Designing for low current draw

Hi Jeffrey,

My colleague will give you response later!

BRs

Zixu

Hi Zixu,

Any update on this?

Hi Jeffrey,

Sorry for late response and thanks for reaching out. 

Is the schematic shown above the from original overspec design or the updated one?

Thanks!

BRs

Lucia

Hi Jeffrey,

It's suggested that you don't need that large inductor. Please follow the equation shown below:

Here the maximum output current means 3A, we don't need to use 0.1-0.2A. And please kindly reselect Cout based on the updated inductor value.

Anything unclear, kindly let me know. Thanks!

BRs

Lucia

Hi Lucia,

The design I included is the updated one. Also, if I understand you correctly, I should be using 3A as the maximum output current, even though we only expect to draw 0.1-0.2A. Is that correct? In that case, can I source an inductor that's only rated for the actual current we expect to see or do I need to find one that can handle 3A? If I ground ILIM2, can I use 1.5A as the maximum output current for that circuit? And wouldn't that mean I need to use 4.5A for the maximum for output 1 since that's where the current limit threshold is?

Hi Jeffrey,

My colleague Lucia will give you reply by tomorrow!

BRs

Zixu

Hi Jeffrey,

Yes, you are right. But please kindly check the rated current and saturation current according to the inductor rms current and peak current.

Thanks

BRs

Lucia

Hi Jeffrey,

I mean that in this equation, we recommend to use 3A*K as the peak to peak ripple. And for the current rating, you need to consider the rated current, the saturation current and also the current protect limit. This ensures that the inductor does not saturate even during a short circuit on the output. When the inductor core material saturates, the inductance falls to a very low value, causing the inductor current to rise very rapidly.

BRs

Lucia

Hi Lucia,

If I'm calculating this correctly (so using 600mA as the maximum ripple but 100mA as the maximum output current), I can reduce the inductors to 15uH, with a ripple current of 475mA for 5V and 357mA for 3.3V. This is significantly larger than my expected current draw; is that going to be a problem?

Also, it looks like that would mean my saturation currents are 338mA and 279mA, so I'm looking for a 15uH inductor with rated current of at least 100mA and a saturation current of at least 340/280mA. Additionally, I'd need 47uF of output capacitance. Does that make sense?

Hi Jeffrey,

My colleague Lucia will give you reply by next Monday!

BRs

Zixu

Hi Jeffrey,  

1. It not only depends on the rated current and saturation current of the inductor but also you need to consider the current protection limit. For example, when the device enter current protection mode,  the saturation current rating of the inductor is at least as large as the protect current limit. This ensures that the inductor does not saturate even during a short circuit on the output. When the inductor core material saturates, the inductance falls to a very low value, causing the inductor current to rise very rapidly. If you want this current protection function work well, please also select the inductor based on this. 

2. What's more, in the equation, you can use 3A*K as the peak to peak ripple.

Anything unclear, please kindly let me know.

BRs

Lucia

Hi Lucia,

I understand the issues caused by inductor saturation; that part makes sense. What's confusing me is that the example in the datasheet calculates a saturation current of 2.25A for both outputs, despite the nominal overcurrent limit for output 1 being fixed at 4.5A. Is that not correct? Or am I missing some other part of the circuit that prevents saturation in the case of a short?

Also, is it a problem that the peak-to-peak ripple is greater than my maximum expected current?

Thanks,

Jeffrey

Hi Jeffrey,

My colleague Lucia will give you response by next Monday!

BRs

Zixu

Hi Zixu,

My apologies, I was out of town for the past two days. I was wondering if Lucia might be able to answer my questions by the end of this week?

Thanks,

Jeffrey

Hi Jeffrey,

Due to the vacation of national holiday, my colleague Lucia will give you response after 10/8.

BRs

Zixu

Hi Lucia,

That helped a lot. I will use 2.25A as my saturation current. One last question: You mentioned using 600mA for the ripple current; Figure 23 in the datasheet includes response curves for 200mA and 400mA as well. Can I use 200mA as my ripple current since that is closer to my expected current draw? It would boost my inductance to ~33uH, which is still significantly less than the 180 in the schematic I posted originally.

Hi Jeffrey,

It depends on your application. Lower inductance corresponds to faster transient response, smaller DCR, and reduced size. But too low of an inductance can generate too large of an inductor current ripple such that over current protection at the full load could be falsely triggered. It also generates more inductor core loss since the current ripple is larger. Typically, we would recommend K to be 20%-60%. You can use 200mA ripple current according to your application if the size of 22uH can meet your requirement. Please also check this on your evaluation board. Thanks!

Anything unclear, kindly let me know. Thank!

BRs

Lucia

Hi Jeffrey,

Please kindly check whether the size of 33uH can meet your requirement. Sorry for 22uH from last reply which is a typo. Thanks!

BRs

Lucia