Driving loads from 2.3Volt uC

A LOW-Side N-Channel FET?

Put the FET between the device and GND, then drive its gate by the MSP. That's the easiest setup. It won't draw any power at all except when changing the current state.
If you want to put the 'driver' between 5V and the device, then things get a bit more complicated. Especially if you want to stay low-power.

Jens-Michael,

Would you recommend any products?

Jens-Michael Gross said:

A LOW-Side N-Channel FET?

Simplest solution indeed. However you shall be extremely careful when you float ground of logic circuits. I would recommend "low-side power switch" only for passive parts/loads. For logic it is advised to implement "high side" power switch which can be made of P-FET as power switch and (tiny) low GS_on voltage N-FET + pull-up resistor as driver for P-FET gate. Yes, current will flow through pull--up resistor while P-FET is on, but you can make it like 47KOhms to minimize quiescent current of the "driver". What's 5V current you want to switch on/off?

My ultimate goal is to drive 12Volt 2Amp inductive load.  To do that, I am using NCV8402 but it still needs 5 volts to be turned on, so I am looking for intermediary device which needs to source just a few mA.

ON the other hand, would there be a TI product that could drive 12Volt 2Amp with 2.3 Volt MSP430 Micro?

Silver Diamond said:

ON the other hand, would there be a TI product that could drive 12Volt 2Amp with 2.3 Volt MSP430 Micro?

Well.. do you want me to specify "manufactured by TI" resistors too? (kidding)

NCV8402 gives bare 2A at 3V VGS (gate-source voltage). This is clearly not enough for your spec. You need just powerful enough N-mosfet with lower VGS ratings, like PMN25UN. Please check SD diode specs of particular mosfet to meet your inductive load.

This possibly will be helpful for you.

Peter, OMG that is awesome, thank you.  I did not even think about looking into this direction but this is exactly what I have been looking for...

Thanks...

Ilmars said:

Simplest solution indeed. However you shall be extremely careful when you float ground of logic circuits.

I agree that two FETs with a resistor are a better, but also larger and more expensive solution.

Silver Diamond said:

To do that, I am using NCV8402 but it still needs 5 volts to be turned on, so I am looking for intermediary device which needs to source just a few mA.

You can use an N-FET with pull-up resistor at the gate of the NCV. THis again is a simple solution with a drawback: the NCV will be active if the N-Fet is off. It is an active-off constellation. When the MSP drives the N-Fet, it will short the pul-lup to GND, which will shut off the NCV instantly. When teh MSP stops driving the N-Fet, it will no longer short the pull-up and the NCV begins conducting. The size of the pull-up determines how steep the NCV will raise the current, but at the same time it determines the current that flows while the NCV is shut off.

To invert this (current only flowing when the NCV is on), you'll need the additional P-Fet, proposed by Ilmars, and (important) a pull-down on the NCV gate. Since shutting-off th eP-fet won't stop the NCV as it has a high-impedance input and an input capacitance that will keep it on for some time even if the gate voltage is removed. It needs to be actively discharged to GND to shut down instantly.

Jens-Michael, in this case, this is not cost sensitive project, but it needs to be low power and robust.  For this reason, I am not using pull up or down resistors, because it seem to me that they would dissipate power.  Is that true?  So the SN74LVC1T45 seems to be a good choice.  What bothers me is that it seems to be optimized for high speeds but I need low power.  I wish I can find an FET that would work for me but I just do not know enough about specifying FET's.  

Silver Diamond said:

For this reason, I am not using pull up or down resistors, because it seem to me that they would dissipate power.  Is that true?

Well, if your device indeed draws 2A when active, I guess some additional active current won't make a difference. It's taken from the high-voltage supply, not from the low-voltage MSP supply.

TheH SN74LVC is indeed not optimized for low power. Its internal logic draws quite some current for the level conversion, even if the output is static and doesn't draw any current. I guess with pullups or  pulldowns in the range of 47k you'd waste less current here. Especially if they are only active while the device is active.

Hey Jens-Michael Gross, I see that u have interfaced MSP430 wih a laser before. I have a similar problem, I am trying to interface MSP430 launchpad with high powered 405nm 150mW laser. The driver circuit needs somewhere around 6-12V. Can you suggest me a solution with connections. It would be a great help.

Thank you,

Chhavi 

It's not a simple circuit. Actually, the modulation was plain analog. The MSP was used as current supervisor, temperature regulator (driving teh perltier elements for temperature control with PWM) and for programming/calibrating the digitally programmable potis in the analog path. including fail-safe circuit and a fast bypass to protect the diode.

BTW, 150mW isn't really high-powered. We had an 8W green laser, pumped with ~90W infrared laser diode. The circuit was modulating 30A with 30kHz. So it i most likely overkill. Sales price for the power supply was in the range of $1000. And besides this, the schematics are company property, of course.

However, if you give me some more details about what you want to do and how, i can mabe make some suggetions. But 'interface' is a bit vague.

my circuit is to turn on 150mW laser diode using msp430. The diode already has a driver circuit, which controls the current flowing through it. 
i am very new to this msp programming. driver ckt needs an input voltage of 6V-18V but MSP pin cannot source that much voltage so i was thinking if i can put some transistor to do the trick ? 

the whole thing is to have a transmitter and a receiver. the transmitter sends signal through rf module. the rf module at receiver receives it and sends the signal to msp, and msp has to turn on the laser diode, which is sufficiently high powered for the application. :D

Best that I can find.

http://ec.irf.com/v6/en/US/adirect/ir;jsessionid=9C126CBBE1AAEE66E9D136FB9A826796?cmd=catProductDetailFrame&productID=IRLR6225PBF

Be sure to pull it to ground with 47.5k to 100k resistor on the gate.  Works well when driven directly from the MSP430 uC.

Chhavi Gandhi said:

driver ckt needs an input voltage of 6V-18V but MSP pin cannot source that much voltage

If you want to have th elaser off by default and only on if the MSP is actively driving the transistor, things get complicated as the transistor needs to be high-side and therefore its control voltages for both, on and off, are above the MSP supply. So you'll need a second transistor.

MSP->puldown and N-Channel Fet -> pullup and P-channel FET -> Driver

yes i need the second case here. The laser by default stays off and only when msp sets that output port to 1, the laser should turn on and stay on. 

so i guess i need an n-channel and a p-channel FET. 

Do you think this will work? http://www.google.com/imgres?imgurl=http://rayshobby.net/blog/wp-content/uploads/2011/10/remote_switch_interface_schematic.png&imgrefurl=http://rayshobby.net/?p%3D936&h=266&w=620&sz=5&tbnid=QdDrCgZHBnmr5M:&tbnh=54&tbnw=126&zoom=1&usg=__yhPKgAxCSgHaYhu9qpeCMe7n2IQ=&docid=MdcXHsPJ5l4CXM&sa=X&ei=cLPNUdGmJIfS9QTgkIDIBg&ved=0CDUQ9QEwAg&dur=7003

Thank you Jens ! Really it means a lot to me !

Chhavi Gandhi said:

Do you think this will work?

thank you Jens :D 
You helped me a lot !

Hello there,

I am also interested in high-side power switiching for the sake of going low-power ...

I have been having troubles switching off, a flash memory (Spansion S25FL512S) and a GPS Chip (Gms-u6b)

The MSP430FR5739 I am using crashes when I toggle the pin connected to the P-Mos Gate (DMP3098)

I thought I might be linked to the problem you mentionned with non passive component ?

Should I go for a N-MOS with pull-up / P-MOS high side switch ?

Any suggestions ?

Thanks

Fred

Fredoo.net said:

Should I go for a N-MOS with pull-up / P-MOS high side switch ?

If voltage you want to control is higher than msp430 VCC - then yes. If supply voltages are equal, like 3.3VDC then you will be fine with single P-MOS, it's suggested to add some 330ohm series resistor between driving pin and gate.

Great ! The series resistor is for what purpose ? the current is limited anyway by the maximum drain current no ? 

could it be that it is because I have no series resistor the current gets too high and therefore the MCU shuts down ?

Yes, series resistor is current-limiter. Gate acts like (is) capacitor which is charged during turn on and discharged when turned off. So at the very beginning of turn-on msp430 pin is effectively shorted to VCC in case of P-MOS, huge upto 40mA current spike happens and if you don't have strong supply and good decoupling capacitors around msp430 power pins, then things can go very bad, as you already noticed:

Fredoo.net said:

could it be that it is because I have no series resistor the current gets too high and therefore the MCU shuts down ?

Yes, it could be so. Always add series resistor, 10mA initial current is good choice (330 ohm for 3.3VDC VCC). For big, powerful mosftets having high gate capacitance it is advised to use gate drivers.

Ilmars said:

Yes, series resistor is current-limiter.