BCCs: Nicish synchronous boost converter for eg uC supply

> > Microchip have just published a data sheet for a boost converter
> > working from a single cell for processors. Not sure if it goes to 5V
> > output though. May just be 3.3V.

Vout is 2 -  5.5 V programmable.
Vin start is 0.65V - 5.5 V
Vin run is >= 0.35V.

Start at 0.65V is only with 1 mA load current so not applicable if you
cannot deload it during startup.
For most purposes Vin >= 1.0 V will suffice.

Claimed "up to typical" efficiency figure is a meaningful but
potentially misleading term.
At 2.5V Vin (and probably 3.3V out) it will deliver > 90% efficiency
at 2ma - 150 mA load.

At 1.2 Vin and ?3.3 Vout? it delivers > 80% at 4-185 mA load.

There are a wide range of Asian sourced parts* which perform a
similar function but which have non synchronous output but which sell
for around $US0.10 in 1000's.  Internal and external FET versions are
available - the latter adding a FET cost BUT also allowing essentially
unlimited output current as desired. Efficiency is down on the
synchronous version but not vastly so.
(At 3V3 an output  Schottky diode adds notionally say 0V4V/3v3 = 12%
losses so you'd expect it to be well down on ideal. )
(At 5V it's 0V4/5V = 8%.) The synchronous rectifier should be far
closer to ideal if done properly.

As Joe notes, the MCP1623/1624
             http://ww1.microchip.com/downloads/en/DeviceDoc/41420A.pdf?from=rss
Appear to be lower rated versions of the MCP1640
   http://ww1.microchip.com/downloads/en/DeviceDoc/22234A.pdf

Peak FET switch current is about double for the 1640.

The 1623 is PWM only while the 1624 can switch between PWM and PFM
allowing greater efficiency at low loads due to pulse skipping.

Both versions offer "true low disconnect" by turning off the
synchronous output PFET during shutdown. This is often very useful as
it removes the battery voltage which is otherwise present on the load
in  a non-synchronous design during converter shutdown (less a
Schottky diode drop). Inability to remove voltage from the output can
be extremely undesirable.

The 1640 has variants which are not available in the 1623/24 which
instead of turning off the PFEt in disable mode, turn it hard on. This
allows the battery to be connected to the output in shutdown. If Vin
is high enough this is most useful.

Eg a system may require > = 3.3 V out and be operated by 3 x NimH
cells. When V p er cell > ~= 1.15 V the battery can be connected to
the output directly and the converter disabled. When Vbat falls too
low the converter is enabled. This mode is not available with the
1623/1624 meaning that it cannot be used with 3 x NimH and a 3.3V
output;. At most 2 x NimH can be used.

Similarly 1 x LiIon cannot be used at 3V3 out but is OK at 5V out.

I haven't compared the 1640 / 1623 specs in detail but it seems like
the 1640 is the superior part unless there are some extra features
waiting to be found (quite possible).

Apart from price, quite useful ICs.


                  Russell

(* CE9908, CE6360 [not a QQE03/12 :-) ], ... )

> If it's the MCP1623/24 I found the datasheet easily enough, but
> can't find the actual device to either buy or sample, which is a bit
> of a tease. And no mention that I can see anywhere of if/when it
> will be available
>
> Press release leads to MCP1640
>
> http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2018&mcparam=en547321
>
> which looks, at least superficially, the same as the MCP1623/24.
> Unavailable for sampling but can be purchased June, $0.46/100
> (compare with $1.80/100 for ST L6920)
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