34063 - Page 1
Forum section: electronics / general

author: hedley
author_rank: Contributor
timestamp: January 05, 2018, 06:43:58 pm
content: Have been using these for 20 years as buck converters from 12vdc to 5vdc. Have a requirement to supply 750 Ma . I have used correct components ie diode and inductor and run all the numbers many times.Observation - when I increase load using a dummy load as soon as I go over ~ 420 ma the chip ‘ shuts down ‘ BUT if I increase the input voltage from my bench supply to 20 Vdc then I get the 750 ma @ 5vdc with no problemAny ideas -?

author: SeanB
author_rank: Super Contributor
timestamp: January 05, 2018, 07:43:58 pm
content: Switch transistor is limited to around 1A internally, so the current limiting is shutting down the chip with increased load. Only solution is to use another newer switcher with higher rating, or use an external switch transistor to boost the current rating of the switch to a higher value, though you will have increased losses in the transistor and will need a base pull down resistor as well to ensure fast turn off of the power device.

author: mariush
author_rank: Super Contributor
timestamp: January 05, 2018, 08:02:43 pm
content: The answers to your questions are in the datasheets... duty cycle, inductor size, switching frequency ... download multiple datasheets and application notes (google "application note 34063 / 33063 " ) and you'll find lots of explanations.Anyway, those chips are crap anyway, time to move on to better more modern chips ...Look at AP65111 or AP65251 or AP6502 for example, they run at 500 kHz (or 340kHz for AP6502) so they need much smaller inductors and they're cheap, under 50 cents if you buy 10 pcs or more .. and you save money by being able to use cheaper inductors and cheaper ceramic capacitors and resistors.AP65111 :https://www.digikey.com/product-detail/en/diodes-incorporated/AP65111AWU-7/AP65111AWU-7DICT-ND/7652826AP65251 :https://www.digikey.com/product-detail/en/diodes-incorporated/AP65251WU-7/AP65251WU-7DICT-ND/7915274AP6502 :https://www.digikey.com/product-detail/en/diodes-incorporated/AP6502SP-13/AP6502SP-13DICT-ND/2767828^ change R1 and R2 ratio and tweak the inductor value if needed (as explained in datasheet) and you got 5v output at up to 1.5A

author: hedley
author_rank: Contributor
timestamp: January 06, 2018, 09:52:59 am
content: Thanks . Datasheet says 1.5 amp .1. I have taken the advice of other posters and ordered alternate parts .2. After reading many data sheets / app notes I still can’t get an intuative feel for why with higher input voltage the chip can deliver up to 1 amp , but with a 12vdc input only 400 mA.3.there seems to be some relationship between the voltage difference between input and output and the load current before shutdown .I will keep looking at app notes but if someone has an insight please reply.

author: hedley
author_rank: Contributor
timestamp: January 06, 2018, 10:29:45 am
content: The setup isInput provided by Rigol DP 832 ( adjustable )Load - Constant current load 388mAOutput from switcher 5VDCObservations - As I increase the input voltage on the DP832 in steps of 1V from 12 -> 25 V the current on the input decreases proportionately so that the POWER stays the same on the input.This is why I think with lower voltage on input ie 12VDC a higher current will be required which must pass through the darlington pair in the switch to result in the same power .?If set as per the datasheet with a 25VDC input then reliable 5VDC @ 1.5 Amps is achievable . My problem is I have a 12VDC source so there is no free lunch - eg Power in - less losses ( efficiency ) out