MC33908NAE's strange phenomenon

[xin_zhao]

Hello,

I designed MC33908 circuit according AN4766 to provide power to MPC5744p. When I debug the hardware, there is a strange phenomenon. When I first turn on the 12v switch power supply, then provide power the MC33908, MC33908 did not give its output. But when I truned off the switch power supply and power supply connector still connet to MC33908, then I  turned on the power supply again. MC33908 can provide power to MCU and AUX devices. The system works fine.  I tried many times, the phenomenon can repeatly appeared. I tested the circuit many times. There is no fault on circuit. Can someone tell me why MC33908 has such a phenomenon? What can I do to improve it?

PS:  The debug pin is active.

[guoweisun]

Hi,

Could you please change the power supply or use the battery replace your previously power supply?

Thanks!

[xin_zhao]

Hello,

I changed my power supply to a battery. It can not work .

I also changed to a adjustable power supply, when the voltage was between 7~10V, it could work. when voltage was above 10v, it failed.

[guoweisun]

Hi,

我觉得是电路的硬件问题，您能参考下面连接的原理图对比下您的原理图看看哪里有错误吗？

Evaluation Board - MC33908, Safe DC/DC|NXP 

[xin_zhao]

Thank you. I found where the error hanppened.

I add a 100nF capacity in buck circuit with diode D1, then  pin VSW1 VSW2 can not generate PWM. After I remove the capacity, it works. But in AN4766, it gives the Vpre characteristics, the  FSWPRE is 440 kHz, I check the frequency is about 300 kHz. Is it OK?  Are there some risks when I use it ?

[guoweisun]

Hi

你可以测试重载的时候的开关频率，轻载的时候就像跳频的工作模式为了效率更高。

多谢！

[xin_zhao]

谢谢！

[xin_zhao]

Thank you . I will check it.