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Issue In MC33063A for USB charger

Other Parts Discussed in Thread: MC33063A-Q1

Hi All,

One of customer is doing USB charger based on MC33063A-Q1 and they are facing below issue. Kindly help to suggest a proper solution to resolve this.

Specifications:

 

Input voltage    - 8 to 18 V

Output voltage - 5V. Output current - 1A

Working Temperature  à  - 10 deg to 80 deg cel

 

Components used :

 

1)      T12 coil Inductor - 200 uH and 1.8A

2)      PNP transistor PBSS5540Z

3)      Rsc - .1 ohm / 2 watts

4)      Ct – 220pF

 

Problem :

 

1) When short happens at the load the Ipk current goes up to 6 to 7 Amp due that the IC or External Transistor getting failed .

2) The Datasheet is saying that the IC is having Short circuit current limitation but it’s not working in their circuit.

 

3) Please help us to identify the proper Rsc value to shut off the IC , when the short happens at the load

 As per data sheet they used Rsc .1 ohm and 2 Watts & Ct 220 pF  , but still short circuit not working properly and external transistor failed in 2 seconds  . When we used an SMD PTC SMD075 short circuit worked , but when doing an endurance testing at more than 30 deg cel , the PTC is getting opening up in 20 minutes which is not acceptable . so they forced to remove the PTC and design the components in such a way IC should do the short circuit current shut off by itself .

Request you to suggest the best value for Rsc and Ct  and the best suitable PNP transistor to resolve this issue 

Regards,

Aravind