Switching on/off L80-R with a transistor

I am using L80-R as a GPS module. There are two different supply pin for L80-R VCC and V_BCKP. For using the module in BACK-UP(7uA current consumption) mode I have to cut off power for VCC and supply V_BCKP. I have just one battery so V_BCKP will be connected to battery all the time but I am thinking about using a npn transistor as a switch to cut off VCC supply. I am not sure how it will effect the IC. Because I think i should place a resistor to collector. You may see my schematic in attachments. Is it the best way to make it? Could relay be a better option? Waiting for your valuable comments.

NPN transistors are usually not suitable for a high side switch, as the base voltage has be to about 0.7V higher than the output voltage. The 1K collector resistor R6 will cause a severe voltage drop and must be removed, or the module won't work at all.

Why are no voltage levels printed on the schematic?

I suggest to buy a high side switch from Pololu, or research better designs. Most use PNP bipolar or P-channel MOSFET transistors.

I suspect the vast majority of the forum would not have read the post, it only had 36 views, because few would recognise the actual subject from the post title;

'Switching on/off L80-R with a transistor'

Might have been an idea to mention in the title that the L80 is a GPS ?

Anyway as mentioned your circuit wont work, its normal to use MOSFETS for this purpose. Choose your GPS wisely and you dont need to use a switch anyway, some GPSs can be shut down quite well with software.

srnet:
I suspect the vast majority of the forum would not have read the post, it only had 36 views, because few would recognise the actual subject from the post title;

'Switching on/off L80-R with a transistor'

Might have been an idea to mention in the title that the L80 is a GPS ?

Anyway as mentioned your circuit wont work, its normal to use MOSFETS for this purpose. Choose your GPS wisely and you dont need to use a switch anyway, some GPSs can be shut down quite well with software.

Thanks a lot for your suggestion about topic title. I will be careful next time. The reasons picking L80-R are price ,compact size with built-in antenna and low power consumption. Also it is easier to find the developer board in my country. However if you have any other suggestion I will consider for sure.

No problems withe the L80, around $5, easy to solder and low power.

There is a equivalent Ublox GPS the SAM M8Q which is lower power, but around 4 times the price.

jremington:
Most use PNP bipolar or P-channel MOSFET transistors.

I am thinking about using this option and I find the design in attachment. However I am not sure which value to use as a R Load. Current consumption is quite important for my project. What will be the best solution?

Screen Shot 2019-02-20 at 18.41.08.png

The circuit posted is not a functional design. Here is a practical design for a high side switch using Arduino. Google for many others.

Replace the motor with your GPS module, and be sure to use a logic level MOSFET.

You dont need a resistor load, its clearly only shown as an example.

Appreciate that there is quite a lot involved in making a low power GPS tracker.

I just finished a write up of a breakout board for a Quectel L70\L76, it has the circuit I would normally use for swithing a GPS ON\OFF;

Quectel L70\L76 Breakout

And if you want some tips on low power operation of trackers you can read this;

GPS Tracker Performance

Happy reading.

srnet:
You dont need a resistor load, its clearly only shown as an example.

Appreciate that there is quite a lot involved in making a low power GPS tracker.

I just finished a write up of a breakout board for a Quectel L70\L76, it has the circuit I would normally use for swithing a GPS ON\OFF;

Quectel L70\L76 Breakout

And if you want some tips on low power operation of trackers you can read this;

GPS Tracker Performance

Happy reading.

Thanks a lot for your help I am thinking of using the circuit you have made(attachment). However I have some questions about current consumption when GPSPOWER is low. VCCIN will be 3.7V and GPSPOWER is a digital output of microcontroller. It might be wrong but it looks there will be 70uA current going from VCCIN to GPSPOWER. So Should I increase R3 and R4 to decrease the current?
Also I read the GPS Back-up part of the tutorial and you have mention about using external battery for supplying VBCKP and it’s disadvantages. I have seen another application circuits also in data sheet they suggest using external battery for VBCKP. What I am trying to do is supplying VCC and VBCKP from same battery but VCC will be controllable to make the module Back up mode. I mean it makes more sense than using external battery right? or Am I doing some logical mistake?

Screen Shot 2019-02-21 at 12.08.53.png

caneradiyaman6:
It might be wrong but it looks there will be 70uA current going from VCCIN to GPSPOWER. So Should I increase R3 and R4 to decrease the current?

By all means reduce the value, but with the GPS consuming between 20mA and 50mA (depending on type) why be concerned with the 70uA ?

If you adjust the values, be sure to check the VCC to the processor for glitches when the GPS is switched on, too large a glitch can trigger a brownout of the processor.

As long as VBCKP meets the voltge requirements given in the GPS datasheet, and can supply approx 10uA, it does not matter much where it comes from, as long as its clean. A logic pin will do for instance.

srnet:
By all means reduce the value, but with the GPS consuming between 20mA and 50mA (depending on type) why be concerned with the 70uA ?

I am planning to power on the GPS in every 10min. So 70uA will be consumed when it is off. I was planning to spend 7uA(back up mode) during that time.

srnet:
If you adjust the values, be sure to check the VCC to the processor for glitches when the GPS is switched on, too large a glitch can trigger a brownout of the processor.

How changing resistor values effect VCC glitches? I have concerns about these glitches but I hope decoupling capacitors will be sufficient.

Your missunderstanding the circuit.

With the logic pin controlling the gate high for GPS off, very little current flows as both ends of the resistor are at much the same voltage.

The combination of the resistors and the capacitors slows down the turn on of the GPS power, reducing VCC side glitches.

Why dont you try it and measure stuff to see what happens ?

srnet:
Your missunderstanding the circuit.

With the logic pin controlling the gate high for GPS off, very little current flows as both ends of the resistor are at much the same voltage.

I am sorry for that you are right so now I have no concern about 70uA

srnet:
The combination of the resistors and the capacitors slows down the turn on of the GPS power, reducing VCC side glitches.

I don't need to change the values anymore if the circuit works fine work L70

srnet:
Why dont you try it and measure stuff to see what happens ?

I do not have the components right now. I am just trying to draw the schematic and layout.

Whilst you may not have the parts at the moment, you would learn a great deal more by trying stuff out for yourself.

Designing a very low current tracker, that survives a long time on batteries, is not so easy. In partiucular you need a very good understanding of what happens in a circuit (to the sleep current) when you start switching bits off the circuit on and off.

Reopened at the request of a member

Hi all, (thank you for reopening topic)

Is the circuit attached in reply #8 of this topic (designed by srnet, thanks for sharing) still optimal? Have there been any revisions to improve stability?

The circuit uses the IRLML6402, I am looking to order some but wanted to confirm this is the correct part/part number: IRLML6402TRPBF
Datasheet: https://www.infineon.com/dgdl/irlml6402pbf.pdf?fileId=5546d462533600a401535668d5c2263c

Forgive me if this is a stupid question, mosfets are new to me.

Many thanks for any advice.

itmoto:
Is the circuit attached in reply #8 of this topic (designed by srnet, thanks for sharing) still optimal? Have there been any revisions to improve stability?

Please explain what problems you have had with that setup ?

I have no problems with the circuit as I haven’t actually built it yet or got parts, I was seeing whether the circuit/components had been tweaked since this post?

I am in the process of purchasing parts and wanted to confirm this was the correct part number for the MOSFET and that the capacitors and resistors were still correct value or whether I needed to buy other values.

The circuit has not changed and you post did imply that it needed to be 'improved'.

As far as I know Infeon now only make one version of the IRLML6402.

Buy some and try them, they are around 2p on Alliexpress.

Apologies if that is how it sounded, that wasn’t my intention. I was just trying to find out if there had been any changes made since you designed it so I didn’t buy the wrong parts.

Good to hear circuit hasn’t changed.

Have now ordered parts on your schematic from eBay, more expensive I know but would rather get the parts quickly.

Maybe in touch to order one of your little pcb’s srnet, thanks.