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I'm using MC33167 as DC-DC buck convertor (with 190uH, 5A Iron powedered core inductor, SB540 Schotky Diode and 4700uF, 50V Electrolytic Cap as other elements). Now I noticed that when I'm feeding the MC33167 from a 24V battery it allows upto 6A of output load current but when I supply it from a Transformer-Bridge-Filter power supply of 33V it allows only upto 4.6A of load current and then starts reducing the output voltage when tried to increase the load, to maintain the current at arounf 4.6A. Any idea why this could be happening? Is it because of the feature 'Cycle-by-Cycle Current Limiting' that acts more for higher input voltages?

For maximum output current especially when the duty cycle is greater than 0.5 (In this case DC max is 0.92), it is suggested that _IL be chosen minimum current limit threshold of 5.5 A. If the design goal is to use a minimum inductance value, let _IL = 2 (IL avg). This will proportionally reduce the converter’s output current capability. And that is what you are seeing when you comparing I out_ rms (load ) with different DC sources (battery and rectified AC source). They do have different peak−to−peak inductor ripple current. Inductor peak−to−peak inductor ripple current can be only affected by changing Inductor L, Frequency _f or Duty cycle. Please see our table 1. for design equations at: http://www.onsemi.com/pub_link/Collateral/MC34167-D.PDF Let me know if you have any other questions. Answered by:
ON Semiconductor
2013-02-11 11:16:08.931