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Cell charger and LDO with minimal leakage current

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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

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.

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)

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.

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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