TPS22918: Usage of load switch output when the external input is applied

Sathyasai Prasad

Part Number: TPS22918
Other Parts Discussed in Thread: TPS22917

Hi,

I am using TPS22918 load switch with 5V as input voltage and ON control is given to external 3:1 Header. In my design the output of load switch is connected from external 5V also as the bypass option and load switch will be disabled in this condition. Is there any chances that if the external 5V is used that will cause reverse leakage current through internal FET or it will be completely isolated from input since the ON control is OFF condition.

over 7 years ago

0 Aleksandras_Kaknevicius over 7 years ago

TI__Genius 15780 points

Hi Sathyasai,

The TPS22918 does not have any reverse current blocking, so if a voltage is applied on VOUT without a voltage on VIN, then reverse current will flow across the body diode of the FET (from output to input). This will be the case even if the device is disabled. The TPS22917 is a very similar load switch that has reverse current blocking integrated, so this device may work better for your application.

Thanks,

Alek Kaknevicius

0 Sathyasai Prasad over 7 years ago in reply to Aleksandras_Kaknevicius

Prodigy 180 points

Hi Alek,

Thanks for suggesting new IC and it is matching to my requirements. Please let me know the OFF resistance(Roff) for this device and why this parameter is not mentioned in the datasheet?

Thanks,
Sathyasai

0 Aleksandras_Kaknevicius over 7 years ago in reply to Sathyasai Prasad

TI__Genius 15780 points

Hi Sathyasai,

I'm not sure what you mean by OFF resistance here, the device has the same characteristics of a typical NMOS. When it is turned off, from input to output you will see a very, very large resistance preventing power from being transferred across the switch. From output to input, there is very little resistance because of the body diode. The forward voltage for the diode is about 0.7V, but we do not characterize its performance since this is not the intended use case for the device.

Thanks,

Alek Kaknevicius

0 Sathyasai Prasad over 7 years ago in reply to Aleksandras_Kaknevicius

Prodigy 180 points

Hi Alek,

What is the resistance across the switch when the device is turned OFF. Since you mentioned very large resistance and just wanted to know resistance value.

Regards,
Sathyasai

0 Aleksandras_Kaknevicius over 7 years ago in reply to Sathyasai Prasad

TI__Genius 15780 points

Hi Sathyasai,

Just to give you an idea of the magnitude of the resistance, I grabbed a TPS22918 EVM and applied 3.3V to VIN and measured the resistance from VIN to VOUT. The reading I got was about 7.8MΩ. Let me know if this helps.

Thanks,

Alek Kaknevicius

0 Sathyasai Prasad over 7 years ago in reply to Aleksandras_Kaknevicius

Prodigy 180 points

Hi Alek,

Thanks for providing the reading which will help us.

In TPS22917 datasheet, there is RCB Activation Current(IRCB) mentioned as 0.5A(typical) to 1A(Max) means reverse current blockage will happen only if current is more than 0.5A otherwise reverse current will flow from output to input. Our front end circuitry will be damaged if the reverse current flows from output to input. Please provide any suggestion for blocking initial current also.

0 Aleksandras_Kaknevicius over 7 years ago in reply to Sathyasai Prasad

TI__Genius 15780 points

Hi Sathyasai,

The only way around this current is to turn the device off before the reverse current condition. When on, the FET needs this current to measure the voltage drop from VOUT to VIN.

Thanks,

Alek Kaknevicius

0 Sathyasai Prasad over 7 years ago in reply to Aleksandras_Kaknevicius

Prodigy 180 points

Hi Alek,

Thanks for suggestion and I will use the same technique.
If the load switch VIN is 5V , Do I need to use the same voltage on ON pin also although it has range from 0 to 5.5V?

Thanks,
Sathyasai

0 Aleksandras_Kaknevicius over 7 years ago in reply to Sathyasai Prasad

TI__Genius 15780 points

Hi Sathyasai,

The ON pin voltage can be separate from the VIN pin voltage. Even though the VIN voltage is 5V, the ON pin voltage only needs to be above VIH for the device to be on.

Thanks,

Alek Kaknevicius

0 Sathyasai Prasad over 7 years ago in reply to Aleksandras_Kaknevicius

Prodigy 180 points

Thanks Alek,

What precaution need to be taken for QOD pin if the external 5V is applied on the load switch output when IC is in disabled state. If the external voltage is applied then it should not discharge through QOD pin.

Thanks,
Sathyasai

0 Aleksandras_Kaknevicius over 7 years ago in reply to Sathyasai Prasad

TI__Genius 15780 points

Hi Sathyasai,

My recommendation in this case would be to leave the QOD pin floating (not connected to VOUT). When the device is disabled, this pin will pull low, so to avoid unwanted power dissipation when VOUT has a voltage source, I would not connect it to VOUT.

Thanks,

Alek Kaknevicius

0 Sathyasai Prasad over 7 years ago in reply to Aleksandras_Kaknevicius

Prodigy 180 points

Thanks a lot Alek for all suggestions..

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TPS22918: Usage of load switch output when the external input is applied