NVT4858 VCCB Power Selection

[asdf103]

Hi !

I am referring to the user manual of NVT4858-4557-EVB evaluation board, UM11582.

Looking at Figure 5, two types of Load switch(U3 NX3P1108 and U6 TPS22913BYZVT) are used to switch VCCB between 3.3V and 1.8v.

What is the reason for using two different  load switch IC?

I think it's okay to use the same IC for these two.

If there is a circuit problem with using the same load sw, could you please tell me?

(For example, the VCCB voltage drops because the IC switching timings are the same, etc...)

[asdf103]

Hi Jozef-san.

Thank you for your reply.

Do you think it's okay to use the same IC for two load switches?

For example, it means that U3 uses NX3P1108 and U6 also uses NX3P1108.

I am wondering why different ICs are used for U3 and U6 in the EVB.

I would like to use the same load switch for U3 and U6 unless there is a problem.

Best Regards

[JozefKozon]

Hi Yamamoto-san,

you can use the same switch for both paths, there is no problem. The use of the different types was probably availability of either of the products. 

With Best Regards,

Jozef

[asdf103]

Hi Jozef-san.

I guess TPS22913BYZVT is used for U6 for reverse current protection.

When 3.3V is selected as VCCB,  the input voltage of U6 is 1.8V  and the output voltage of U6 is 3.3V.

TPS22913B specifications are as follows.

• Vo max voltage is 6V(Absolute Maximum Ratings) 
• Full-Time Reverse Current Protection

on the other hand, NX3P1108 specifications are as follows.

• Vsw max voltage is Vi (Absolute Maximum Ratings) 

If  NX3P1108 is used for U6, isn't the current flowing back through the parasitic diode?

Best regards,

[JozefKozon · Accepted solution]

Hi Yamamoto-san,

yes, you are absolutely right. I missed that. I apologize for my inconsistency. The NX3P1108 does not contain the reverse current protection and if the voltage on the NX3P1108 VOUT is higher, than VIN+0.5V, the a leakage current would flow form VOUT to VIN. Maximally 50mA, which is significantly higher comparing to the TPS22913BYZVT 10uA, with reverse current protection.

With Best Regards,

Jozef

[asdf103]

HI Jozef-san,

Thank you for your confirmation.

Now I can design with confidence.

Best regards,

Yamamoto

[JozefKozon]

Hi Yamamoto-san,

I believe the reason for two load switches is, that you can easily switch between the 3.3V and 1.8V source. You can have both voltage sources connected to the the VIN pins and control with of them is connected to the VCCB pin by logic level on the pin 4 of the J7 connector. By connecting high logic level, the NX3P1108 will be enabled and the TPS22913B will be disabled and vise versa, by connecting low logic level to the pin 4 of the J7 connector, the NX3P1108 will be disabled and the TPS22913B will be enabled.

Please refer to 74AUP1G04GW datasheet for the logic levels. 

You can also constantly keep the same logic level on the pin 4 of the J7 connector and change the voltage applied to one of the load switches. 

With Best Regards,

Jozef