Auto switching power supply between USB power and external supply
- From: ganeshokade@xxxxxxxxx
- Date: 26 Apr 2005 08:24:37 -0700
Dear Friends,
We are on to designing a communication board based on the NEC V850
microcontroller that will have a USB port for communicating with a PC.
The board requires a supply voltage of 5V and draws at the most 300 ma
of current. We want to design the board to use two sources of supply -
one is from an external power supply if available and the other is from
the USB port. Power should be drawn from the USB port only if the
external power supply is not present. The circuit should be able to
sense this automatically. We have some ideas but each of them has one
or the other hurdle.
1) Use a connector for connecting the external power supply which will
mechanically disconnect the USB ports supply voltage when plugged in.
We are looking for a connector assembly (similar to the circular ones
present on most cell phones) that can do this job. The catch here is we
do not know a source. If we get to know a source, this is the most
acceptable option.
2) Use a simple diode based switch over circuitry where in the diode on
the USB ports line will reverse bias when the external supply voltage
is present and hence disconnect the USB supply. The problem we have he
is that the USB power can go as low as 4.4V (for bus powered hubs) as
per the specs and a further forward voltage drop of around 0.4 volts
will leave us with an effective 4.1 V for the circuitry. We have an
isolated DC-DC converter (5V-5V) at the input section which requires a
minimum of 4.5 volts to function. So the 4.1 V input will not guarantee
a 5V at the output of the DC-DC. We are trying to locate a DC-DC which
has a wider input range, preferably 3.5 V onwards which will resolve
this problem. If this DC-DC is found, this is the second desired
option.
3) Use a mechanical switch which the user sets to one of the two power
sources. This requiring end user intervention is not desired.
I would like to request the readers:
a) Let us know of any source for the type of connectors mentioned
above.
b) Let us know of any source of DC-DC that can output 5V with input
voltage as low as 3.5 V. The DC-DC should be rated for atleast 500ma.
We have searched under C&D Technologies, Tyco Power and Datel but could
not find anything suitable.
c) Let us know of any other trick to achieve this.
Thanks and Regards,
Ganesh Okade
Sunlux Technologies Ltd.
Bangalore, India
www.sunlux-india.com
.
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