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MC33063A: Calculation of PWM frequency

Tao_2199
Genius 5770 points
Part Number: MC33063A

Hello,

Regarding to PWM frequency on MC33063A, my customer is asking some question.

According to datasheet(page4), PWM frequency is a following on Ct=1nF.

 


 

(Question)

(1)Due to calculation of PWM frequency, they refered Figure1(page6) and 9.2.2.2 Detailed Design Procedure of step-up (page14).

But they couldn’t calculate it from above these information.

Could you please tell me how to calculation of PWM frequency from Ct(Timing Capacitor) in detail?

 


 


 

(2) About Figure1(page6),

For example, when Ct=1nF, ton time is approximately 20us.

This is meaning that ton time need to set more than 20us at Ct=1nF.

My understanding is correct?

 

(3) According to datasheet, PWM frequency is up to 100kHz.

I’m understanding that PWM maximum frequency as specification.

If so, could you please tell me how to lead about it with Figure1(page6)?

 

Regards,

Tao2199

  • 0
    TI__Guru* 78270 points
    I agree that the datasheet is not very clear and straightforward about how to determine the switching frequency. Figure 1 is typical IC performance data (tested) and the design procedure equations are 1st order approximation. From the three rows: ton/toff, (ton+toff), and CT, do some math process then you can cancel out ton and toff and get the direct relationship between f and CT. By that relationship, you can directly decide CT for your targeted f.