PCB Tech - Programmable power supply optimized for PCB design

PCB power management is generally about all aspects involved in supplying power to the PCB. Some commonly involved issues are:

1. Choose various DC-DC converters to supply power to the PCB;

2. Power opening and closing sequencing/tracking;

3. Voltage monitoring;

4. All of the above.

In this article, power management is simply defined as: the implementation of management of all power on the PCB (including: DC-DC converter, LDO, etc.). Power management includes the following functions: manage the DC-DC controller on the PCB. For example, hot swap, soft start, sequencing, tracking, tolerance and regulation; generate all relevant power state and control logic signals. For example, reset signal generation, power failure indication (monitoring) and voltage management. Figure 1 demonstrates a typical power management function on a PCB with a CPU or microprocessor; hot-swap/soft-start control function is used to limit the inrush current to reduce the starting load of the power supply. This is an important function for the PCB inserted into the active (live) substrate; the power sequencing and tracking function is used to control how to turn on/off multiple power supplies under the premise of meeting the power-on sequence requirements of all devices on the PCB. All voltages are monitored for faults (over/under voltage) to warn the processor of impending power failures. This function is also called the "supervisory function".

When the processor is powered on, the reset generation function provides reliable startup conditions for the processor. Some processors require the reset signal to remain for a period of time after all working power supplies of the processor are stabilized. This is also called reset pulse stretching. The function of the reset generator is to keep the processor in the reset mode when the power fails to prevent undesired errors from the on-board flash memory.

Limitations of traditional power management solutions

Traditionally, each power management function on the PCB is implemented by a separate functional IC. For different voltage combinations, these ICs have different models. In this way, there are hundreds of single-function IC models from different manufacturers to meet different power management needs. For example, in order to select a reset generator IC model, the following information must be provided:

1. The number of voltage circuits that the reset generator IC needs to monitor;

2. The combination of voltage (3.3, 2.5, 1.2 or 3.3, 2.5, 1.8, etc.);

3.% of fault detection voltage (3.3V-5%, 3.3V-10%, etc.);

4. Accuracy (3%, 2%, 1.5%, etc.);

5. The reset pulse extension function controlled by an external capacitor;

6. Manual reset input

In order to deal with all the possible changes of these parameters, just a single reset generator IC, only one manufacturer can have hundreds of models. In addition, if the engineer needs to monitor another voltage (probably) during the design process, he must choose another product of a different model. Similarly, many single-function ICs have many models, such as hot-swappable controllers, power sequencers, and voltage monitors/detectors, even if they only have the same function and have many models based on different parameters. Each PCB of a system composed of multiple PCBs requires different groups of these single-function ICs, which also increases the material cost.

The complexity of PCB design continues to increase

If the use of a single-function power management IC was manageable, then that would be an old story. Many PCBs now generally use several multi-voltage devices, and each device has a different power-on sequence. The finer the process node, the lower the voltage required for the device, but the larger the current. Designers often need to use one load point of each multi-voltage power supply IC. In this way, the number of power supplies used on the PCB will increase. With the increase of power supply voltage loops and the need for multiple sequencing management, power management becomes more complicated.

As PCB design becomes more and more complex, traditional power management solutions become more difficult to parry. At present, designers who use traditional single-function ICs to implement power management may have to give up monitoring certain voltages or select multiple single-function devices for each power management function. The following two methods are not advisable.

1. Increase the PCB area and reduce the reliability

The increase in the number of single-function ICs and the subsequent interconnections not only increase the PCB area, but also reduce the reliability of the PCB from a statistical point of view. For example, it may increase the probability of assembly errors, leading to unpredictable (definitely bad) results.

2. Second supply channel and design compromise

If the single-function devices are purchased from different suppliers, it increases the risk of production delays caused by even one of the devices not being in place on time. This in turn leads to demand for a second supply channel. However, the second channel will reduce the device availability of design engineers, so that these unavailable devices force designers to sacrifice PCB fault monitoring coverage.

Assembly and test costs are proportional to the number of devices used in the system. The unit cost of the device is inversely proportional to the quantity purchased. Because many devices are required in a given system, and each device required to construct the system is reduced, the overall system cost is increased. For example, if a system has 10 PCBs, 1,000 such systems will be manufactured every year. If each PCB uses a single-function IC to implement power management, about 10 different single-function ICs are needed to complete the design. The annual demand for these single-function ICs is 1,000. The unit price for a batch of 1,000 is of course higher than the unit price for a batch of 10,000. Therefore, the cost of the former power management solution is definitely higher than that of all PCBs using the same single-function power management IC.

The traditional power management scheme implemented by multiple single-function IC devices has become an old thing in the 1980s. At that time, digital designers used TTL gates to implement logic functions. As the complexity of the PCB increases, designers have to choose between the two options of choosing a fixed-function ASIC or increasing the number of TTL gates used. Not surprisingly, the number of TTL devices used in system design is increasing dramatically.

The emergence of programmable logic devices (PLD) allows designers to achieve more functions within a given PCB unit area and also shortens the time to market. As the number of devices used in the system is reduced, the overall system cost is also reduced. Because the same PLD can be used in multiple designs, the number of devices used in the system is reduced. The company can standardize a small number of PLD devices without sacrificing the functions required by each PCB.

It is much easier to manage a small number of PLDs than to manage many TTL gates. The same PLD can be used for multiple PCB designs, thereby reducing or even eliminating the need for a second supply channel. The designer can use software to simulate the design before designing the project board, thus increasing the chance of success. Currently, the use of single-function power management ICs is as old-fashioned as using TTL gates in the past. Designing today's complex PCB requires "power management PLD". Indeed, the use of this device should now be an offer for PCB design.

A typical PCB power management implementation using a single programmable power management device. Programmable power management devices require programmable analog and digital parts to simplify the integration of multiple traditional single-function power management devices. Designers can configure the programmable analog part to monitor a set of voltage combinations without having to resort to a specially configured, factory-programmed single-function device.

Need to use the programmable digital part of the power management device to define the logic for the PCB, this logic combines with the programmable power monitoring function to realize such as reset generation, power failure interrupt generation and the sequencing of each power supply. A software-based programmable design methodology enables power management devices to provide a variety of power management functions for specific PCBs.