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The TPS78233 is probably industry’s LDO with the lowest quiescent current. While working on a boosterpack for TI’s Launchpad I took the chance to measure actual current consumption of this LDO, as well of the voltage supervisor: ON semi’s NCP301LSN30T1.

The TPS78233 promises a typical no-load quiescent current of 420nA. Current into the EN pin is tipically 40nA at 5.5V. This yelds a pretty impressive figure of 470nA typical supply current.

TPS78233 datasheet

TPS78233 datasheet

I am working on a small board that uses this chip, so I decided to give it a test and check if it lives up to its expectations. This prototype includes the TPS78233 fromĀ  Texas Instruments and the NCP301LSN30T1 from ON Semiconductor. Both devices promise extremely low supply current, which is precisely what I am looking for. For this test I used a Lithium Ion battery. The battery’s voltage was measured and registered 3,84V.

Picture of the test board

Test board, featuring the TPS78233 from Texas Instruments and the NCP301LSN30T1 from ON Semiconductor. The NCP301LSN30T1 isn’t mounted yet to allow measurement of supply curent into only the TPS78233

My multimeter can’t go below 1uA or 100uV, which is still high (!!!), compared to the current that I’m trying to measure. So, in order to measure currents as low asĀ  few nA, I decided to mount a current sense resistor with 10kOhm. This yelds 10mV at 1uA, which isn’t much.

Picture of the current sense resistor

To allow measuring such low currents I used a regular 10k 5% resistor.

Now we are all set to make a few measurements and check how low is the supply current and power dissipation of this little device.

The first measurement was made with the switch the EN pin tied low, ie, with the regulator disabled.

Picture of quiescent current with the LDO disabled

With the LDO disabled supply current lowers to 30nA, which causes 300uV on the sense resistor.

0,3mV through a 10kOhm resistor give 30nA of current. Quite impressive, although not as low as the 18nA referenced in the datasheet. Still, this board wasn’t clean at all, there were still flux residues and even breath caused this value to oscillate. For very short moments, the multimeter showed 0,2mV, so the actual value is probably somewhere in between 0,2mV and 0,3mV.

Now let’s see what happens when we activate the TPS78233…

Current sense voltage with LDO enabled

Current sense voltage with LDO enabled. 4.5mV on a 10k resistor, which yelds a current of 450nA!

450nA quiescent current! It turns out the TPS78233 really meets the expectations! I’m impressed! With a supply voltage of 3.84V this LDO’s power consumption is only 1,73uW. Wow…

So, the next step is to add the voltage supervisor into the mix and see the effect it produces on overall power consumption.

Picrutr of the landpattern for the NCP301LSN30T1

Mounting Landpattern for the NCP301LSN30T1.

The voltage supervisor mounted on the board:

Picture of the NCP301LSN30T1 in place

The NCP301LSN30T1 mounted on the board

Now, we’ll repeat our measurements. First, supply current with the voltage regulator disabled.

Quiescent current with LDO disabled and supervisor mounted

Quiescent current with LDO disabled and the supervisor mounted

Current sense voltage is now 2.2mV. This means that supply current is 220nA. The voltage supervisor added 190nA to the circuit’s supply voltage. Measuring the supply voltage and protecting the battery against over-discharge with 190nA is more than a fair price to pay.

The supply current of the NCP301LSN30T1 shouldn’t depend on the ON/OFF state of the TPS78233 LDO. We’re just about to confirm that…

Current sense voltage with LDO enabled and supervisor.

Current sense voltage with LDO enabled and supervisor.

Turning the regulator ON gives a not susprising 6,4mV, which means supply current is now 640nA. This was already expected: an increase of 190nA in the supply current of the whole circuit.

Conclusion

One might think that the TPS78233 seems to good to be true, after reading the datasheet. However, this test shows that the TPS78233 really does deliver the performance it promises.

The voltage supervisor is also a remarkable device in terms of running with a residual supply current.

Summary of supply current per device:

POWER SWITCH POSITION
OFF ON
TPS78233 30nA 450nA
NCP301LSN30T1 190nA 190nA
TOTAL 220nA 640nA

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