Design and analysis of point-of-load power supplies for digital TV applications

Current digital TVs present several challenges to power designers, as these TVs are flat panel designs consisting of separate modules located in different locations within the chassis. These modules include the tuner unit, main system board, display driver, audio subsystem, LCD backlight driver, and other components. Each subsystem contains sensitive analog circuits, processors, CPUs, ASICs, and other various circuits, each with its own unique power requirements. The AC voltage is first converted to an intermediate bus voltage and then supplied to the various modules within the TV. This voltage typically ranges from 12 to 15V. Point-of-load regulators in each module convert the intermediate bus voltage to the voltage required by each component.

As shown in Figure 1, the LCD TV block diagram shows each major subsystem in the chassis along with key component modules, including the front-end tuner, motherboard, display driver, and audio board in this case. These components are powered by a 12V offline power supply. In order to provide the required operating voltage, some form of load point regulation must be provided in each module. Table 1 lists the voltage and current requirements of each subsystem and module, as well as any additional requirements such as sequencing or tracking.

High-performance signal processing devices such as FPGAs and DSPs used in motherboards and display driver units require multiple power supplies that can generate different core voltages and I/O voltages. The power-up and power-down sequence of the power supplies is critical to the operation and long-term reliability of the device. Power sequencing can

Three methods are used: sequential sequencing, ratio-metric sequencing, and simultaneous sequencing. The correct sequencing method depends on the requirements of the processor. This example uses a sequential sequencing method, in which the output of one power supply starts to step up and stabilize to the final voltage, and then the output of the second power supply starts to step up after a certain time delay.

Figure 2 is an example of a sequential sequencing method. A good way to implement this sequencing scheme is to use a device with a power good indication and enable pin, connecting the power good output of the first supply to the enable pin of the second supply to be sequenced, making sure the signal has the correct polarity. Most DC/DC converters that include the above features need to be used in this way, they have consistent polarity, and the signals can be directly matched. In addition, additional supplies can be controlled in this way.

Figure 1: LCD TV internal circuit structure diagram

Figure 2: Sequencing power supply waveform

Taking double data rate (DDR) memory as an example, DDR systems require both Vddq and Vtt voltage rails. The Vddq output can use standard power supply types, but the bus termination voltage Vtt requires a different power supply type. For these DDR bus termination supplies, the Vtt voltage generation circuit must be able to accurately track the reference voltage Vref, which is 50% of the output supply voltage Vddq. Although Vddq can vary from the 2.50V nominal value to 200mV, Vtt must remain within 40mV of Vref under any load and transient conditions. The circuit that generates Vtt must also be able to sink current when the output buffer (line driver) is in a logic high state and source current when the output buffer is in a low state. In current-sinking mode, the current flowing through the output inductor reverses the normal direction in the buck configuration.

Table 1: Voltage and current requirements for each subsystem and module

The switching process for sink mode is similar to that of a boost converter, except that the current flows into the output of the power supply and flows out to the input rail after being stepped up. The control circuitry must function properly both when sourcing and sinking current. Dedicated converters work well in these applications because they are designed to be used in bus termination circuits where voltage tracking is required.

Figure 3 shows a possible power solution that combines the power requirements for the various subsystems so that a single regulator can be used at each load point and only a 12V intermediate bus voltage needs to be distributed through the chassis.

Figure 3: Power system architecture

Conclusion

For analog tuners, linear regulators are ideal for power supplies. Although efficiency is reduced, sensitive analog stages may require the noise suppression capabilities of linear regulators. If power consumption is important, the intermediate bus can be pre-regulated to a lower voltage and a low-dropout linear regulator can be used. DC/DC converters are the best choice for powering digital circuits in processors, ASICs, and FPGAs. Compared with linear regulators, DC/DC converters provide higher efficiency and lower power consumption, and can provide higher current. Using converters with additional features such as enable and integrated power good signals can greatly simplify the sequencing of core and I/O voltages. Engineers need to pay special attention to the special requirements of bus termination voltages when designing.