Dual Power Source: Effective Blocking [SOLVED]

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jpanhalt

Joined Jan 18, 2008
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Shown below is a supply design for a battery operated device with a GLCD. Backlight LED's for the GLCD require 50 mA. Other devices in the total design draw minimal current (less than a few mA). Originally, the device was to be battery operated only and not operate while charging. The isolation in that case was with a P-mosfet. After re-consideration, I would like the device to be operable while charging and would like opinions on this circuit. The MCP73831 and very similar MCP73833 have back voltage protection. The USB charger can be disconnected and the battery left connected without damage or discharge of the battery.

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The MCP73831 IC is commonly used for charging single cell Li batteries with exception of LiFePO4. The MAX 16054 is a push-on/push-off controller and switch debouncer. An MIC5318 voltage regulator is used instead of the more common(?) MIC5219 as the former has a lower drop-out voltage and is stable with ceramic low-esr capacitors. The battery will plug into CN2, a JST connector. SW1 is a push button switch on the main board.

Questions:

1) Aside from battery temperature detection and query #2 below, does the MCP73833/4 offer any advantages? (Asking for any personal experiences.) SOLVED

2) The datasheet for MCP73831 was revised in July 2014 (years after its introduction) to include the following:
6.1.1.2 Input Overvoltage Protection (IOVP)
Input overvoltage protection must be used when the input power source is hot-pluggable. This includes USB cables and Wall-type power supplies. The cabling of these supplies acts as an inductor. When the supplies are connected/disconnected from the system, large voltage transients are created which may damage the system circuitry. These transients should be snubbed out. A transzorb connected from the V+ input supply connector to the 0V ground reference will snub the transients.
The datasheet for the MCP73833/4 was not so revised. Clarification of that statement and recommendations have been asked several times on the web, including on the Microchip forum and StackExchange without any answer. None of the evaluation boards, including those from Adafruit, Sparkfun, and Microchip currently include one. If included in this circuit, I am considering either the SP3031 or SP1003 from Littlefuse (or similar). The main question in my mind is the "clamp voltage." The former is 6.9V @ 1A and the latter is 11.4V @ 6A and 8V @1A. Absolute maximum for the MAX 73831 is 7V. Both show about 1 mA reverse breakdown current at 6V. I have no experience designing with a TVS.

3) Do you think D1 and D2 will provide adequate isolation, or should I consider a more complicated circuit, such as one involving a P-mosfet?

John
 
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