Why am I seeing a small output voltage when load switch is off? - Page 1
Forum section: electronics / projects

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 21, 2025, 09:08:40 pm
content: I've previously posted about how I came up with a circuit using a high-side load switch's RDS(on)as the shunt resistance for a current-sense amplifier to measure current draw of the switched load. Below is the circuit and components I settled on.(Before anyone says: yes, I know such current-sensing isn't accurate due to variations, temp. coefficients, blah, blah... I'm only doingrelativecurrent measurements, between an active and idle state, so I don't care about absolute values or accuracy.)I got this all assembled on a board and it works perfectly for my intended purpose. However, there's one thing I've observed that puzzles me: when I measure voltage of the output load terminals while the load switch is in the 'off' state (EN is low), I measure approx. 0.76V. I'm not sure where this voltage is coming from, and I'm surprised it's present, especially given that the TPS2051C has an output discharge feature, which means the load side should be pulled down to ground via ~470Ω resistance whenever 'off'.It seems suspicious that the voltage is roughly the same as a diode drop, so perhaps something to do with the D2 ESD diodes? Leakage current? But if If I put a small resistance across the load output, 1K, it drops to ~0.5V, and with 100Ω 0.18V. This tells me it's not a super high-impedance voltage source like you might get with leakage current, right?The other confounding aspect is that I also assembled another board with a different high-side load switch, a Silergy SY6288A, which to all intents and purposes performs the same function as the TPS2051C, and I don't see the same off-state voltage - I only see <0.02V on my meter. However, one difference, according to the datasheet, is that the SY6288A has a 10Ω output-discharge resistance (versus 470Ω for the TPS2051C). I guess that probably explains why the same voltage level isn't seen, but the question of where this voltage comes from in the first place still stands.

author: thm_w
author_rank: Super Contributor
timestamp: February 21, 2025, 10:00:50 pm
content: So about 1.5mA leakage current, seems like a lot.Did you remove R9 and see if the leakage stays or goes away?High side load switches are available with current sense built in, but I think your solution is much cheaper, since its using common parts.

author: Konkedout
author_rank: Frequent Contributor
timestamp: February 22, 2025, 03:38:48 am
content: I am looking at the INA180 datasheet. The only thing...I wonder whether you get significant current between the input pins when the differential input voltage is abnormally high. I think that is the case when your load switch is off.Keep in mind that the INA180 is intended to sense voltage across a low value current sense resistor such as maybe 100 mOhms. But if I understand correctly, you are asking about a situation when you have a differential voltage of ~5V across those INA180 inputs.I cannot tell that TI specifies the differential input current under those conditions.Given TI's expectation that you are using a sense resistor, there is no reason for a few mA of current to be a concern under your conditions. These conditions do not represent a normal measurement of current through a low value resistor.Does this make sense?

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 22, 2025, 05:03:25 pm
content: Quote from: thm_w on February 21, 2025, 10:00:50 pmI removed R9, and there was zero volts on the output. I then replaced R9, and removed D2; the 0.76V on the output returned.So, it is nothing to do with the ESD diodes, and seems to be caused by the current-sense amp.Quote from: Konkedout on February 22, 2025, 03:38:48 amSo this relatively high voltage differential is causing a current to flow out of the VM pin? (Presumably in through the VP, or maybe VCC, although which exactly is a moot point.) And so through the discharge resistance of the load switch is where the 0.7V manifests. That makes sense, and I didn't realise until now that the two different output voltages with the two different load switches do indeed roughly correspond when taken in context of the current and resistance the voltage was measured across - TPS2051C with 470Ω discharge resistance, 0.76V implies 1.6 mA leakage current (0.76/470); SY6288A with 10Ω discharge resistance, 0.02V implies 2 mA leakage current (0.02/10).Quote from: Konkedout on February 22, 2025, 03:38:48 amBasically, you're saying that this leakage current is a natural consequence and nothing to be concerned with? I'm not really concerned about it from a functional point of view - 0.7V isn't anywhere near enough to drive the connected load. But I'm wondering whether there is any concern about out-of-spec operation of the INA180. TI don't list any maximum current in/out of the VP/VM pins (only OUT pin). But I assume they expect such current to be okay, seeing as elsewhere in the datasheet they talk about how when the output is driven to the supply rail due to excessive differential input voltage, no damage will occur so long as maximum ratings are not exceeded. And the max. differential input voltage is rated as ±26V, which I imagine would result in far more than a couple of mA of current in/out the inputs!

author: thm_w
author_rank: Super Contributor
timestamp: February 24, 2025, 11:11:24 pm
content: I think you are right they specify -26 to +26V differential, so its ok.And 5V is present, so even a normal opamp could likely handle this condition. If no supply voltage were present INA180 is better protected than a regular opamp.Rint for INA180A3 is 5kOhm, so 10kOhm from Vin+ to Vin-Must be some other circuitry we are missing, they do show "Bias" circuitry in one of the diagrams, but not a full schematic of the IC anywhere.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 25, 2025, 02:55:08 am
content: Have you tried removing R8?

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 25, 2025, 08:22:16 pm
content: I get zero volts with R8 removed and R9 still present. I guess that proves the current is going in through VP (not VCC) and out of VM.Edit:Hmm, an interesting observation: after replacing R8, the measured off-state output voltage is now 0.62V. It can't be a temperature thing, as it's been at least 5 minutes since soldering, but I did clean the board a little more thoroughly...

author: Sassy Taste
author_rank: Contributor
timestamp: February 27, 2025, 09:35:40 am
content: Electrical ground carries electronics too from the negative supply voltage. It conducts electricity as a conductor. Storage to the switch is from that source. Probably a capacitor near by.