LM5060: Simple Hot-swap controller LM5060 or LM5069?

Hi Marek,

The short answer is the LM5060 is meant more as a high-side switch for undervoltage, overvoltage, and gradual overcurrent protection where accuracy is not a concern and short circuit protection is not needed.

The LM5069 is meant for robust protection, such as from short circuits, and in many cases can be used to replace a fuse.

We have a selection tool at www.ti.com/hotswap --> "Tools & Software" which may help with part selection. For example, if you don't need overvoltage protection, the TPS2490/91 may be suitable.

The long answer:

The LM5060 has an adjustable overcurrent threshold. It uses a comparator to see when the SENSE pin is greater than the OUT pin and then it will begin to run a timer, then shut down after that timer expires.

It relies on the Rds-on of the MOSFET. If you have a 10mohm Rds-on FET and you want to set the overcurrent threshold to 10A, then you want to set the LM5060 to shut off when there is 100mV across the MOSFET's Vds.

So the SENSE pin has a bias current of 16uA (typical). If you add a resistor in series with the SENSE pin, then this will produce a voltage drop and offset the signal the LM5060 see's.

Since the LM5060 uses a comparator and triggers when SENSE > OUT, you want this offset to shift the 100mV (the voltage across the FET @ overcurrent threshold) down 100mV to 0mV.

So you would pick resistor of 100mV / 16uA = 6.25kohm.

This is the derivation of the equations 1 & 2 in the datasheet:
www.ti.com/.../lm5060-q1.pdf

This may be a clever technique and work for some applications, but it is important to understand its limitations.

The LM5060 does not have current limiting. It only has a threshold, after which it will run a fault timer and eventually shut off. But during the fault time, there is nothing to regulate or limit the current. This means there is no short circuit protection. If you turned on with a short circuit present at the output, nothing will prevent the MOSFET from blowing up.

Additionally, because the accuracy depends on the SENSE bias current and the MOSFET Rds-on, there is quite wide variation in the overcurrent threshold. Most MOSFET manufacturers will include a typical and maximum Rds-on spec, as well as curves to indicate the variation over temperature. But there is often no minimum Rds-on spec, which means the maximum overcurrent threshold may not be well defined.

The LM5069 on the otherhand uses an external sense resistor. Using an internal amplifier, it can very accurately set an overcurrent regulation point. This means it can provide true current limiting (if the load tries to draw more current than the set overcurrent protection, OCP point, then the LM5069 will regulate the MOSFET by reducing its GATE voltage until the control loop is stable and current is reduced to the OCP point). In extreme cases like a short circuit, the LM5069 has a fast comparator to quickly shut off if the OCP regulation loop cannot limit the current fast enough.

Finally, the LM5069 offers MOSFET power limiting. This means that the OCP / current limit regulation point will be reduced when there is a large voltage difference, Vds, across the MOSFET. This helps survive the most extreme fault events, such as trying to power on the MOSFET when there is a short circuit at the output.

Thanks,
Alex

Edit: To address your question about short pins, a device like the LM5069 does not need it as it has a built in "insertion delay". It waits for a period of time before turning on the MOSFET. This is to allow disturbances during plug in to dissipate before turning on the MOSFET.

Finally, depending on your voltage and current range, you may be interested in devices like the TPS26600 which can go up to 55V, 2A and uses an integrated MOSFET and current sense element. At TI we call these integrated solutions, eFuse (electronic Fuse). Integrated solutions are compelling because of the simplicity and protection they can offer. Because the MOSFET is integrated, there will often be an internal temperature sensor which can shut off the FET if it gets too hot and prevent it from breaking. This is the somewhat "dummy proof" method. An LM5069 can offer similar level of protection without a temperature sensor, but you need to pay much more attention to component selection.

If you go the route for LM5069, we recently updated the datasheet to include a detailed, step-by-step design example. We have calculation tools available on the product folder or at www.ti.com/hotswap --> "Tools & Software" to help with component selection.

If you have any questions about how to use our design tools, we have video tutorials available at www.ti.com/hotswap --> "Support & Training".

And finally, we are always here on the E2E forum to help.

Thanks,

Alex