Take the launchpad everywhere

The MSP430 Launchpad is a very promising piece of hardware, it’s very affordable, and its power saving features allow it to run from batteries for a long time with minimal power consumption.

Oddly, there seems to be little options to power the launchpad when it comes to making it portable. The only solution is to attach it to a USB cable. So I decided to start a hobby project that allows the launchpad to run from batteries – the Rocketfuel Boosterpack. The Rocketfuel Boosterpack should come in two variants:

  • Rechargeable – works with Lithium Ion batteries;
  • Non rechargeable – works with primary batteries with voltage from 3 to 5.5V;

The non-rechargeable rocketfuel is based on the TPS78233 from Texas Instruments, that I wrote about a few weeks ago. This LDO regulator runs from an ultra low quiescent current – about 450 nA – with no load.

The rechargeable variant adds an NCP301LSN30 voltage supervisor from On Semiconductor. This chip disables the LDO when the battery’s voltage drops below 3.0V to prevent over-discharge and damage to the Lithium Ion battery. Charge management is accomplished by the BQ2057C, a simple and effective Li-Ion battery charger from Texas Instruments. There are two LED indicators, one for charging and the other for when the charge is complete.

Status of the Current Design

The current prototype works as expected but has a bug that affects the idle performance of the rechargeable rocketfuel boosterpack. There is a reverse current path through the pass transistor that makes current flow back into the input side and increase battery drain when the DC supply is disconnected. Idle current is roughly 170uA, which is way beyond the acceptable for low power and idle applications.

The measured idle current of the rocketfuel contributed by the LDO and voltage supervisor is 640nA. Mounting the pass-transistor raises current to 170uA. If the current backflow is fixed, the rocketfuel should be able to drain no more than 2 uA from the battery, or even less.

Here are pictures of the prototypes using a Nokia battery as a test case:

Bottom side of the rechargeable rocketfuel boosterpack

Bottom side of the rechargeable rocketfuel boosterpack. Note the unpopulated JST connector’s landpattern due to the battery being soldered directly to the board using wires.

Top Side of the rechargeable boosterpack

Top side of the Rechargeable Rocketfuel Boosterpack, with a Nokia BL-5B battery attached.

The next design iteration is in progress and adds a FET transistor working as a synchronous diode. This should allow current to flow forward, with minimal power losses, when the power supply is connected and completely block backward current when the power supply is removed and the system is left running from batteries.

Design files for the current version should be arriving soon.

Link

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

Useful Links

The Launchpad

The launchpad can be seen as Texas Instruments’ own arduino.

The cool thing about the launchpad is that is comes with the debug interface included onboard. This means that we simply connect the to board with a common USB cable, fire up the right software and get ready to program and debug the processor.

The 3 variants of the launchpad have different base processors (in performance, features and cost) but have compatible form factors and pinouts. This is pretty cool and gives options to choose which device is most suited for a particular application.

More information about the launchppad can be found here: http://www.ti.com/launchpad

Another great site dedicated to the MSP430 is http://43oh.com