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I'm using a lipo battery in a project, and I want to include a sophisticated battery fuel gauge in my design. I have a battery input stage with a TI BQ24072 charge management IC and a linear down-regulator to 3.3v.

For the fuel gauge, I've chosen the TI BQ27421-g1 as I've had good luck with their battery charging IC. The typical application circuit is shown below:

enter image description here

I'm a little confused about the function of the SRX pin. The datasheet only has about 2 sentences about it. To make things more confusing, this diagram on page 20 says that it should be shorted to the BAT pin (sorry if the pin names are tough to read):

enter image description here

Here are my questions:

  1. Am I mis-reading the diagram on page 20?
  2. If I am reading the diagram on page 20 correctly, then how does the coulomb counter even work?
  3. If I don't understand the diagram on page 20, then how should the SRX pin be hooked up?
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  • \$\begingroup\$ That has to be the world's most confusing datasheet. Look at 10.2.2.3, which discusses the selection of an external sense resistor (unnecessary) connected to two pins that don't exist. I think the layout picture's completely misleading and you should not connect those pins. Just work from all the other schematics, etc. in the DS. \$\endgroup\$ – user1844 Nov 3 '15 at 18:58
  • \$\begingroup\$ @WillDean I'm glad I'm not the only one confused by this thing. Usually TI does better than this with their datasheets. If this chip wasn't so nice and small, I would probably jump ship and get a different one. \$\endgroup\$ – johnny_boy Nov 3 '15 at 19:00
  • \$\begingroup\$ @WillDean I would go off the schematic in 10.2, but that doesn't really make sense either. \$\endgroup\$ – johnny_boy Nov 3 '15 at 19:01
  • \$\begingroup\$ Fuel gauge: Ah it isn't just me: en.wikipedia.org/wiki/Fuel_gauge - is it a UK only confusion? \$\endgroup\$ – Andy aka Nov 3 '15 at 19:20
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    \$\begingroup\$ It might be worth asking in the appropriate forum at the TI E2E Community. IME, TI can be very good about providing support on their forums. \$\endgroup\$ – Tut Nov 3 '15 at 21:08
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The BAT pin should not be shorted to the SRX pin. The SRX pin goes to the system VSYS and the charging source. The charging source cannot be a raw USB+5v, that will kill the LiPo, it needs to be the +ve output of a LiPo charger. The BAT pin goes to the battery pack.

Figure 9 on page 20 is not meant to be a 'layout', it is a hybrid abomination which shows the footprint of the device, and a schematic of its connections. Unfortunately, the data sheet shows evidence of having been cobbled together hastily from a previous gas-gauge IC that used an external resistor. Section 10.2.2.3 discusses the selection of a suitable resistor. The tracking under the IC on figure 9 appears to show a trace between the BAT and SRX pins, which could be an alternative implementation of this resistor. This should not be there, and is obviously a hangover from the previous data sheet.

The resistance inside the chip between SRX and BAT is typically 7m\$\Omega\$ which produces a voltage for the ADC to digitise when battery current flows in or out. You do not add any extra resistance externally between these pins, or you will upset the calibration.

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  • \$\begingroup\$ +1. SRX crosses over the heavy line going to the battery. The layout 'example' looks pretty terrible to me. \$\endgroup\$ – Adam Lawrence Nov 3 '15 at 19:03
  • \$\begingroup\$ It's not the crossover, etc, which is the confusion, it's the picture of the tracking under the device. \$\endgroup\$ – user1844 Nov 3 '15 at 19:04
  • \$\begingroup\$ @user44635 Ahhhhh, I see. But a 6 mil trace between those 2 pins would be on the order of 100 micro-ohms, not a few mili-ohms. Would that still be ok? \$\endgroup\$ – johnny_boy Nov 3 '15 at 19:04
  • \$\begingroup\$ @johnny_boy: The 7 mΩ are inside the IC, between SRX and BAT; it's shown on both of your diagrams. You don't provide it with this resistance using a PCB trace. Also, this datasheet's use of a crossover bump is purely optional: two lines crossing on a schematic only connect if there is a heavy dot at the junction. That's been a strong convention for 50-some years. (Some really old good schematics, and some newer bad schematics indicate a connection by just touching lines together, but it's a bad practice today.) \$\endgroup\$ – Warren Young Nov 3 '15 at 19:08
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    \$\begingroup\$ @johnny_boy: You're not supposed to connect these two at all. The "crossover" being mentioned is the semicircular "bump" in the thin line where it crosses the thick line. It's an old-school alternative to the heavy dot convention, where two crossing lines were shown as not-connecting by having one "jump over" the other. You rarely see this used in modern schematics. The graphic artist may have chosen to do it in this case because of the thickness of the other line. \$\endgroup\$ – Warren Young Nov 3 '15 at 19:13

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