Finding a Switch

Hi,

I am trying to find a switch with below characteristics. Can you make some suggestions:

1- Current only flows from input toward output

2- Control signal is digital signal with 0V=LOW, 5V=HIGH

3- Input is analog 0V to 5V.

4- input = Output if Control = 5V.

5- Output is high impedance if Control =0V.

Questions which will help determine most suitable options:

• How much current are we talking about?
• How much current can logic control sink or source?
• How much voltage drop can you tolerate across the switch?

This is for a low power device. The current flow is max 200mA. The control logic is 5V from USB-C so it can provide reasonable currents. The voltage drop across the switch can not be above a few 10’s mV.

OK, those analog switches might be out, as the lowest impedance is in the 200 milliOhm range.

One option would be a small solid state relay (SSR) such as one of these:

TLP3553A(F 50mOhm impedance

TLP3107A(TP,F 40mOhm impedance

You can also use something like standard small signal relays.

I was going to also suggest reed relays, but their contact resistance seems to be a bit on the high side.

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Thanks.
In case lets say I decide to use FSA1156L6X.
Does it isolate output from Input if Control signal is 0V?

Thanks.
How much is a typical leakage current in output when input is turned off?

Regarding the FSA1156L6X, there is a very high impedance between input and output data ports (on the order of 175MOhms with the stated +/-20nA leakage, if I calculate correctly) when off, but it is not galvanic isolation.

Again, keep in mind that this part has an on impedance in the neighborhood of 0.75 Ohms, typically, which means that 150mV will drop across it with 200mA of current.

To which part are you referring here?

Yeah, when VCC=5.5V the isolation is good and leakage is low, but what happens when VCC=0. That data does not seem to exist in the datasheet for FSA1156L6X.

For example the leakage current between output pins for TLP3107A when inputs are grounded.

For the TLP3107A, leakage current with maximum voltage differential between input and output (60V) at 25°C is about 15nA. With lower voltage differential, it would be much lower, but the leakage increased exponentially with temperature increase.

From page 5 of datasheet:

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Correct, I do not see such a specification either. However, it seems highly likely that the pass element is a MOSFET, though I cannot find a definitive statement of such, and if so, I would expect that the leakage should be the same whether or not the device is off, or on with control input low.

However, it also does not definitively state that one is allowed to apply any voltage to either A or B when the device is unpowered. I suspect it would be fine, but I would not guarantee it.

Thanks David. A Review of similar MOSFET switches shows that the leakage current increases with one or two order magnitudes when VCC = 0. For example see:
1-Ohm SPDT Analog Switch 5-V/3.3-V Single-Channel 2:1 Multiplexer/Demultiplexer datasheet (Rev. A)

Hmmm, that’s interesting – 166x the leakage over full temp range when no Vcc. Based on that, there’s a good chance I was wrong about the FSA1156L6X. Another reason to lean toward the SSR solution, albeit a larger and more expensive option.

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Hi Kaveh,

would fit to your specification. Most logic gates can drive this directly (may require a reverse parallel diode to protect the output stage.)

Cheers, heke, AsamaLab

Thanks.
Is this normally open?

Yes, it is. When there are no voltages applied to the primary side of the circuit, the switch is an almost a perfect insulator at dc, no leakage. Please check against to your possible timing specification, as this device is “slow” actor…

Thanks. Timing should be ok.
Is there an SMD version smaller package of this part? Maybe half to 1/3 height.

I am quite sure there are. I think Digi-key people can recommend a good selection to choose from.
Meanwhile, would this come to close? (coil current is 20mA, a bit highish).

Cheers,
heke, AsamaLab

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