Solve design challenges calmly and meet power sequencing challenges
“As boxer Mike Tyson said, “Everyone needs to be flexible in order not to get knocked down.” When good boxers see through their opponent’s strategy, they win. However, good boxers tend to be happy to take the first punch and then adapt to the situation until they find the winning formula.
By Celi Palhua Flores
As boxer Mike Tyson said, “Everyone needs to be flexible in order not to get knocked down.” When good boxers see through their opponent’s strategy, they win. However, good boxers tend to be happy to take the first punch and then adapt to the situation until they find the winning formula.
Likewise, system designers must learn to be flexible when encountering power sequencing issues. Often, the easiest and most cost-effective solution is to add a multiplexer with power-down and fail-safe protection.
What is power failure protection?
Power loss protection is a key feature in some analog switches and helps solve power sequencing challenges. Today’s industry is keen on low-power systems, often with multiple power rails powering multiple integrated circuits (ICs). As you might guess, this can clutter the system. You have to account for possible failures during power up and down when adding or changing power rails. In general, preventing conditions such as back-powering or latch-up can be very annoying because these problems can happen at the last minute. A flexible and cost-effective solution can solve problems quickly, like a boxer’s stopgap after a hit.
Signal switches and multiplexers are solutions for protecting system analog and digital signals. More specifically, a switch with power loss protection, such as the TMUX1511, is an excellent tool for isolating two subsystems and protecting downstream components. Integrated brownout protection prevents reverse power supply, protecting downstream components from digital signals. In Figure 1, you can see a basic signal switch that isolates the 3.3V digital control signal from Subsystem A to Subsystem B.
Figure 1: Switches for Signal Isolation
A switch in the open state can leave the control signal on subsystem A before safely passing the signal to subsystem B. Generally speaking, the purpose of a switch is to indicate when a control signal is passed to turn on a critical part of Subsystem B. As simple as this sounds, failures occur when mismatch problems occur unexpectedly during power-up and power-down. In this case, the power rails of Subsystem A can be powered up before the switch and Subsystem B, as shown in Figure 2.
Figure 2: Reverse power supply during power sequencing
In contrast to Figure 1, the 3.3V signal from Subsystem A back-powers the internal ESD diode, which turns on the switch. This closes the signal path to Subsystem B, allowing the signal to leak out of the switch (yellow arrow), resulting in a false logic state that could damage Subsystem B and interrupt system startup.
To address this issue, TI power-down protection switches maintain a high-impedance state on the input/output (I/O) pins, preventing back-powering of the unpowered rail (VDD) and SEL pins.
Figure 3 shows the TMUX1511 successfully isolating the digital signal and protecting itself and Subsystem B during power sequencing. With power-down protection, the switch isolates the signal path from the power rails. This adds a critical layer of system protection because switches can now isolate energized or de-energized subsystems. Ultimately, this gives you a flexible way to solve the power sequencing challenge by adding protection to the signal path.
Figure 3: TI switch with power loss protection
What about the logic signals in the SEL pins?
In the same way that power-off protection isolates the power rail and I/O signal path, the fault-protection logic isolates the power rail and the SEL pin logic signal, as shown in Figure 4.
Figure 4: Fail-safe logic switches prevent reverse power supply
Without failsafe logic, a logic signal in the SEL or EN pin could back-power VDD, damaging Subsystem B and the switch itself. This switch acts like a power-down protection, maintaining a high-impedance state on the SEL logic pins, preventing power from flowing through VDD during power sequencing.
Choose the right switch for you
TI’s portfolio of switches and multiplexers covers a wide variety of channel counts, configurations and voltage levels. This feature-rich portfolio includes switches with power-off protection and fault-protected logic, including the TMUX1072.