AN2937 Datasheet, PDF (32/41 Pages) STMicroelectronics – Power system demonstration kit based on the PM6686 dual

Representative waveforms
AN2937
11.1.3
Fault management: overvoltage and undervoltage
The PM6686 provides fault protection for the switching section against undervoltage and
overvoltage.
Figure 21 shows the undervoltage fault management. If, during regulation, the output
voltage drops below 70% of the nominal value, an undervoltage latched fault is detected.
The controller performs a soft-end procedure. The undervoltage fault is reset by toggling the
EN pin or by cycling the VIN (this action performs triggers a power on reset [POR]).
Figure 21. Undervoltage protection (UVP) management
The PM6686 provides latched overvoltage protection (OVP). If the output voltage rises
above the +111% (typ.) from the nominal value for section 1 and above the +116% typ. for
section 2, latched OVP protection is activated. The controller attempts to pull down the
output voltage to 0 V, working in PWM. The current is limited by the negative current limit.
The low-side MOSFET is kept on when the output voltage is about 0 V. This management
prevents high negative undervoltage of the output rail that may damage the load (Figure 22).
If the EN signal is pulled low, the low-side MOSFET is still high, keeping the output to
ground, thus preventing any damage to the load if the cause of the overvoltage is still
present (Figure 23).
The protection is latched and this fault is cleared by toggling the EN pin of the section, or
cycling the V_IN.
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