LM3485 current limiter ($R_{adj}$) value selection

I have what I believe is a working buck converter design centered around the LM3485. It's intended to be a Raspberry Pi power supply.

The schematic is at Pi Power schematic, which I copy below. (note that in the image there are two errors: L1 is 15 µH instead of 33, and the MOSFET is a Si3443CDV in the latest version).

The parts selection came largely from an online TI design tool. I changed some of the values for availability and cost, and the resulting circuit seems to perform well on the bench.

But since I have some time before the first production boards come back, I've been going over and over the design with a fine toothed comb, and the one thing I can't quite work out is the selection of $R_{adj}$ - the current limiter comparator resistor.

According to the datasheet, the comparator compares $V_{isense}$ with $V_{adj}$, and when $V_{isense}$ is lower, it triggers the over-current shutdown.

Well, $V_{isense}$ is the voltage drop across a sense resistor, which for this design is 25 mΩ. The maximum rated current for the design is 2 amps, and the datasheet says the peak amps is $(I_{max} + I_{ripple}) \times 1.1$, and that for currents over 2 amps, $I_{ripple} \le I_{out} \times 0.3$.

On the $R_{adj}$ side, there is a calibrated 3.0 µA current sink that's connected on the opposite side of $R_{adj}$ from $V_{in}$.

Put it all together and the current limit should be

$$R_{adj} \times .000003 = I_{max} \times .025$$

or

$$R_{adj} = \frac{I_{max} \times .025}{.000003}$$

For an $I_{max}$ of 2.53A ( $= 2.3 \times 1.1$), $R_{adj} = 22k$ (roughly).

So why did TI's design software recommend 39k?

I have to assume TI is smarter than I am. What did I miss?

• Go for it. Sorry. Too many cooks. :) – nsayer Jul 18 '14 at 2:24
• There! See if you like it this way - also check for errors, please. – Ricardo Jul 18 '14 at 2:26
• Looks great! Thanks. I'll have to look at how that syntax is done. – nsayer Jul 18 '14 at 2:41
• Question, why are you using a sync-fet for your top-gate? – Funkyguy Jul 18 '14 at 2:56
• I guess I don't know what a sync-fet is. I picked it by basically searching through DigiKey. I went for the cheapest part that would meet the current and thermal requirements. – nsayer Jul 18 '14 at 3:07