MC34063 converter part value selection confusion

[chmorgan]

Posted on April 02, 2013 at 02:13

Hello.

I've gone through the process of selecting part values for a few other dc/dc converters but I'm stumped with the steps to do so for the MC34063. I'm starting with 12V and converting to 5V and 3.3v so I just copied the example circuit values for the 12V->5V step-down but I have no idea where to start on the 3.3V one.The calculations table seems like it has the key information but I'm not sure how to interpret it. Some of the calculations are asking about forward voltage but don't specify if this is if using an external transistor or if its a part characteristic or what, same for ton/toff etc.

[Vincenzo]

Posted on April 08, 2013 at 15:50

Hi Christopher,

all you need is to change high side resistor (R2)

V

OUT

= Desired output voltage, |V

OUT

| = 1.25 (1 + R

2

/R

1

) you can find the formula at page 13/23 of DS. Regards Vincenzo

[brianj]

Posted on April 16, 2013 at 18:05

Hello,

I had a follow up question to this one. I've noticed several differences between the ST datasheet for the MC34063A and other manufacturers (i.e. TI, ON Semi, etc). For example, the ST one is missing several parts of the calculations table on page Also, the Saturation Voltage (darlington) says that the test conditions are connecting pins 7 & 8 in the ST datasheet, as opposed to pins 1 & 8 in the other datasheets. I haven't combed over every detail of the rest of the datasheet to compare with others yet, but I am wondering if these are just documentation errors on the side of ST, or if, in fact, the ST version of MC34063A is functionally different and not equivalent to the industry standard MC34063A part (originally developed by Motorola)? We're considering spec-ing the ST version for our design BOM because of the lower price, but we don't want to sacrifice compatibility, performance, and behavior. I've attached a copy of the ST datasheet (obtained via Digikey) and the TI datasheet for your reference. I'd appreciate a response to this concern ASAP. Thanks, Brian ________________ Attachments : 497-7852-1-ND_Switcher.pdf : https://st--c.eu10.content.force.com/sfc/dist/version/download/?oid=00Db0000000YtG6&ids=0680X000006Hu03&d=%2Fa%2F0X0000000abD%2FmHSq20Iaye3ElX4Cvh16lPcp.d1.Lal7bKSSMtx1PWE&asPdf=falsemc33063a_TI.pdf : https://st--c.eu10.content.force.com/sfc/dist/version/download/?oid=00Db0000000YtG6&ids=0680X000006Htpf&d=%2Fa%2F0X0000000aXT%2F7Ol3qk3aZaOuSUZw9OGkrQKRiB_GFv88c9NaEhF_grk&asPdf=false

[brianj]

Posted on April 16, 2013 at 18:26

I also just noticed that the ST value for Ct (in the calcs table) is different.  ST shows it to be 4.5 x 10^-5 x ton, and the standard (TI, ON Semi) datasheets show it to be 4.0 x 10^-5 x ton.  Again, the same question, is this a typo or doc error by ST, or a difference of behavior for the ST part?  If the ST part does, in fact, behave differently then it means that it's not a compatible swap-out part, which means that we would need to go with a different part for our design. 

I look forward to your prompt reply. Thanks, Brian

[Vincenzo]

Posted on April 17, 2013 at 16:26

Hi Jones,

thanks for your e-mail, I confirm a typo for the test condition and as soon as possible it will be fix in DS. Basically our 34063 can be swap with other competitor, the difference for Ct calculation could be minimal with no big impact for the final application. Best Regards Vincenzo

[brianj]

Posted on April 17, 2013 at 22:47

Vincenzo,

The only way that the part is completely compatible and swappable is if the ST part has the ''exact'' same behavior and datasheet specs.  If the Ct spec is different then the part is not swappable, since that has a direct impact on switching frequency and overall power supply efficiency and performance.  It's either 100% compatible or it's not.  Can you check with your team and supervisors and confirm definitely if the ST version of the MC34063 is 100% compatible?  We don't want to design our system based on the idea that it's ''probably'' compatible. Thanks, Brian

[Vincenzo]

Posted on April 18, 2013 at 13:51

Brian,

could you please share with us the schematic, PCB layout and volume (pcs) for your application? So I can better help you. Best Regards Vincenzo