LPW5210B5 Power Switch Issue

[MotoDan]

Has anyone used the LPW5210 power switch? I recently inherited a design which uses this device to power/protect a USB port. The issues I'm having are 1) there's a huge voltage drop at the device's output - approx 0.8V or more depending on the load. Needless to say the USB device is not happy with such a low voltage and 2) the chip gets hot when drawing much over 0.5A. The part claims to have a fairly low Rds of 160 mOhm and is rated at 1.5A. I will be trying another board to see if the problem still exists before calling the chip bad. Just wanted to see if this is a known good solution before looking for a replacement.

Any help is appreciated.

[aliarifat794]

 The LPW5210 getting hot when drawing over 0.5A suggests that it might be operating close to its thermal limits.If possible, provide better heat sinking or airflow around the LPW5210.

[HwAoRrDk]

What is the value of the R_ISET resistor on pin 3? The value of that resistor dictates the current limit. It may be that the wrong value of resistor has been fitted, and the current limit the chip is trying to enforce is lower than you think it should be. For a 1.5A current limit the resistor should be about 4.5k.

The way these chips tend to work is that they will limit the current by increasing the on-resistance of the pass transistor to throttle current, which will dissipate power and increase temperature, and then rely on a thermal cutoff to shut off the load. So there will naturally be some zone between the current limit threshold and complete shut-off where the device will get really hot, and I could imagine there might some pathological scenario where the load being applied is just enough to hit the limiting but not get it hot enough to shut off. Or it's just faulty...

If you want a reliable USB load switch, I can recommend Richtek devices. They have a pin-compatible 1.5A adjustable limit device, the RT9722. I have tested and abused their 3A-rated RT9742 and it performed flawlessly. Regardless though, if you can, I would recommend using a device that is over-specified for the current load you want (e.g. 3A-rated device for 1.5A), as these SOT-23 package devices are rather marginal in the power they can dissipate at loads at or approaching their rated maximums. Look at the R_DS(on) values, not the maximum current rating.

[MotoDan]

Rset is currently 4.53K which sets the current limit to 1.5A - the maximum of the device. I agree that 1.5A is probably not a good choice for this device due to heat dissipation, etc.

Thanks for the info on the RT9742x device. While not pin compatible, the 3A part should be able to operate at 1.5A without overheating. The Rds is about half of the LPW5210 we currently are trying to use.

[thm_w]

It is slightly concerning that the RdsOn in the datasheet is only given as typical, 0.16R. But that should be way way less than what you are measuring.
Definitely check a few more devices and see if the voltage drop is consistent.
Measure the actual value of Rset on the board.
Scope the input/output/enable pins.

Looks like they have quite a few variants
https://www.lcsc.com/product-detail/Battery-Management-ICs_LOWPOWER-LPW5210B5F_C2935097.html
https://www.lcsc.com/product-detail/Power-Distribution-Switches_LOWPOWER-LPW5209AB5F_C387727.html
https://www.lcsc.com/product-detail/Power-Distribution-Switches_LOWPOWER-LPW5208B5F_C517265.html
https://www.lcsc.com/product-detail/Power-Distribution-Switches_LOWPOWER-LPW5213B6F_C517134.html

[HwAoRrDk]

Thanks for the info on the RT9742x device. While not pin compatible, the 3A part should be able to operate at 1.5A without overheating. The Rds is about half of the LPW5210 we currently are trying to use.

Well, it's sort of pin-compatible if you use the non-M/V/U pin-out and active-high EN versions - the FLG pin is in the same place as RSET on the LPW5210, so if you don't need the FLG functionality, just leave the RSET resistor off the board! Or even leave it - FLG is an open-drain output.

Referring back to my notes on testing the RT9742, I got 88mV drop across the IC at 1.5A, which corresponded approximately with the datasheet figures for R_DS(on) (58mΩ vs. 55mΩ). It would take at least 1A over the current limit to trip within 1 second; with only 0.5A over it would take >5 seconds to trip. So the current limit shouldn't be expected to be very exact.