| Electronics > Projects, Designs, and Technical Stuff |
| Is this datasheet wrong? |
| (1/1) |
| 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: --- Quote ---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. --- End quote --- 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:
--- Quote from: ebastler on January 05, 2019, 08:56:24 pm ---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. --- End quote --- 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:
--- Quote ---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? --- End quote --- 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. |
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