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Dear Sir/Madam,
Pls help to recommend the suitable level shifter from 1.8V to 3.3V for interface signals of SPI, UART, I2S, GPIO & I2C.
Thanks.
Hi Clemens Ladisch,
Which family is suitable for all the interface signals that mentioned (GPIO,SPI, I2C, UART & I2S) ?
Because Iintend to used same family (8 bit) for all the signals.
Thanks.
I²C requires a device with open-drain outputs (LSF or TXS). This often works with the other protocols (if you do not have pull-down resistors), but is not the optimal choice for those.
Hi Clemens Ladisch,
My application need heavy pull up for I2C which is 2K at input and output of level shifter.
which family suitable for this application, LSF or TXS family ?
There are some limitation for both family on this application.
Thanks.
The TXS has built-in 10 kΩ pullups (and edge accelerators), but can add external resistors for more pull-up current. The LSF always requires external resistors.
Hey Lee,
I would personally recommend the LSF family since it provides a little more flexibility. The implementation of devices in this family is also made easy with these training videos.
Hi Dylan Hubbard,
According the explanation on the video, there are some limitation for LSF family as shown below:-
1) There is bias sink current from VCCB to VCCA through 200K resistor. How do I know that the power supply (VCCA) on my system can handle this sink current ?
2) If the pull up resistors at both transmitter and receiver are 2K, the maximum sink current (IoL) will be increased and cause VOL to be higher.
what is the max IoL is allowed to b be sink so that it would not exceed threshold ?
Thanks.
Hello,
lee peng kiat said:1) There is bias sink current from VCCB to VCCA through 200K resistor. How do I know that the power supply (VCCA) on my system can handle this sink current ?
The most common case where this won't work is when you use an LDO regulator to provide VCCA that doesn't actually power anything else. Linear regulators generally don't like it when you send a reverse current into them. Above you can see one example from a common LDO reg.
Most systems will have enough load on the supply to 'soak up' that small amount of current and it will never actually make it back to the supply. If you aren't certain, you can add a small load resistor to the supply directly to sink the current.
lee peng kiat said:2) If the pull up resistors at both transmitter and receiver are 2K, the maximum sink current (IoL) will be increased and cause VOL to be higher.
what is the max IoL is allowed to b be sink so that it would not exceed threshold ?
It really depends on your receiving device and driving device. If, for example, you're translating from 3.3V to 1.8V, then the 1.8V receiver probably needs a VIL of 0.63V (max). If you're using 2k resistors on both sides, the total current (in the worst case) would be (1.8+3.3)/2k = 2.55 mA. If your driver has an output impedance of ~25 ohms (which is fairly common), it would have a V_OL of 0.064V, which gives quite a bit of headroom for the receiving device. In this case, we'd expect V_AB to be up to ~30mV, so the total output voltage would be around 0.1V (giving some margin for error).
In other words - unless you're using very small pull-up resistors to get really fast up-translation, it won't make a huge difference. If you're trying to use 100 ohm resistors and push the limits of speed on the device, you'll start to see your V_OL and V_OL+V_AB get much larger & become a factor in your design.
I would recommend to put the 2kohm resistor only on the 3.3V side, as the 1.8V side will not require it. Many times you can get away with having no pull-up at all on the lower voltage side, except for open-drain devices (I2C). For those, I would put a larger pull-up than normal since there will be several in parallel essentially. 10k often works well.