Adding Voltage Tracking to a DC-DC Converter Module

Feb 1, 2004 12:00 PM
By Tony Ochoa, Senior Marketing Manager, Xicor, Milpitas, Calif.

In many systems today, dc-dc converter modules offer anywhere from two to four outputs with a voltage input that varies from -48 V to 12 V. In addition, some include trim pins. Consequently, with such multiple voltages on a single board that houses ASICs, microprocessors and other sensitive ICs, there's a need to provide voltages that track each other on power-up and on power-down to ensure that each supply turns on at the appropriate time. However, the sequencing of the voltage outputs is normally left for the external components.

The task to integrate internal power sequencing can be challenging, because the power-sequencing requirements from one system to another can differ greatly. Hence, any integrated solution must offer the flexibility to provide various power-sequencing configurations to regulate the amount of bulk current delivered into the end application over time.

To speed time-to-market and offer scalability to such dc-dc module designs, Xicor has developed a simple solution to provide user-selectable flexible power sequencing within a dc-dc converter module, such as silver boxes, bricks and custom dc-dc converter designs. As shown in Fig. 1, this voltage-tracking and sequencing circuit, labeled X80200, can be added to provide power sequencing. It provides voltage sequencers with power-up system monitoring that is used to drive three external N-channel FETs, which provide some of the tracking logic. The X80200 also can connect the system to the SMBus. The X80200 contains separate charge pumps to control external N-channel power FETs for each of the supplies. The charge pumps provide the high gate-control voltage necessary for efficient operation of the FET switches. Furthermore, the device includes control of up to three voltage supplies and can be cascaded to control additional supplies. It also contains independent undervoltage lockout for each controlled voltage.

Housed in a 20-pin TSSOP package measuring 6.4 mm × 6.4 mm × 1.05 mm, the X80200, along with simple logic circuits, can be combined to create a voltage tracker and sequencer (Fig. 2). The circuit implementation shown tracks 5 V, 3.3 V and 1.2 V on power-up. Other voltages can be used with the appropriate selection of components. Because the X80200 is an open-loop control element, controlling the outputs is performed by adjusting the inputs. Fig. 3 illustrates the performance of this circuit where the outputs are trimmed using potentiometers to achieve maximum deviation from one supply to another to less than 100 mV.

The sequencing can be either voltage-based or time-based. For more creative switching schemes, the Enable Sequence (ENS) pin on the X80200 is used for time-based voltage sequencing of supplies VDDM and VDDL. Also, the voltage feedback (V_FB) pin can be used to create point-of-load sequencing between voltages. These pins allow for flexible design of switch sequencing and management. The circuit can be monitored and controlled by the designer on the SMBus through the use of the X80200.^[1] For example, this feature allows the designer to monitor the operation of the switch through a status bit read on the SMBus. The X80200 Status Register contains faultdetection bits that indicate the status of the charge pumps.

The complete solution enhances reliability and safety with the addition of a system monitor connected to the SMBus.^[2] For instance, the microcontroller can monitor for system status and then send a signal to the X80200 to shut the charge pumps off. The small package of the X80200 is suitable for small electronic boards where minimal space is a factor.

References

1. Refer to the X80200 data sheet for a logic table.
2. Refer to application note AN165 for more details.

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