Design An Adaptable Power Supply For XENPAK

XENPAK is a multisource agreement (MSA) for a 10-Gbit transceiver package. As part of the XENPAK MSA, each XENPAK module requires input power from an adaptable power supply (APS) that resides on a backplane. Figure 1 shows a diagram of the APS and a module.

A control resistor on the XENPAK module sets the output voltage of the APS. The table lists the specified output voltages for different control resistances. These resistor values were developed with the idea that the APS control IC contains a 0.8-V internal reference, which severely limits the choice of controllers. However, adding a pull-up resistor and using the control IC's external reference enables the use of controllers with any internal reference value.

Figure 2 displays an APS circuit using a TPS54310 3-A synchronous buck regulator, which contains a 0.9-V internal reference. This circuit provides a high-efficiency solution with a minimal amount of components and minimal amount of board space.

The interface between the APS and XENPAK module consists of five signals: APS_POWER, APS_SENSE, APS_SET, MOD_DETECT, and GND. Power is supplied to the module through the APS_POWER signal and GND. The APS_SENSE signal provides remote sensing of the output voltage. The APS_SET signal sets the regulation point of the APS output voltage, and the MOD_DETECT signal enables a hot-swap capability.

All signals on the left side of Figure 2 connect to the XENPAK module. The APS input voltage is supplied on the backplane where the APS resides. This APS solution is fully functional with input voltages between 3 and 6 V. Thus, this solution can be powered from either a 3.3- or 5-V bus. Over 3 A of current can be supplied to the XENPAK module through the APS_POWER signal and GND. Short-circuit current is limited to around 4-A peak by the regulator.

While power is supplied to the XENPAK module through the APS_POWER signal and GND, the voltage on the module is sensed through the APS_SENSE signal. This improves the voltage regulation on the module by compensating for a voltage drop between the APS output and the module. But the voltage drop in the return path (GND) isn't compensated by the APS. The return-path voltage drop is controlled by the XENPAK MSA requirements on the impedance of the ground connections. To provide the tightest regulation possible, the APS_SENSE signal should be connected as close as possible to the devices being powered on the XENPAK module.

Resistor R7 offers a fail-safe mode in case the APS_SENSE pin doesn't make contact with the module. In this situation, the APS will provide regulation locally.

Perhaps the most unique requirement of the XENPAK APS is that while the supply resides on the backplane, the resistor that controls the output voltage resides on the module. The circuit of R1, R3, and R4 adjusts the voltage on the sense pin of the controller to meet the output-voltage requirements. This solution can yield an output-voltage accuracy of ±3% if 0.1% precision resistors are used for R1, R3, and R4. The XENPAK MSA also requires the control resistor to have a tolerance of 0.1%.

This solution can also be easily configured as a fixed-voltage supply by populating resistor R9 instead of using the XENPAK module's control resistor. For any desired output voltage, implement the same value for R9 that would be used as the control resistor.

Inserting the XENPAK module starts up the APS. When the module is connected to the APS, a 1-kΩ resistor located on the module pulls the MOD_DETECT signal to ground. This releases the controller's enable pin and triggers a slow start of the APS. Such a feature permits a hot-swap capability, while preventing large inrush currents.