit may be working, but your data collection setup prevents you from seeing that because the oscilloscope is pulling the TS2 pin voltage down.  The oscilloscope itself is affecting the pin. 

The circuit I drew you in the last post shows how to add an external voltage source to counteract the effects of the oscilloscope probe.  If you do not change your oscilloscope setup, then any further data collection you run with the oscilloscope will show information that is not representative of what the circuit is doing without an oscilloscope probe present.  Hopefully this explanation is a bit clearer.

Best,

Andria

Andria,

While I understood there is a need to use innovative way to probe the 4.6MOhms internally pulled up TS2 pin, the additional 5V voltage source placement was not clear to me. It still isn't. Is it supposed to be an external pull-up on TS2? 

The other question I was looking for a response for is TS2 configuration related. Posting it again here: What according to you should be the ideal configuration setting for the TS2 pin if we only need to use the WAKE functionality (and not thermistor functionality)?

Sincerely,

Hi Dhruv,

The 5 V external voltage supply is placed in series with the WAKE circuit, and after the oscilloscope to counteract the effects of the leakage current and consequent voltage drop caused by the oscilloscope probe.  The external, additional, 5 V source should help maintain the voltage level of the TS2 pin, as if the probe wasn't there at all.

In one of my previous posts, I gave 3 circuit options for the TS2 pin if you are just interested in WAKE functionality.  There are multiple options, and there is not one "best" option.  Depending on what components you'd like to use or personal experience with certain configurations, you can select any of the 3.  

The links I have provided direct you to our product page, which contains reference designs as well, and you can look at these for TS2 WAKE pin ideas.

Best,

Andria