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Hello, and welcome to The Logic Minute.

In this video, the fifth installment of the LSF family series, we will explain how LSF devices achieve down translation.

This is a simplified schematic for down translating with an LSF device. A device powered from the B supply is transmitting into the B side of the LSF, and a device powered from the A supply is receiving the signal after translation.

To understand how this works, we have to look inside these devices. This schematic provides a more accurate view of what is going on inside the transmitter, receiver, and the LSF.

The transmitter is on the high side of the LSF, and has a push-pull output, which will either be driving the B1 input of the LSF to VB or to ground. The receiver has a high impedance standard CMOS input.

Before we get into how the translation works, we need to add a few more components to this circuit. A pull-up can be added on either side of the device. Each line also has a parasitic capacitance associated with it, coming from the trace and attached devices. The parasitic capacitors are shown here grayed out as a reminder that they are not real components.

We're going to break down the translation into two components, when the transmitter is driving the input low and when the transmitter is driving the input high. When the transmitter is driving the input low, the channel 1 FET of the LSF is turned all the way on and is essentially acting like a resistor. The input at the receiver is driven low as well through the FET.

It's important to know that in this state current will be generated by both pull-up resistor, and that current will sink into the transmitter. The current IA passing through the FET will produce a small voltage drop, shown here as a VAB. And therefore, the voltage at A1 and the input to the receiver will be slightly higher than the voltage at B1.

When the transmitter drives the line high, the output voltage will follow the input until the FET turns off, which happens at approximately VA in this case. Once the FET turns off, the output goes into a high impedance state.

The voltage at A1 will remain at approximately VA due to the parasitic capacitance CA1. The capacitor's voltage can't change instantaneously.

Now that the A1 voltage node is essentially disconnected, the passive RC circuit will drive the receiver's input up to VA and hold it there.

There are special circumstances that allow the removal of one or both of the pull-up resistors. If the signal is always going to be down translated from a push-pull transmitter, then the resistor RB1 can be removed.

If the leakage current into the receiver is less than 1 microamp, then the resistor RA1 can also be removed. Recall that CB1 and CA1 are parasitic capacitances, and are shown here only as a reminder.

This is the simplest application of the LSF series of translators with no external components being required. This arrangement can be used when down translating from a push-pull output to a low-leakage input.

For an open drain transmitter, the pull-up resistor RB1 will have to be added back since an open drain output can't drive high by itself.

This is a typical schematic for the LSF0102 down translating from a 3.3 volt device to a 1.8 volt device, including both a push-pull output and an open drain output.

Please click on the links below to jump to the video of interest, and thank you for watching.

This video is part of a series