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Cell charger and LDO with minimal leakage current

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Wilson__:
Appreciate advises, pointer to information and insight for choosing cell charger chip and LDO regularor for limited size project. 

Input 5 volts from USB wall socket.  After charging done, charger chip will leak minimum current from cell. 

The LDO will step down to 3.3V for MCU which is 1 uA stop mode.  The LDO should draw around 1 uA or less. 

Wonder if traditional linear LDO or Switch Mode, MCU take 2.7V minimum which give 90% duty cycle for minimum cell 3V.  Seem SMPS need 90% or less duty cycle to work.  Any pointer to cook book for inductor size (ampere handling without magnetic core satuation) and inductance   TIA

Peabody:
The typical single-cell lithium charger is the TP4056.  I measured one and found essentially no current flowing back through the charger when the charger is not powered.

In addition, you will have some kind of protection circuit, such as a DW01, which is always on.  I think those draw about 3uA.

The MCP1700 regulator has quiescent current of about 1.6uA.

Switch-mode regulators usually are not very efficient at very low output currents.

jbb:
If looking for switch mode controllers, you want to search for “Low Iq” or low quiescent current regulators.

An example: https://www.ti.com/product/TPS62737

Generally, a lower max output current will also correlate with a lower Iq. So if you only need 100mA max for your project then a 0.5A buck converter is likely a better choice than a 3A buck converter. But check the quoted Iq value because they vary wildly.

A big downside of a low Iq switch mode (ie buck) converter is that it probably achieves the very low Iq by going into a pulse skipping mode. This means such a buck converter will output a LOT of voltage ripple (could easily be 100 mV)

jbb:
Oh yes, I also suggest you come up with an estimate of what your device will draw, like “it’ll draw 100 mA for 0.1% of the time, 10 mA for 0.5% of the time and about 2 uA otherwise.”

Use this information to make models of what the proposed buck converter would do versus what a basic LDO would do.

aldi:
If the max current draw is low (<150mA) take a look at the NCP170 series. It's my jellybean low-power LDO. The Iq is around 500nA, typical.

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