STEVAL-ISA174V1

The key feature of this offline converter is Zero-Power Mode (ZPM). It is an idle state during which the device is totally shut down and the residual consumption from the mains at 230 V_AC is kept below 10 mW. The IC enters ZPM by pulling the OFF pin to SGND for more than 10 ms and exits ZPM (resuming normal switching) by pulling the ON pin to SGND for more than 20 µs. This function can be tested by acting on the tactile switches connected to the ON and OFF pins.

The negative voltage setup is extremely simplified thanks to an embedded error amplifier, whose accurate 1.2 V internal reference (FB pin) is referred to a separate ground (EAGND pin) that can drift down to -12.5 V with respect to the signal ground of the IC (SGND). The negative output (OUT1) is obtained by just connecting EAGND to the negative rail and a voltage divider between FB, EAGND and SGND. The positive output (OUT2) is obtained from OUT1 through the turn ratio of the secondary windings of the transformer.

To achieve extremely low input power consumption, the IC is biased through a diode coming from the OUT2, which maintains the V_CC voltage high enough (> 4.5 V) to prevent the activation of the internal HV current source.