At 07:28 PM 16/09/2012, you wrote:
>Hi,
>
>I'm using the MCP1640 converter:
>http://www.microchip.com/wwwproducts/devices.aspx?dDocName=3Den547080
>to provide power supply for a device, which is powered either from 2
>recharge'able Ni-MH batteries (2,4V), or from USB (5V) (both power
>sources were separated via Schottky diodes).
>The feedback circuit is set so, that the MCP1640 delivers 4.2V when
>running form batteries.
>According to the datasheet it is acceptable to provide higher input
>voltage, that the regulated output voltage (Note 2 on the page 3 of
>datasheet: For VIN > VOUT, VOUT will not remain in regulation.). So it
>seems to be OK to power the device from USB.
>The output voltage after the MCP1640 (c.a. 4.2V when using batteries,
>or c.a. 4.7V when running from USB via Schottky diode) is further
>stabilized at 3.3V using the linear voltage regulator LP2950.
>
>Usually everything works fine, but in 2 cases the MCP1640 got burned
>after the device was connected to the USB.
>The chip was heated to a temperature sufficient to detach connected
>tracks from the PCB.
>The post mortem diagnosis showed, that possibly the capacitor at the
>output of the MCP1640 was incorrectly soldered and got disconnected.

This is plausible. You are running very close to the abs. max
input voltage for the part, and removing the output cap could
allow the switch voltage to rise well above the 5.5V abs max
value.

I'm a little uncomfortable with using the regulator that way,
(out of regulation) but maybe it's okay.

>Has anybody experienced similar problems with MCP1640?
>Is it possible, that temporary disconnection of the output capacitor
>was the only reason why MCP1640 "exploded"?
>What's interesting, the LP2950 regulator survived...

Not surprising- the LP2950 can withstand +30V in continuously
(abs max). The MCP1640 will protect it by kamikaze methods before
the output voltage could rise that far.

--sp


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