Hi...
My customer has designing LSF0204 for voltage translation.
But output level (B1, B2, B3) has occurred a problem such as dc offset (more than 0.5V) of output level.
So main system has stopped due to MCU control problem. (Failure rate : 1/100, 1%)
The schematic of my customer is as follows.
I need your advice for LSF0204 application.
Thank you.
Hi...Emrys
I'm late for your response due to delay of my customer's response.
The input and output waveform of LSF0204 are as follows.
(Connection : UART Reset -> A1 -> B1 -> MCU, UART TX -> A2 -> B2-> MCU, UART RX -> A3 -> B3 -> MCU)
1. The input waveform of LSF0204 (VREF_A=3.3V)
a dc bias of low level is about 0.6V to 0.8V.
2. The output waveform of LSF0204 (VREF_B=5V)
a dc bias of low level is about 0.6V to 0.8V.
if pull up resistor value of 5V is changed 680ohm to 1.5kohm, a dc bias of low level is changed 0.6V~0.8V to 0.45V.
also a dc bias of input waveform is changed 0.6V~0.8V to 0.45V.
My customer want to know why.
3. Waveform with FT232 instead of LSF0204 (VCC=3.3V, UART TX Line)
a dc bias of low level is about 0V. But a dc bias of UART RX is about 0.45V. My customer want to know why.
We need your advice and help.
Thanks.
The LSF0204 is using a simple FET for translation. This image shows the basic internal functionality:
If the input voltage does not go to zero volts, then the output also will not go to zero volts. I am unfamiliar with the FT232, but just based on the waveforms, I can tell you that it is a buffer style translator, using a standard logic buffer to translate between voltage levels. Since the input is below that part's V_IL, the output is V_OL. TI also makes many translators that work in a similar fashion if the customer would prefer that functionality (such as the SN74LV1T125).
