Need help to design DC-to-DC step down SMPS using MC34063A

[Carl]

Hi,

I would like to design a SMPS to make a step down DC to DC converter to
supply my digital camera with power. The camera needs 3,3 V and draws a
max of 1,1 A at power-on and when charging the flash, and about 500 mA
when idling. I have the SMPS IC MC34063A and would like to use it if
possible.

The datasheet is here;
http://www.onsemi.com/pub/Collateral/MC34063A-D.PDF

Application data is here;
http://www.onsemi.com/pub/Collateral/AN920-D.PDF

The initial idea was to use 8 pcs NiMH cells for a total of 10 V as
input voltage, but I can get more NiMH cells if the input voltage need
to be higher.
I have tried to follow the design example in the application data but
got a switching current between 2 and 3 Amps, and the spec indicates a
max switching current of 1,5 A for the IC. Still there are examples in
the datasheet with circuits delivering up to 5A, so I don't know what
I'm doing wrong. What I really need is a drawing that indicates the
values of the surrounding components of the MC34063 IC.
If the IC MC34063A is not suitable for this application could you
please direct me to a circuit that would be suitable?
Thank you in advance for your help.

marty

 

[John Popelish]

Hi,

I would like to design a SMPS to make a step down DC to DC converter to
supply my digital camera with power. The camera needs 3,3 V and draws a
max of 1,1 A at power-on and when charging the flash, and about 500 mA
when idling. I have the SMPS IC MC34063A and would like to use it if
possible.

The datasheet is here;
http://www.onsemi.com/pub/Collateral/MC34063A-D.PDF

Application data is here;
http://www.onsemi.com/pub/Collateral/AN920-D.PDF

The initial idea was to use 8 pcs NiMH cells for a total of 10 V as
input voltage, but I can get more NiMH cells if the input voltage need
to be higher.
I have tried to follow the design example in the application data but
got a switching current between 2 and 3 Amps, and the spec indicates a
max switching current of 1,5 A for the IC. Still there are examples in
the datasheet with circuits delivering up to 5A, so I don't know what
I'm doing wrong. What I really need is a drawing that indicates the
values of the surrounding components of the MC34063 IC.
If the IC MC34063A is not suitable for this application could you
please direct me to a circuit that would be suitable?
Thank you in advance for your help.

I hate to use any IC so close to its maximum ratings. And this
particular chip is not very efficient in step down mode, because of
its darlington output, that must drop over 1.2 volts when on.
However, using a larger value inductance will lower the ripple current
and make the peak current closer to the average output current.

Keep in mind, that with your input voltage several times the output
voltage, the output diode will carry the load current most of the
time, so its forward drop is a big part of the total losses. Choose
one with a low forward drop.

Perhaps:
http://www.irf.com/product-info/datasheets/data/10bq15.pdf
http://www.irf.com/product-info/datasheets/data/30bq015.pdf
http://www.microsemi.com/datasheets/msc1300.pdf

 

[Jochen Rapp]

Hi Carl,

i used this IC for a step Up Converter.
Take a look to national semiconductors.
They have a lot of SMPS IC'S with current of a few Amperes.

For Example LM 2576.

Best regards Jochen