Is this datasheet wrong?

[smbaker]

I built myself some USB power supplies using Figure 10 from https://www.mouser.com/ds/2/631/ACT4533A_Datasheet-346085.pdf, which is supposed to have a current limit of 2.4A. Measuring the supplies I built, I'm getting a current limit of around 3A.

Here's a description of setting the current limit:

ACT4533A/B  constant  current  value  is  set  by  a 
resistor connected between the ISET pin and GND.
The CC output current is linearly proportional to the
current  flowing  out  of  the  ISET  pin.  The  voltage  at 
ISET  is  roughly  1.1V  and  the  current  gain  from 
ISET  to  output  is  roughly  21000  (21mA/1μA).  To 
determine  the  proper  resistor  for  a  desired  current, 
please refer to Figure 2 below.

Using ohm's law, I'm calculating that the recommended 7.87K resistor would yield a current limit of 2.94A (consistent with the ~ 3A that I'm seeing) and that for 2.4A current limit the appropriate value would be 9.63K.

Can someone check my math and verify that I'm not being an idiot?

Scott

[ebastler]

I guess they mean that the power supply is designed to supply up to 2.4A reliably and robustly. Then all components will provide for some headroom beyond that current; and don't want your current limiter to kick in at exactly 2.4A, but a bit above that limit. So, no real contradiction.

[smbaker]

I guess they mean that the power supply is designed to supply up to 2.4A reliably and robustly. Then all components will provide for some headroom beyond that current; and don't want your current limiter to kick in at exactly 2.4A, but a bit above that limit. So, no real contradiction.

Hmm, I suppose you could read it that way.

The description of the IC is that it provides constant voltage and/or constant current regulation, so my expectation reading that example in the datasheet is that it would transition to constant-current mode at 2.4A.
I should find a few other chargers around the house and see how they behave when they reach their stated current limit. Maybe having it limit at 25% over the stated rating is standard practice.

[Wimberleytech]

Using ohm's law, I'm calculating that the recommended 7.87K resistor would yield a current limit of 2.94A (consistent with the ~ 3A that I'm seeing) and that for 2.4A current limit the appropriate value would be 9.63K.

Can someone check my math and verify that I'm not being an idiot?

Your math is correct.  Moreover, the datasheet figure gets closer to the 2.4 number as shown in my attached image.

[David Hess]

±7.5% CC Accuracy

No other accuracy specifications are given for Iset and current ratio.  It is not meant to be a precision constant current source.