nRF52832 efficient power management solution

RE: nRF52832 efficient power management solution

dingari — Wed, 05 Apr 2017 21:34:28 GMT

Yeah, I totally understand. No hard feelings :) Hmm, I'll have to get some clarification on the GPIOs then.

RE: nRF52832 efficient power management solution

AmbystomaLabs — Wed, 05 Apr 2017 21:29:12 GMT

Sorry, didn't mean to suggest you weren't familiar with the need for level shifters. It just gets overlooked often, so I stress it.

On the open drain aspect of the gpio, the spec isn't clear on quasi versus true open drain. Most things that say they can be configured as open drain are only quasi open drain.

The electrical specifications for gpio says max input voltage is VDD. I would encourage you to verify this with Nordic switch to ground 3v from logic when VDD is only 1.8v.

RE: nRF52832 efficient power management solution

dingari — Wed, 05 Apr 2017 21:08:42 GMT

Yes I understand level shifters and why and when they are needed. In my application the nRF52 is talking to two I2C slaves (a MARG sensor and a haptic driver), each operating at 1.8V slave logic level (regulated internally by each chip). So no level shifting should be necessary. The main reason I'm eager to go with the TPS62740 is the high efficiency at low currents. And another thing: My understanding is that the nRF52 GPIOs can be configured as proper open-drain/collector, so writing 0 closes, and 1 leaves the pin open. That should allow supplying LEDs with higher supply voltage than the nRF52, without having voltage drop (LED_VDD - nRF52_VDD) across the LED in an open/high state (writing 1 to the pin).

RE: nRF52832 efficient power management solution

AmbystomaLabs — Wed, 05 Apr 2017 18:35:16 GMT

I just wanted to clarify the logic bit, since vdd is 1.8 you cannot try to talk directly to 3v logic. The 3v logic will have internal pull ups to create 3v high. 3v on the nRF gpio will just turn on the high side fet of the nRF and drive the nRF and all your other circuitry with 3v from the logic. The only way for you to talk to your haptic is via the gate of an N channel fet that does low side switching of the power on the haptic.

RE: nRF52832 efficient power management solution

AmbystomaLabs — Wed, 05 Apr 2017 18:08:19 GMT

Even if you are using 1.8V for all your logic you still need to populate the onboard dc/dc of the nRF. It will reduce the current consumption by 1.3/1.8 or 72% vs. the internal LDO which doesn't reduce current consumption at all.

The load switch on the 62740 does provide a convenient way to handle disconnecting your unused circuitry, however you could easily implement this with a p channel fet in a 0.6x0.8mm package.

Make sure you can control your haptic motor with low side switching. Since gpio vdd is 1.8v you can't high side switch unless you level shift.

RE: nRF52832 efficient power management solution

dingari — Wed, 05 Apr 2017 17:32:56 GMT

I've posted an update in the original post, any thoughts?

RE: nRF52832 efficient power management solution

dingari — Wed, 05 Apr 2017 09:42:25 GMT

Hi @AmbystomaLabs, thanks for your input. Really appreciated. I'm working with a LiPo whose cut-off voltage is 3.0V, and I plan to integrate standard protection circuitry on my board (Texas BQ297xx or equivalent). The other chips on the board use 1.8V logic levels so that shouldn't be a problem, and any LED's I'll configure as active low. I'll have a look through ST's site for some regulators.

RE: nRF52832 efficient power management solution

AmbystomaLabs — Tue, 04 Apr 2017 18:41:15 GMT

This series of parts from TI has a nice small 1x1.5mm uson package, very low quiescent current of 22uA and will operate at 100% duty so you can let it go unregulated if the lipo voltage is below VDD.
www.ti.com/.../tps62230

The UVLO (ie, brownout) of 2.05V is a little too low for most lipo's so you will still need to choose a lipo with a built in PMC to protect it. Otherwise the charger side of the system may have a UVLO with load disconnect.

RE: nRF52832 efficient power management solution

AmbystomaLabs — Tue, 04 Apr 2017 17:18:49 GMT

These seem like reasonable choices. They are all synchronous buck in uson or similar packages.

Though, I would probably choose from a component line that offers several VDD choices with internal feedback. The feedback circuit for an adjustable regulator can take up quite a bit of board space. ST has a large selection of DC/DC you can look at.

You should be careful if you choose 1.8V for the nRF rail. This forces the GPIO VDD to be 1.8V and you may create problems with off chip communications.

If I were doing this, I would also look for a converter with a brownout function. LiPo's don't recover from full depletion. Most LiPo's have a built in PMC to protect them from being fully depleted or excess current demands. Your LiPo may/may not have this.

RE: nRF52832 efficient power management solution

bjorn-spockeli — Tue, 04 Apr 2017 09:25:01 GMT

No, you're definitely not the only one ;) Hopefully some of the other users will comment if they've used one of the ICs you listed and give you a recommendation.

RE: nRF52832 efficient power management solution

dingari — Tue, 04 Apr 2017 09:10:18 GMT

Hi Bjorn, thanks for the reply. I was really just hoping to get the discussion going, since I can't be the only one doing a design like this.

RE: nRF52832 efficient power management solution

bjorn-spockeli — Tue, 04 Apr 2017 09:06:28 GMT

Hi Dingari,

as far as I know we do not have any reference designs that incorporate a dedicated power management IC, so we can't really recommend a specific IC based only on the occurrence of it in our customer designs.

Its primarily the RADIO TX and RX currents that are reduced when the internal DCDC is enabled, please refer this section of the nRF52832 Product Specification. We've only included specifications for 3V with and without the DCDC enabled, for which there is a considerable current consumption reduction with the DCDC enabled.

Generally, the efficiency of the DCDC will be reduced as you reduce the input voltage, i.e. the difference between the LDO and the DCDC is probably marginal at 1.8V. However, I do not have any specs on this and the actual current consumption is highly application specific, so I advise you to perform some measurements on your own before concluding.

Best regards

Bjørn