nRF9160 TX current duty cycle (configurable?)

[alcornb]

Hello,

Let me start with some background information and end with some questions.  

Background

We are using the 9160 in cat-m0 and locking to AT&T and Verizon bands used in the USA as per the link below:

https://www.phonearena.com/news/Cheat-sheet-which-4G-LTE-bands-do-AT-T-Verizon-T-Mobile-and-Sprint-use-in-the-USA_id77933

I am taking measurements of the system current in order to size our batteries, super cap, and power supply.

I have found the following worst case scenario during the initial connection:

The yellow data in the scope shot above is current to the 9160 @ 3.5V from a benchtop supply.  The conversion is V *0.02 = A.

• Peak currents =  ~380mA
• Worst case duty cycle is 50% (8ms on / 8ms off). 
• Transmission "cycles" can go for ~1s on with 0.5s rest.  This repeats for a few seconds.

Zoomed out view of the full connection process:

Our alpha design uses two Li-SOCl2 AA batteries in parallel (3.6V typical).  I've found that I'll almost definitely need a boost converter to keep the voltage above 3.3V.  These batteries are rated to put out 50mA each (100mA total in parallel) continuous current.  They are capable of higher peaks, but knowing duty cycle is critical here.  The power supply design is proving challenging with these high duty cycle transmissions for long durations.

Questions:

1.  What can I expect for worst case transmit duty cycle and overall communication time?

2.  Is the duty cycle of transmissions configurable in the 9160 modem?  For example, could we limit the duty cycle to 5% in order to reduce stress on our battery?

3.  Any examples of power supplies on small batteries with this device would be a nice to have

Happy holidays!  Thanks in advance for the guidance!

[ketiljo]

1. Typical CAT-M1 TX case would be 1-3ms TX and 5ms minimum gap before next 1-3ms TX. In cell edge areas the TX periods can be significantly longer - worst case up to 32 ms. The current draw will vary with the distance to the base station. So expect a worst case peak current draw of 6-700 mA. 

2. No, this is solely decided by the network

3. Not other than that your battery and capacitor combination must be able to supply the current needed by the modem. The battery selection is key to make a robust system. Also check the temperature spec. of the battery. 

[alcornb]

Hi Ketiljo,

Thanks for your reply.  I can't design the power supply based on the typical case, it needs to be worst case.  To clarify, you're saying:

1. Worst case duty cycle is 32ms on and 5ms off?

2. Worst case peak current draw is 6-700mA @ 3.6V?  That doesn't align with the spec for the 9160.  The highest current consumption stated in the datasheet is 390 mA @ 3.5V.  Are you saying the highest current consumption is higher than that which is spec'ed in the datasheet?  What am I missing?

[ketiljo]

1. The duty cycle will be 3 ms on and 5 ms off for a duration of max 32 ms. 

2. There's a difference between gen1 and gen2 of the nRF9160. The current is lower on gen2 due to more efficient regulators. 6-700 mA is too high, sorry for that. For gen1, peak current is ~525 mA for the modem. This is down by up to 19% on gen2,depending on the band, see https://infocenter.nordicsemi.com/pdf/pcn_134_v1.0.1.pdf

For new design, use the numbers for gen2 in the current PS

[alcornb]

Hi Ketiljo,

Phew!  Thanks for the response.  Is there any way to add a rest time between the 32ms cycles?  This seems to restart immediately from what I can tell.

[ketiljo]

Not unless the network lets you do that. All timing is decided by the base station.