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PICList Thread
'Cheep DC/DC, any idea ?'
2003\06\17@011148 by Mike Singer

picon face
Tal wrote:
> I need to implement a DC/DC converter for a cost sensitive
> design. Input voltage around 7-9VDC, output voltage 4 to
> 6VDC, at least 500ma. Ideally the converter should use
> components not heigher then 3-4mm, a relativily small foot
> print 1-1.5 square intch) and use surface mount
> components. Efficency is not a major requiremnets.

How about dropping input voltage requirements down to 2.5-4V
to enter mobile system's step-up DC/DC converters world?
Cheep & compact.


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2003\06\17@105428 by Tal

The input voltage is also used generate 5V (low current) to operate a
PIC and some digital and analog circuits. This is done using a low power
linear regulator. If we drop the voltage, we will need another step up
to 5V.


> {Original Message removed}

2003\06\18@015354 by Mike Singer

picon face
Instead of a low power linear regulator, you may use
regulating charge pump, MAX1759 mentioned in "Battery Level Monitor" thread for example.


The MAX1759 is a buck/boost regulating charge pump
that generates a regulated output voltage from a single
lithium-ion (Li+) cell, or two or three NiMH or alkaline
cells for small hand-held portable equipment. The
MAX1759 operates over a wide +1.6V to +5.5V input
voltage range and generates a fixed 3.3V or adjustable
(2.5V to 5.5V) output (Dual Mode™). Maxim’s unique
charge-pump architecture allows the input voltage to be
higher or lower than the regulated output voltage.
Despite its high 1.5MHz operating frequency, the
MAX1759 maintains low 50µA quiescent supply current.
Designed to be an extremely compact buck/boost converter,
this device requires only three small ceramic
capacitors to build a complete DC-DC converter capable
of generating a guaranteed 100mA (min) output
current from a +2.5V input. For added flexibility, the
MAX1759 also includes an open-drain power-OK
(POK) output that signals when the output voltage is in
The MAX1759 is available in a space-saving 10-pin
µMAX package that is 1.09mm high and half the size of
an 8-pin SO.

Tal wrote:
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2003\06\18@032401 by Tal

Thanks Mike.

This is an interesting chip but seems to be expensive (my guess 1.5 to
2$ in large quantities, please correct me if I am wrong).
The direction we are considering now is to use LM3485
( which is about 50C + 15C for
a power P Mosfet. This chip seems to work up to 1Mhz and therefore can
use a smaller inductor.

We are now in the process of studying this chip. One interesting point
is that its frequency depends on the ESR of the output capacitor
(strange, does it make the component selection more tricky?). It also
seems that its design result in noticeable ripple but I am not sure.

Does anybody has any experience with the LM3485 or can comment on it ?



> {Original Message removed}

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