NVT2008PW,118 Level Issue

[manishbuttan]

Hello, 

We are using the NVT2008W,118 level translator processor to interface with a Quectel BG95 LTE cellular module. The module communicates at 1.8V and our application processor communicates at 3.3v. I have attached the circuit of the module. The ref on the module side is supposed to be 1.8V and the other side is supposed to be 3.3V. When the EN pin is disabled, the voltages are 1.8 and 3.3. When the EN is enabled, the voltage goes to 2.4V on the input and the output side. Why is this happening?

Thanks for your help.

Regards,

Manish

[manishbuttan]

Thank you for your reply Josef. I am not sure I understand about the  'The Ax pins are referenced from the VREFA pin. You need to bring 1.8V also to the VREFA pin'. My VREFA pin is already at 1.8V. Can you please explain further what is wrong in my circuit?

[JozefKozon]

Hello Manish,

now, when you mention it, I see, that you have the B_VDD_EXT=1.8V on the VREFA pin. But I wasn't sure what is the voltage of the B_VDD_EXT, so therefore I rather mentioned it.  

With Best Regards,

Jozef

[manishbuttan]

Hi Jozef, Since these boards are already manufactured, we are trying to do a minimum correction. The lowest step was to put the 3V3 VREFB through a 10K res. (Currently, it is connected directly to 3V3). This caused the module to work fine. In the furure versions, we will put the 200K res and reduce the pull up res values to no pull up on the 1.8V and 1.15K on the 3V side. But currently, I have 10K pull ups on the 1.8V side and 10K pull ups on the 3V3 side. I also have 10K between the EN and 3V3 and the same 10K between VREFB and 3V3. We have tested the board and this configuration is working fine with the UART communication, as well as the voltages.

Can you confirm what is the importance to no pull up (instead of 10K) on the 1.8V side, 1.15K pull up (instead of 10K) on the 3V3 side and the 200K between the VREFB and 3V3 (instead of 10K)? The reason all were 10K is due to the ordering volume benefit. Thanks. 

[JozefKozon]

Hello Manish,

according to what voltage you will apply to the VREFA, VREFB, Ax and Bx pins, you need to choose correct pull-up resistors for required current for internal drivers. The current requirements depends also on required communication speed. Please refer to Figure 42. in the UM10204 attached. For the Standard mode (up to 100kHz) and Fast mode (up to 400kHz), the minimum required current is 3mA, for the Fast mode plus (up to 1000kHz) it is 20mA. 

The 200kOhm on the EN pin is required for low current flowing to the EN pin. I had one customer who connected the EN pin directly to VREFB and the NVT20xx didn't work, adding 200k pull-up resistor solved the issue. 

With Best Regards,

Jozef

[JozefKozon]

Hello Manish,

please refer to the Figure 7. in the NVT2008 datasheet. The Ax pins are referenced from the VREFA pin. You need to bring 1.8V also to the VREFA pin. Please use 200k resistor from the 3.3V to the EN pin.

Please decrease the pull-up resistor values on the Ax and Bx pins. Please refer to the Table 6. and Table 7. and Table 8. for the recommended pull-up resistor values. 

With Best Regards,

Jozef