8.2.2.17 Advanced ORing Techniques

See Advanced Adapter ORing Solutions using the TSP23753, TI document number SLVA306A for ORing applications that also work with the TPS23754 device. The material in sections Adapter ORing and Protection, D1 are important to consider as well. The following applications are unique to the TPS23754 device with the introduction of PPD.

Option 2 ORing with PoE acting as a hot backup is eased by connecting PPD to V_DD per Figure 33. This PPD connection enables the class regulator even when APD is high. The R-Zener network (1.8 kΩ – 24 V) is the simplest circuit that will satisfy MPS requirements, keeping the PSE online. This network may be switched out when the APD is not powered with an optocoupler. This works best with a 48-V adapter and the APD-programmed threshold as high as possible. An example of an adapter priority application with smooth switchover between a 48-V adapter and PoE is shown on the right side of Figure 33. D_APD is used to reduce the effective APD hysteresis, allowing the PSE to power the load before V_VDD1-V_RTN falls too low and causes a hotswap foldback.

Figure 33. Option 2 PoE Backup ORing

Option 1 ORing of a low voltage adapter (for example, 24 V) is possible by connecting a resistor divider to PPD as in Figure 34. When 1.55 V ≤ V_PPD ≤ 8.3 V, the hotswap MOSFET is enabled, T2P is activated, and the class feature is disabled. The hotswap current limit is unaffected, limiting the available power. For example, the maximum input power from a 24-V adapter would be 19.3 W [(24 V – 0.6 V) × 0.825 A].

Figure 34. Low-Voltage Option 1 ORing