Calculating the oscillator frequency

Question

I am trying to calculate the frequency of the internal oscillator, in order to go through the design procedure listed in the datasheet. The problem is that it gives me answers that contradict the 100kHz max f specification. Here is my calculation with the example values in the step-down converter example:

Here is the datasheet I am using:

http://www.ti.com/lit/ds/symlink/mc33063a-q1.pdf

To find the discharge time (tdis), I am using the following values:

C = 470pF (CT on figure 13)

dV = 0.5V (Section 7.4)

Idis = 200mA (Section 7.4)

I then calculate:

Idis = C * dv/dt ----> dt = C * dv/Idis

dt = 470pF * 0.5/200mA = 1ns

since the charge time is 6 times the discharge time, the period T is 7ns, which gives a frequency of approximately 143 MHz. What is happening here?

Answer 1

Section 7.4 is wrong.

It says mA, but the actual tabular data is talking about µA. So there is an error of factor 1000 there.

With that you end up at 143 kHz. Calculating a bit more precisely, you end up with 128 kHz.

But there is some non ideality in this chip. If you look at Figure 1, for 500 pF you end up with 3 µs on and 20 µs off time, which results in 44 kHz.

Answer 2

You've spotted a mistake in the data sheet text!

If you look at the later section you can see they've got the currents in the text description in mA where they should be in uA:

if there seems to be a mismatch between text and tables, I'd always go with the tables.