LDO Regulator

A low-dropout or LDO regulator is a voltage regulator that has a very low input-output voltage drop, thus is able to operate at input voltages very close to the output one. Read more at Wikipedia.

Schematics

Below is the schematics of a simple LDO regulator with a over-discharge protection circuit for the battery.

The voltage regulation is done with the popular TL431 adjustable shunt regulator, by controlling 3 P-MOS transistors (Q1,Q2 and Q3) placed between the input (the battery) and the output (the load). When the voltage across R1 is above 2.5 V, U1 starts to conduct current through its cathode, increasing |V_GS| on Q4 P-MOS. Q4 turns on lowering |V_GS| on Q1, Q2 and Q3, which in turn will start to increase its R_DS, lowering the output voltage, automatically lowering the voltage across R1.

This negative feedback loop will keep the voltage across R1 at 2.5 V, which means the output voltage will be:

where V_REF is TL431 typical voltage reference, 2.495 V.

Things to consider when adjusting the circuit:

• don't go very high with R1 and R2; TL431 reference input needs 4 uA bias current
• keep C2 large/low to maintain stability; check the Stability Boundary Conditions graph (fig.16/18) in TL431 datasheet
• Q1, Q2, Q3 and Q4 needs to be P-MOS transistors with low |V_GS(th)| like: BSH207, IRLML6401, Si2305, PMN40UPE or PMV33UPE.
• make sure that (V_IN-V_OUT)*I_OUT power can be safely dissipated by Q1, Q2 and Q3; depending on how much power you need to dissipate you can reduce/increase the number of transistor in parallel; I wouldn't dissipate more than 200 mW in each one of those tiny SMD transistors; check their datasheets for thermal resistances
• also make sure that Q1, Q2 and Q3 have low RDS(ON) < 50 mΩ

Below are two simulations that shows the voltage drop caused by the regulator (excluding V_DS on Q6) at 100mA and 1A, by setting the input voltage equal to the regulated one, 3V.

At 100 mA the voltage drop is only 3.38 mV (plus 10.14 mV caused by the over-discharge protection transistor Q6).

At 1 A the voltage drop is 33.1 mV (plus 99.3 mV caused by the over-discharge protection transistor Q6), 10 times more, as expected.

The over-discharge protection circuit cuts off the battery when its voltage drops below a certain limit. This is done by a very low power comparator with an internal voltage reference, LT6700-1. Only one comparator from the package is used, and when the voltage across R7 drops below 400 mV, the output is pulled high turning off Q6, cutting off power to the regulator and load. The cut-off voltage is:

where V_TH(F) is the falling input threshold voltage of LT6700 internal hysteresis, 393.5 mV.

When the power is cut off, it is important to keep quiescent current as low as possible to prevent any further battery discharge. In the off state the quiescent current is formed by:

• 3.24 uA Q6 off state leakage current
• 11.4 uA U2 quiescent current
• 241 nA voltage divider current

and it totals to 14.9 uA.

For convenience, I've made a small form that lets you determine the output voltage and the cut-off voltage by adjusting specific resistors: