Comparators can be powered-down without loading source

STMicroelectronics has designed a pair of rail-to-rail comparators that can be power cycled without dragging down circuits connected to their inputs. Both industrial and AEC-Q100 qualified automotive versions are available.

For dependable use in power-saving applications where the power rail is periodically cut, they have a guaranteed maximum start-up time of 15μs.

“Inspired by industrial robots that use sensors running from a back-up battery to maintain positional awareness while the main system turns off saving power, these comparators fulfill diverse automation applications,” according to the company.

Static power consumption is typically 73μA (155μA max over full temperature range of -40 to +125°C).

Operation is from power rails between 1.7 and 5.5V (6V abs max), and they are specified at 1.8, 3.3 and 5V.

The devices are:

• TS3121A with input offset of ±2mV max (25°C), ±4mV max (Tmin-Tmax)
• TS3121 with input offset of ±6mV max (25°C), ±8mV max (Tmin-Tmax)

In both cases, the typical offset is 0.5mV at 25°C, and a hysteresis of typically 1mV has been designed in to reduce the chances of output chatter from slowly-changing inputs.

Propagation delay varies with supply voltage (and transition direction), with a worst case around 250ns across temperature with a 2.5kΩ resistor on the open-collector output.

To prevent loading of input and output circuits if power is removed, input and output protection is lead to Zener diodes instead of the power rails (diag left).

The input stage is also modified, drawing a maximum of tens of nA at high temperature with up to 6V on the inputs and 0V on the supply pin, or tens of pA at room temperature.

As the output is open-drain, and not linked to the power rail with a diode, it can be connected to circuits operating at up to 5.5V regardless of device supply voltage, allowing the comparator to be used for level-shifting.

Electrostatic discharge protection is up to 4kV HBM (human body model).

Input offset voltage drift across the full temperature range is typically 3µV/°C (20V/°C max). The associated data sheet defines this drift with an equation to allow external compensation to be applied.

Packaging is SOT23-5, or SC70-5 to save space, and the comparators are included in ST’s 10-year longevity program.

“They are ideal for use in industrial controls, building automation, power tools, and smart metering,” said ST. “Automotive-qualified devices are suited to use in powertrain and body-control modules.”

Find the TS3121, TS3121A datasheet here, and an there is anexplanation of the input diodes here