# Value Selection MP2636

We would need 120mA Charge current (and therefore 12mA Trickle Charge) if its possible to do whit this chip? If yes, what are the formulas to calculate the relations for RS1 and Riset.
Example: The 2400 / RS1 x Riset could be solved like
120mA=2400/ (20mOhm x 1MOhm) if the 20mOhm is a limit
or
120mA=2400/ (166.66mOhm x 120kOhm) if the 120kOhm is a limit
I assume for RS1 the only Limit is the Efficiency and Power loss over it. Therefore it should be as low as possible. But how high can Riset be choosen?

Hi,
Thank you for utilizing the MPS Technical Forum,

The minimum TC current supported by this device is 200 mA for Riset >= 60kohms. Therefore it will not be able to support 120mA of CC current and 12mA of TC current.

Regards,
Nouman

Hi Nouman,
Just to clearify for me. If the Riset will be > 60k then the trickle charge stays at 200mA. Right?
So the other line in datasheet with 10% of ICC is only valid von CC from 2-3A ICC with RS1 20mOhm?

So if i could accept that, is it right, that when i use an 600kOhm for Riset and 20mOhm for RS1, i would get an starting current of 200mA rising to 300mA till VBatt_full is reached and then the current also only drops to 200mA to terminate Charging?

Still the question is, how high could Riset be choosen for a stable operation of the chip? becaus the datasheet shows typical values up to 120kOhm? Is 600k or 1.5MOhm somthing that would work?

Hi gmu,
Confirmed with our internal team - the ICC is recommended to be not set lower than 1A, lower current will make it difficult for the current sense signals to be detected with high accuracy.

Regarding the minimum I_TC current, it is constant/fixed 200mA with RS1 = 20mohms and RISET >=60k.

Can you please provide your requirements/specs? we have other linear charging devices that might support your current specs.

Regards,
Nouman

Hi Nouman,

Thank you for your clearification on this. The question is for my now, how would the accuracy go down if we go to 300mA.

We have some special requirements. But may you can give us a better recommondation:
We come in from USB with 15W (5V / 3A)
With this we charge a small Batterie with 300mA (Limitted by the Batterie) to 4.2V.
For our device behind we need permanent 5V, thats why we we need to boost the batterie voltage from 3-4.2V to 5V.
On the Device we must be able to get 5V / 3A in any circumstance either by the power supply (if connected) or from the Batterie, if available indipendent of the SOC. Its just for short time (smaller than 1s) but must be possible. That is how we came to this solution because it is not very common to get 15W out with both sources. But may you can suggest a better solution?
Regards, Gmu

Hi Gmu,
I have looked into your request - unfortunately, we do not have a device that supports such low of ICC for your case. The devices that support other all other specs but the only limitation is on charge current.

The lowest Icc in the devices I have found are ~450mA.

For reference - please look into MP2617A/B, and MP2617H

Kind Regards,
Nouman

Hmmm… very bad news for us, but thank you for clarifying.