MC34063 minimum current in circuit? - Page 1
Forum section: electronics / general

author: aeberbach
author_rank: Regular Contributor
timestamp: December 17, 2020, 09:52:47 am
content: I need to supply VGH = 18.9V and VGL = -7.8V to an LCD panel. The datasheet for the panel tells me required current is 1mA. Available voltage inputs are 12V, -12V and 5V. My voltage is lower than required so I looked at the MC34063. The EEVBlog video was useful and everything seems to work out. It's not expensive and it will also invert the input to get the -7.8V switching a few components around.But why do all the online calculators only give me reasonable coil values when the output current goes significantly above 1mA? Is the lesson that if the datasheets don't show performance at that level, don't try it?Can I get away with simply attaching a suitable resistor to the output, bringing output current up to the region calculated values become more sane?For 18.9V, 50mA, 390R 5W, for -7.8V, 50mA, 150R 5W? With the $1.50 or so these resistors add maybe it is better to choose something else.What is a better solution here? Maybe LT3463, a dual positive and negative converter.

author: Ian.M
author_rank: Super Contributor
timestamp: December 17, 2020, 11:33:42 am
content: MC34063(A) converters are usually stable with no external load, as the feedback divider loads the output sufficiently to handle switching transistor leakage in a high input voltage buck configuration. At low loads they'll simply cycle skip until the output voltage drops below the threshold.IMHO you need to be operating near the high end of the frequency range covered by the graphs in the MC34063A datasheet, so select a 330pF timing cap or specify 48KHz min. oscillator frequency, and 10mA output current, run the design equations on that basis and see what pops out. A couple of online calculators agree you'd need a 2.7mH choke, and it looks like a Bourns RLB0812-272KL would do nicely.For the -7.8V negative rail, if its also low current, just use a linear adjustable regulator from your -12V rail.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 17, 2020, 11:28:03 pm
content: With MC34063, and you want go go from +12v to +18.9v at 1 Amp....So loads of inductors will do as long as you don’t saturate them…….makes sure Rsense is 0.3V/Ipk where Ipk is less than I(sat).The thing with MC34063 is that the switching frequency is often not determined by the timing capacitor. The timing cap gives you a free running frequency…but whenever your current sense voltage hits the 0.3V reference, then the charge current which flows into the timing capacitor suddenly rapidly increases and so the period comes to an end and a new switching period begins, and thus the f(sw) ends up being often much more than the timing cap would suggest.There is an MC34063 app note which explains this (its under MC33063 which is basically the same part just different temperature range).Anyway, at 1mA load it’ll be cycle skipping most of the time so you wont get a high overall f(sw).There'll be lots of different timing caps and inductor values that you can use.Essentially the Vout divider comparator will sort it all out for you.....just put a big enough output cap on so you dont get much ripple...but make Rsense big ohmic value as poss , which also will tend to reduce vout ripple.And here is the holy grail on the MC34063 (AN920)https://www.onsemi.com/pub/Collateral/AN920-D.PDFIve attached an representative MC34063 sim for you in the free LTspice simulator...just hit the running man icon and experiment with different sense resistor values and inductor values.....or leave it as it is it probably doesnt matter

author: David Hess
author_rank: Super Contributor
timestamp: December 18, 2020, 02:59:54 am
content: Quote from: aeberbach on December 17, 2020, 09:52:47 amI suspect the online calculators are assuming continuous conduction mode which requires larger inductance at lower output currents.There is no reason you cannot operate the MC34063 in discontinuous conduction mode because the constant off-time operation can be unconditionally stable.At low output currents, a switched capacitor converter may be a better choice.

author: aeberbach
author_rank: Regular Contributor
timestamp: December 18, 2020, 06:04:52 am
content: Fantastic help, thanks all very much. Because I need to control the power up and down sequence of the various standby/reset/voltages I am going to use the LT3463 this time, it has shutdown on both outputs and makes it easy. Also requires smaller inductors and is barely more expensive all things considered. I'm still buying a couple of MC34063 to keep around though.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 18, 2020, 10:10:00 pm
content: QuoteWell said David Hess for calling a spade a spade...none of the datasheet or app note calls MC34063 a constant off time converter but that is essentially what it ends up being. It can operate continuously, or in burst mode, and when it does switch, it does so in essentially constant off time mode.