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Here is a schematic of the "Typical Application" for the Maxim MAX1873 -

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Can anyone tell me what "To system load" means? Does it mean I can connect this to let's say a 12V door lock (my input is 15V)? And would this device automatically switch to the battery backup when Vin goes down (essentially a power failure)?

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    \$\begingroup\$ That's not a schematic of the 1873, but a schematic using the 1873. \$\endgroup\$ – Olin Lathrop Oct 22 '13 at 14:13
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This charger has what's commonly called as power path. Basically, the current coming into the charger is split between powering the system and charging the battery. The actual split depends typically on system current requirements and some settings in resistors.

The System load is the physical system you want to power (microcontroller, etc). DO NOT connect the battery to this terminal. Doing so could damage the battery. The battery has its own dedicated terminal called BATT.

Power Path allows the system also to switch from being DC powered to battery powered if the charger supports it.

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  • \$\begingroup\$ It's not a very well documented device after reading the data sheet and I'm not sure that "system load" is for what you say it is - it is an unregulated output that will always be between a volt (approx) and maybe nearly 20V higher than what the battery voltage is. Doesn't it make sense that the "regular" load is across the battery and that "system load" feeds more supervisory circuits such as disable and the current limit output and possibly switches the "normal" load off when charging the batteries via another FET (unshown)? \$\endgroup\$ – Andy aka Oct 22 '13 at 14:43
  • \$\begingroup\$ @Andyaka - I haven't used that part but I have done designs with battery chargers from TI. The System load is the actual system. The battery needs a very specific voltage for charging as well as a constant current, constant voltage patterns. This cannot be applied to the whole system. That's why it has its own pin. Some chargers have more integrated switches so its not as difficult to use. \$\endgroup\$ – Gustavo Litovsky Oct 22 '13 at 15:25
  • \$\begingroup\$ @Gustavo yes these chargers can be really complex (as I've come to realize). Could you suggest chargers with integrated switches? These are external FET switches for the charger or switches for transferring between regular power supply and battery supply? \$\endgroup\$ – hpb Oct 23 '13 at 17:56
  • \$\begingroup\$ I've had very good experience with TI's charger chips. The new ones are quite integrated and will do everything you need in a pretty small package. \$\endgroup\$ – Gustavo Litovsky Oct 24 '13 at 3:23
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It's not very clear I have to say so here's my take on it.

The connection called "system load" is an unregulated output that I believe is intended to feed some other supervisory chips that might control the MAX1873 in other ways such as disabling the chip (see ICHG/EN pin) or monitoring the current fed into the battery. I say this because the input voltage for the charger circuit is 9V to 28V and it does not make sense that this is used for your load.

It makes sense that your load is connected to the battery as in pretty much any other battery powered application. I'm not 100% but that's my take on it after a few minutes trying to make sense of the topology.

I will add this too. The connection called "system load" will also receive battery voltage minus the volt drops of the external P channel FET and the current limit resistor so it looks like this connection will always receive power. In fact the FET volt-drop could be quite small because I see no reason why the FET should not remain active when there is no external input supply.

This answer is speculation in the main part because the data sheet doesn't seem to really say what is going on.

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  • \$\begingroup\$ I got in touch with folks at Maxim and they told me this - "To "system load" means that you have a path from the input power supply to a load. A control loop limits current drawn from the input source so that the input supply (AC adapter) size and cost can be minimized. Connect a current-sense resistor between CSSP and CSSN to limit total current drawn from the input source. This device does not automatically switch to the battery backup when Vin goes down." \$\endgroup\$ – hpb Oct 23 '13 at 4:44
  • \$\begingroup\$ @hpb Hah - doesn't sound like maxim were very helpful then. Ask your self this... if you wanted to connect your battery to YOUR load how would you do it - would you connect it to so-called "system load" or would you connect it to the battery directly (maybe via another FET so that YOUR load can be turned off via your battery management system). I favour the 2nd because anything else doesn't make sense and the extra voltage regulation requirements of connecting YOUR load to "system load" make no sense! Good luck finding out and I'm interested to hear anything else you find out. \$\endgroup\$ – Andy aka Oct 23 '13 at 7:14
  • \$\begingroup\$ Thanks Andy! I was thinking of connecting the "System load" output of this device to one of the inputs of a voltage switcher (LTC4353) and the battery to the other input of LTC4353. This chip basically allows higher of the two inputs to pass through at its output. Should work dont you think? \$\endgroup\$ – hpb Oct 23 '13 at 15:12
  • \$\begingroup\$ @hpb that sounds more feasible dude. It's a shame maxim couldn't be very helpful. \$\endgroup\$ – Andy aka Oct 23 '13 at 17:04
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MAX1873 divides the input current between load and battery. It won't automatically switch power source in case of input failure, it will only power down charging to avoid reverse current on the battery. From my understanding of your question MAX6326 used along with MAX1873 will reach what you desired. MAX6326 it's a smooth source switch.

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  • \$\begingroup\$ You have not answered the OP's question by not explaining how the extra parts will help. The MAX1873 is a Li-ion charger only. \$\endgroup\$ – Sparky256 Jul 28 '16 at 1:22
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It looks like that is where the system that is being powered is connected to. That line is the power output of the overall power system. That overall system includes the batteries, the battery charger, and the optional external input power.

Think of the power system as having three ports, the optional external input power, the battery port, and where it supplies power to the system being powered. The "system load" line is this third port.

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  • \$\begingroup\$ He should never connect the battery to the system load port. This can destroy the battery by incorrectly trying to charge it. \$\endgroup\$ – Gustavo Litovsky Oct 22 '13 at 14:27
  • \$\begingroup\$ @Gustavo: That's not what I said, but I'll reword it to make that more clear. \$\endgroup\$ – Olin Lathrop Oct 22 '13 at 14:31
  • \$\begingroup\$ I think thats where devices such as the LTC4353 come in. This is a dual input voltage monitor that switches between the highest of voltages at its inputs and feeds it to a single output. \$\endgroup\$ – hpb Oct 23 '13 at 4:57

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