MC33063A: MC33063A Overcurrent protect issue

kevin hsu

Part Number: MC33063A

Tool/software:

Dear TI expert,

We have a voltage inverting concerter design with MC33063A as show in below. The Vin is +12V and desire Vout is -12V. We would like to know what is the ocercurrent protect value or how to set it?

There is our experiment to share with you. It looks like no current protection in this chip. It use total output watt=1.8W as protect strategy instead

Test condition

Vin: 12V

Iin: Form 12mA-->240mA(The maximum chip sink current, The power supply OCP sets in 1A)

Result

Vout: -12V@ Iout: 0~150mA-->1.8Watt(@150mA)

Vout: -10.8V@ Iout: 170mA-->1.836W

Vout: -4.5V@ Iout: 400mA-->1.8W

Thanks

Kevin Hsu

over 1 year ago

0 Brigitte over 1 year ago

TI__Guru 65170 points

Hello kevin hsu,

The resistor from IPK to VCC defines the current limit.

Best regards,
Brigitte

0 kevin hsu over 1 year ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte,

Based on MC33063A datasheet, the Current-limit sense voltage is 0.3V(Typical) and there is a 0.33 ohm resistor between the IPK and VCC pin. Then the current limit should be 0.3V/0.33ohm=0.909A. But our experiment result shows that Vout decrease when Iout over 0.15A. It is faw away from the current limit setting. Would you help to clarify it if any misunderstand? Thansk!

Kevin Hsu

0 Brigitte over 1 year ago in reply to kevin hsu

TI__Guru 65170 points

Hello Kevin,

Could you please measure the inductor current?

Best regards,
Brigitte

0 kevin hsu 11 months ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte,

Sorry for late reply due to current measurement equipment issue. Please check inductor current measure result as below. Thanks!

Iout=0A Vout=-12.31V Voltage gap between IPK to VCC = 4.4mV

Iout=100A Vout=-12.14V Voltage gap between IPK to VCC = 51mV

Iout=200A Vout=-9.7V Voltage gap between IPK to VCC = 77mV

0 Brigitte 11 months ago in reply to kevin hsu

TI__Guru 65170 points

Hello Kevin,

To me this looks as if your converter is hitting current limit. The current limit is limiting the peak current. What happens if you reduce the resistor on IPK?

Another test would be to increase the inductance for a slower current rise.

Best regards,
Brigitte

0 kevin hsu 11 months ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte,

We observed the voltage gap(Yellow and Purple) between VCC and IPK by scope. So I would like to confirm with you if the current limit happened in every switch cycle even in Iout=0A condition? Thanks!

Kevin Hsu

+1 Brigitte 11 months ago in reply to kevin hsu

TI__Guru 65170 points

Hello Kevin Hsu,

Please have a look at this app note: Application of the MC34063 Switching Regulator (Rev. B MC33063A behaves the same.

The device is regulating the switching with the peak current limit.

Best regards,
Brigitte

0 kevin hsu 11 months ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte,

Thanks for your spport. We have no more question.

Kevin Hsu

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MC33063A: MC33063A Overcurrent protect issue