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I am designing a device which has an integrated Li-Ion battery (single cell, for simple low power portable digital device) and normally charges by way of a buck converter which goes from 12-16V Automotive supply to 5V, then into the single cell linear battery charger. If in the event the battery goes 'flat' and is not desirable to wait and rely on automotive input (inconsistent supply) or during user interaction by for example diagnostics/testing/data logging I need to also power the battery charger IC via the same power bus.

When the buck converter operates, it is intending to charge at up to 1-1.2A if possible during the short times it has energy available. In the normal use case of this device, the buck converter is the primary input to the Li-Ion charger. I will therefore make the standard case for the charge current setting of my charger IC to be 1.2A.

In the case when the USB is connected, it is assumed the device has been removed from the automotive input, but just in case I will have the USB activate the "Shutdown" Pin on my Buck converter IC. The only remaining issue (as far as I can see) is the charger IC attempting to draw 1.2A to charge the battery, from a USB port on a standard computer!

I have come up with a way to have a "normally ON" N-channel MOSFET based switch for a resistor to be in parallel to give me the required R_PROG value for ~1.2A. When USB is plugged in, the buck converter is disabled and the switch which parallels the two resistors is turned "OFF", leaving only the high value resistor and therefore the low (200mA) charge current for safe use with a PC or laptop.

I have attached a picture showing my plan below, which uses a MOSFET, NPN transistor, and 3 resistors to implement my plan. Note the blocking schottky diode D1 for the USB bus to the battery charger input node, that is there for protecting the USB power bus if the buck converter is in operation but the USB bus is not powered/floating, and to make sure the NPN transistor does not then also turn off the MOSFET.

proposed normally "ON" parallel resistor with USB power turning off the switch

The question is: Is there an easier way that uses less components than my intended N-MOS + NPN + 3 resistors to achieve a context sensitive resistance value for the charger IC, where the resistance is normally low, and when a certain input is connected, the resistance goes higher? This must be done in hardware, for a fast response. Slow responses may attempt to draw too much current for too long.

I am not actually sure if the charger IC can deal with an adjustable resistance like this on its PROG pin, as it may just sample the resistor initially and lock in the charge current setting until power-off. Any thoughts on that part too? The charger is MCP73113/4. Thanks for any insights guys/girls!

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  • \$\begingroup\$ Why not use one of those "OTP" NV digipots? Set it for low current at start, and adjust up. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 25 '14 at 18:09
  • \$\begingroup\$ The questions seems quite complex. I'll try to summarise to check that I have got it. 1. when the system is connected to 12-16V, the battery is charged at 1.2A, 2. when it is connected to USB, and not 12-16V, it charges the battery directly from USB at 200mA. Is that it? To control the charging current, the MCP73113/4 must be 'programmed" with two different resistance values. Yes? The rest is about how to 'program' the two resistances, or your reasoning and rationale. Yes? \$\endgroup\$ – gbulmer Sep 25 '14 at 18:12
  • \$\begingroup\$ @IgnacioVazquez-Abrams I'm not sure what you mean, do you have an example component perhaps, that I could look at it's datasheet? \$\endgroup\$ – KyranF Sep 25 '14 at 18:13
  • \$\begingroup\$ @gbulmer Yes, but the 12-16V is stepped down through a Buck converter to 5V, and goes to the battery charger IC. When USB is connected, it's assumed the 12-16V input is NOT connected, but just in case I have the USB voltage shut down the buck converter. Otherwise, yes you are right. \$\endgroup\$ – KyranF Sep 25 '14 at 18:14
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    \$\begingroup\$ This seems to be closer to your price range (I looked at Mouser): ti.com/lit/ds/symlink/lm3658.pdf and charges with two different currents, depending on which power source is active. So there may be something which balances cost and functionality. \$\endgroup\$ – gbulmer Sep 25 '14 at 21:12
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I searched for "Li-Ion battery charger dual current".

It looks like ICs are designed for exactly this task, e.g. ISL9221.

Or much of the task, e.g. LTC4077

And finally LM3658 which can be preset to 100mA or 500mA for USB.

They adjust the battery charging current when when one source is disconnected (they assume a high current wall outlet is on one), and assume the other is USB, where the default current is 100mA.

I think disabling the buck converter when USB is detected may be optional for the ISL9221, but might still be needed for the LTC4077.

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  • \$\begingroup\$ I have looked at mouser and digikey, neither of those parts are in stock/manufacture/supply anymore, or have very limited stock, or are x3-4 times the cost in units of 1k compared to the one I have currently selected. I do agree thought that those ICs can do the job I am trying to achieve, but perhaps a little more complex and expensive than is necessary.. I will be getting tens of thousands of this device made in the future, do you have any ideas for a way to do it along the lines of discrete components? \$\endgroup\$ – KyranF Sep 25 '14 at 19:09
  • \$\begingroup\$ @KyranF - I'm not a PSU person, so I'd dig around for alternative parts; their are lots of battery charging IC manufacturers. Parts from Intersil direct appear to be much lower-cost than Mouser. As you wrote, it is not clear from looking at the datasheet for the MCP73113/4 how it will react to changing the programming resistance. That does seem a valid question to ask Microchip though. If it continuously monitors the resistance (which seems reasonable), setting the 100mA USB charge current with a fixed resistor, then switching in a second one in parallel using an N-MOSFET looks plausible. \$\endgroup\$ – gbulmer Sep 25 '14 at 19:39
  • \$\begingroup\$ Indeed I will take the time to investigate cheaper charger ICs designed for dual inputs like the examples you showed. The uncertainty comes from the fact that you can use the PROG pin on this MCP73113/4 charger as a charge disable, by applying a voltage direct to it. Perhaps it uses a weak current source to measure the resistance (voltage built up over it) for the current setting, and when external voltage is applied it changes over to more of a "enable/disable" function input. Thanks for your help mate! \$\endgroup\$ – KyranF Sep 25 '14 at 19:42
  • \$\begingroup\$ @KyranF "The uncertainty comes from the fact that you can use the PROG pin on this MCP73113/4 charger as a charge disable, by applying a voltage direct to it" - yes, that made me very unsure, especially when I couldn't find more details about how it works (grrrr). Ask Microchip on one of their forums, or something? \$\endgroup\$ – gbulmer Sep 25 '14 at 19:47
  • \$\begingroup\$ I shall indeed! Their forum seems a bit cluttered with microprocessor issues though, i'll try to contact them directly. I was wondering though, maybe I can just make a little SPST slide switch and try to enforce users to ensure it's in the correct "PC" mode before plugging into PC USB port? A phone charger USB will still be able to use the "full power" mode. I think too many idiots will fry their PC USB ports and I'd rather avoid that.. \$\endgroup\$ – KyranF Sep 25 '14 at 20:30

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