• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 2174 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

MCP33063 Step down converter circuit overheating

Fidel
Prodigy 50 points
Other Parts Discussed in Thread: MC33063A

In the above circuit, the voltage is set by R7 to be about 4.1V. The coil L sets the output current to 1.2A. Input voltage is limited by C1 at 16V (approx. 14V in reality).
The circuit is designed to provide about 500mA to the rest of the circuit and one device can require 1.2A bursts.
After D2 is a 1A/3.7Vnom lithium battery that can either power the rest of the circuit or be charged if VIN is applied.
Although the MC33063A is cooled by copper in the board as per design recommendations, it gets pretty hot along with the coil when charging a battery. I'm suspecting my choice of diodes could be the culprit.
Is the choice of schottky diodes with 2A forward current incorrect? Can I replace D2 to slightly choke the max current down to say 800mA or would it be wiser to change the inductor to 47uH/800mA Irms?

  • 0
    TI__Guru**** 164456 points
    RSC is too low. Maximum switch current is 1.5A. Minimum Rsc is 0.3V/1.5A = 0.2 ohms
    With Ct = 470 pF, figure 1 (in data sheet) says on time is 13uS
    Current rise in coil is (VIN - VOUT -VLOSS) * Ton / L = (14V - 4V -1.5V ) * 13uS / 22uH = 5 amps.
    This means that current limit need to work intra-cycle which it can with some propagation delay.
    However , Rsc allow too much current now.

    Higher Rsc (I suggest at least 0.3 ohms) and larger inductance (I prefer at least 100uH) will help a lot.
  • 0 in reply to Ron Michallick
    Prodigy 50 points

    Thank you very much for your answer Ron!

    I will test some different coils asap.