I have designed board with Mp24833-A according boots converter circuit form data sheet with small changes. The question is why current is not stable? R sense ( R3+R4=0,0685 Ohm) should set output current 2,92A but is not. I have measured much more, over 5A and is increasing with load.
What can be wrong?
Hello Mr. Weis,
a number of reasons, can be wrong.
Most critical with boosters is, that the supply is providing enough current at all times. When the low side fet of the booster is on, then current has to flow. If for some reason the current flow is restricted, then the booster cannot work as intended. Connect an oscilloscope to the VDD and VSS pin of the MP24833. You will have to solder small wires to the traces close to the pins and hook the probes up to these wires. The supply voltage has to be stable at all times. If not, my first action would be to solder a big electrolytic like 1000uF in parallel to C3.
Other than that, the routing of the FB trace is also critical. There should be no vias in the trace and it should stay away from the SW trace as much as possible.
How many boards do you have?
Just one? Then you have to check all solder joints and all components for correct soldering.
If you have more than one board and all are behaving the same way, then one important matter is to check if all component values are correct. Sometimes it happens, that the wrong value is placed and soldered by the EMS and people have grown a lot of grey hair finding such problems.
Did you assemble the boards yourself? Or was it done by an EMS?
If the MP24833 was soldered manually, then is the exposed pad soldered correctly?
Is the inductors saturation current well above the current limit?
Did you calculate the inductor value or did you just pick 68uH because the DS suggests it?
Hope that helps.