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I've got two circuit modules, let's call them master and slave.

The master controls whether the slave is powered (5V and 3V3 rails). Both modules share the same power supply. The slave will be built by somebody else, so I'm planning to use a LTC4217 hot swap controller IC to protect the master from any faults on the slave.

From reading through the LTC4217's datasheet I couldn't find a simple way of using a microcontroller to switch its output on or off. Perhaps there is an obvious example that you can think of though!

Instead, I'm trying to find an alternative method. The slave will be on the vast majority of the time, and the master will be switching it off for approx 0.25s every few minutes.

MOSFETs seem ideal, however they seem generally used to switching things on momentarily - i.e. normally off. My slave is normally on. I've heard that 'depletion mode' MOSFETs could be a potential solution but I am not knowledgeable with regards to them.

Am I overthinking this? Is there some simple normally used solution staring me in the face?

Cheers.

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  • \$\begingroup\$ I don't get why you object to using a mosfet \$\endgroup\$
    – PlasmaHH
    Commented Nov 24, 2015 at 14:19
  • \$\begingroup\$ What power are you concerned about wasting? Why aren't MOSFETs suitable? \$\endgroup\$ Commented Nov 24, 2015 at 14:35
  • \$\begingroup\$ Field-Effect Transistors (FETs), including the specific construction of Metal-Oxide Semiconductor (MOS), require no current to remain on or off. The gate is effectively one plate of a capacitor that attracts or repels charge on the other "plate", which is actually the channel. The presence or absence of this attracted/repelled charge overcomes the doping (or not) to control the effective size and therefore the resistance of the channel. Steady-state power loss when fully on is determined solely by that resistance, and is usually pretty low. Look at Rds_on in a couple of datasheets. \$\endgroup\$
    – AaronD
    Commented Nov 24, 2015 at 20:56
  • \$\begingroup\$ The reason I asked this question is because I had read through this article and thought that as the slave will be on 99.99% of the time, it would be a better solution to use something that won't constantly draw power and get hot. \$\endgroup\$
    – BenAdamson
    Commented Nov 25, 2015 at 19:00

1 Answer 1

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You can use the hot swap controller and use the UVLO (under-voltage-lock-out) input pin to turn the output off to your slave. You have to be aware the current limit setting used because this can cause your slave to power up a little bit slowly - it all depends on load capacitance and the current limit value.

BTW it uses a MOSFET.

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  • \$\begingroup\$ One caveat with the LTC4217 UV pin is that it has two thresholds -- getting the chip not to auto-retry may require sticking a 1N4148 in series with the pulldown FET in order to present correct voltage levels to the UV pin... \$\endgroup\$ Commented Nov 25, 2015 at 1:25
  • \$\begingroup\$ Thanks Andy - so perhaps it would be simplest to just stick a MOSFET in the path between the LTC4217 output and the slave VDD. \$\endgroup\$
    – BenAdamson
    Commented Nov 25, 2015 at 19:02
  • \$\begingroup\$ @BenAdamson yeah it's tempting but inside the chip they use an N channel MOS device with a voltage boost circuit so that the gate can be driven significantly above the incoming power rail level so it's not all that simple internally. However, you could use a P channel mosfet (easier to drive) but then you ought to consider current limit protection and the like. I'd still be sorely tempted to stick with the controller. Maybe shop around for a cheap little one? \$\endgroup\$
    – Andy aka
    Commented Nov 25, 2015 at 20:40

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