Low-cost, complete, non-isolated power solution for powered devices (PDs)

Spot Remarks 2017+

SMD aluminum electrolytic capacitor

Summary:

The circuit in Figure 1 is a complete PD power supply circuit with a DC-DC converter that delivers up to 0.85A at 12V output. The MAX5953A includes a high-side, low-side power-switch FET, and the low-side FET cannot be configured as a synchronous rectifier diode. Therefore, the buck converter uses only high-side FETs. Because the current-limiting circuit inside the IC operates with the voltage drop generated by the low-side FET current, the circuit does not have an automatic current limit function. Fuse F1 provides short circuit protection at startup.

Figure 1. PD schematic with a 12V, 0.85A buck converter

The MAX5953 has the following features:

• The TVS diode D1 is used to suppress the instantaneous spike voltage and reverse voltage.
• The circuit operates in three modes depending on the input voltage: PD detection mode, PD classification mode, and PD power supply mode. The voltage threshold with or without a diode bridge is compliant with the IEEE 802.3af standard.
• In PD detection mode, the power supply device (PSE) applies two voltages of 1.4V to 10.1V and a minimum step size of 1V at VIN, and records the current measurements corresponding to these two points. The PSE then calculates ΔV/ΔI, confirming the presence of the characteristic resistance R1 of 25.5 kΩ. In this mode, most of the internal circuitry of the MAX5953A is turned off and the bias current is less than 10μA.
• In the hierarchical mode, the PSE classifies the PD according to the power consumption requirements of the PD. The resistor R2 (255 Ω) informs the PSE that the PD will operate in a 3-level mode with a maximum power of 6.49W to 12.95W. When the power supply enters the power supply mode, the grading current is turned off.
• When VIN rises above the 38V UVLO threshold voltage, the MAX5953A enters power-supply mode and gradually turns on the internal MOSFET, suppressing inrush current.
• When the turn-on process is completed and VOUT - VEE = 1.23V, PGOOD enters the open-drain mode. The soft-start capacitor C15 is charged by an internal 33μA pull-up current to provide a soft-start to the DC-DC converter. By setting the voltage dividers of the divider resistors R6/R7 and 1.33V DCUVLO, the dc-to-dc converter does not start operating until VOUT = -30V (vs. V+).
• Because the 3-level power limit maximum power is 12.95W, when the output voltage is 12V and the power conversion efficiency is 80%, the load current is limited to 0.85A.

See the uploading documentation for details (click to download)