Out-of-Bounds (OOB) overvoltage protection protects the output load at a much lower overvoltage threshold above the target voltage. OOB protection does not trigger an overvoltage fault, which is an early non-fault overvoltage protection mechanism.
This application note presents a detailed introduction to this OOB feature based on TPS56C230, including OOB operation principle, the differences between OOB and overvoltage protection (OVP), OOB operation logic and OOB advantages in load transient.
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TPS56C230 has the Out-of-bounds (OOB) overvoltage protection that protects the output load at a much lower overvoltage threshold of 8% above the target voltage. OOB does not trigger overvoltage fault protection. OOB protection operates as an early no-fault overvoltage protection mechanism, differs from fault overvoltage protection (OVP).
During the OOB operation, the device operates in forced PWM mode by turning on the low-side FET to discharge the output capacitor, thus causing the output voltage to fall quickly toward the target value. Turning off low-side FET logic is controlled by either triggering the cycle-by-cycle negative overcurrent (NOC) limit or output voltage falling below target value (FB voltage falling below reference voltage). For TPS56C230, OOB control logic allows maximum 16-cycle NOC triggered in order to ensure the safe operation of internal FETs. After 16-cycle NOC triggered, the device would stop switching to wait OVP or output falls to target voltage.
OOB operation is blanked in forced continuous conduction mode (FCCM), since the control logic of FCCM-mode itself allows continuous current until the output voltage decreases to target value.
OOB is an early non-fault overvoltage protection when output voltage is higher than OOB threshold. The OOB can help device fast recovery to normal operation. OVP is a fault protection when OVP threshold is triggered.
Taking TPS56C230 as an example, OOB threshold is 108% of output/reference voltage, while OVP threshold is 125% of output/reference voltage. When OOB is triggered, device is forced PWM controlled with maximum 16-cycle NOC triggered. PG will be high if output is lower than 115%. While, when OVP is triggered, the output will be discharged after a wait time of 120 µs and PG is low.
OOB operation only gains benefit in Eco-mode by forced PWM control with maximum 16-cycle NOC triggered. OOB operation is blanked in FCCM-mode. OVP is the protection no matter Eco-mode or FCCM-mode.
Table 2-1 summaries the differences between OOB and OVP based on TPS56C230.
| OOB | OVP | |
|---|---|---|
| Triggered Threshold | FB>=108%*Vref | FB>=125%*Vref |
| Triggered Behavior | Forced PWM, maximum 16-cycle NOC triggered and PG High (FB<115%*Vref) | Output discharge and PG Low |
| Fault Behavior | Non-fault | Fault |
| Operation Mode | Eco-mode | Both Eco-mode and FCCM-mode |