MC7700 Power Requirements


#1

I am working on marrying a Raspberry Pi Zero and a MC7700 to provide GPS time corrections, send /receive SMS. I realize the MC7700 is an older product but it’s readily available and inexpensive. If I get my prototype hydroponic/aquaculture control system working with AT commands, I can step up to a current product in the future.

My issue is the MC7700’s 5 VDC power requirement. My 5 VDC supply is provided by a battery backup circuit with a maximum current rating of 1.1 A. The following table is from the MC7700 Product Technical Specification.

MC7700%20Power

The title states the values shown are “averaged” current requirements and the LTE value is 1100 mA, my entire power budget, leaving nothing for the Raspberry Pi Zero. If I limit operation to HSUPA, how much headroom do I need above the 800 mA average current from the table? Will I have 150 mA left to run the Raspberry Pi Zero?

Regards,
George


#2

Let me add that I looked at the SIM series devices by SimCom. They only tout the standby and shutdown current values in their specs but won’t provide operating current values other than to state a 2 A supply is required to provide sufficient headroom. That is a very inefficient chip or a LOT of headroom!

Regards,
George


#3

@wizwork,

I am guessing that the SIMcom device is also 2G capable, that is going to be where the 2A peak comes from, 3G/LTE is a continuous modulation rather than a hard time slot based one, it was one of the points of 3G to stop the brutal hammering of power supplies/batteries. Also 2G has a peak power of 33dBm where 3G/LTE is 23dBm.

Having read the spec I think if you can do 1.15A then it will be ok.

Regards

Matt


#4

“includes USB bus current”

USB 3.0 supports only up to 900 mA, USB 2.0 up to 500 mA. Everything else is a violation of the standard. Drawing 2 amps from a USB port is insane.

However, GNSS should only draw a few mA.


#5

The 2G point is well taken Matt and their device is capable of 2G. The USB current is a good point but is shaded on the MC7700 by the PCI Express bus that may provide more current.

It occurred to me there are two ways to interpret “average” in the table above: the instantaneous current of a single device or the average across several different devices. The later would be better for me.

Regards,
George


#6

GNSS should work flawlessly but 2G might cause the modem to crash due to low voltage. My PI Zero reboots every time when I attach a power hungry device (within specs) to its USB port. I am using a 5V/8A PSU…

Using the PI Zero for GNSS is a great choice. I bought the Zero for the same purpose.


#7

Check the notes. Configuration is 100/50Mbps at 23dBm.
These are the absolute max values on B2.
The modem was forced to transmit at max power while transmitting UL/DL max throughput (callbox)

Your actual power consumption is going to be significantly less.
If you worry about 2.5G I’d add a bigger supercap. But I’d lock the modem to 3G and LTE.
If you have a multimeter you can measure the current yourself while transmitting data.
The Sierra devkit has an option to supply power through USB. When not testing max throughput this option worked OK, never experienced resets.
An average PC can supply around 1A (or more) through the USB, this should be enough.

“My PI Zero reboots every time when I attach a power hungry device “
Are you drawing power from the PI? Then your bottleneck will be the PI’s regulator.


#8

Thank you to everyone that replied; you convinced me that Sierra Wireless was the way to go!

After two unsuccessful attempts to get SimCom to provide me with active current requirements and despite their more favorable geometry, I have switched to using my tried & true MC7700. I have several in use but none in equipment of my own design. A Mini PCI Express card to USB adapter will handle the power conversion, SIM card and mounting to the breadboard. I am building in a jumper so I can monitor the current being drawn for awhile just to make sure it’s within the capacity of the 5V “UPS” with the Pi Zero operating.