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How to select output current for USB source?

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Mandar Joshi

Member level 2
Mar 4, 2015
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I am using LM3658 for my project which is battery charger IC. It has USB_sel pin which allows to select USB charging current i.e. if it is pulled high, current limits to 500mA and if pulled low current limits to 100mA. So my question is which is good option?
Also one more question which puzzles me if mobile requires charger with output 5V and 1A rating then what will happen in following cases:
1. Charger with output 5V and 0.5A selected, will it charge slowly or won't charge at all?
2. Charger with output 5V and 2A selected, will it damage mobile or charging will be fast?
One might think its silly question but correct answer will be more helpful to me clear my doubts.


It seems to me you are mixing some information. A picture could clarify things.

In an USB system there is
* a host. Usually a PC. It uses "A" type connectors
* and a device. Maybe a printer or a camera. It uses "B" type connectors.

When you connect a device to a host, then first the communication speed is determind, then the device "asks" the host for a supply current. The PCcan refuse this, then the device must not draw that much current. If i remember right, then in ctive mode the PC should not refuse up to 100mA, but it may refuse more than 100mA.

Exeptions are 5V USB style power supplies with type "A" style connectors. There usually is no USB communication. If it can supply up to 1A then the supply connects both D+ and D- datalines to show the high current capability.

I guess your charger chip is on the device side. So higher charge current reduces charging time. I'd prefer this.
So if you keep on USB standard you first have to check if there is a true host. If yes, then you have to check if 500mA is available.
If it is a 500mA supply, then 500mA is always possible...


KlausST, sorry but I forgot mention one thing my first question regarding LM3658 has nothing to do with my second mobile charger question. These are two different questions.

Using LM3658 select the option of 500mA if the USB is connected to a desktop PC, and 100mA if it's connected to your laptop.
If you charge your phone with 2A charger your phone will take only 1A from the charger and it will be OK.
If you connect 0.5A charger then your phone will charge slowly but you are taking a risk in damaging the charger or even causing damage to the phone.
Vbase, thank you very much for this clarification I was thinking exactly opposite of your answer for my second question regarding mobile charger i.e. charger with 0.5A will be safe to use instead of 2A charger was my assumption. Thanks once again.


Often USB supplies (connected instead of host , PC) are called "chargers" but indeed they are constant voltage power supplies.

The charger circuit is always in the "device".

A power supply has rated supply current. This is the max current the device may draw from it. But if the device needs less current everything is OK.
The device must not draw more current than the supply/host can deliver.


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