Power Supply Design Tips For Embedded Applications

By Shamibrota Kishore Roy, Cypress Semiconductor

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Due to the advancement of the semiconductor industry, the technology node keeps on decreasing. This leads to lesser core voltage and higher current requirement. Also, the voltage regulators need to have good transient performance due to fast load switching. It is very challenging for the power domain engineers to design such very low conversion ratio high current converters with desired transient response, efficiency and other performance indexes. On the other hand, when supplying power for different ICs on a board with different voltage and current requirement, some power delivery architecture needs to be followed. Special care is required for these high frequency components, as RF noise can screw up the whole power delivery system. Grounding is another part for power delivery system. Proper grounding system is necessary, otherwise the board may not work as intended. Special attention needs to be paid on layout design too, as it is going to decide the overall system performance. Let’s look at some power supply design tips starting with regulator design guidelines before going into power delivery architecture.

Power supply design: regulator guidelines

Linear voltage regulators can be used when it requires to stepping down the input voltage to a regulated output voltage. These are having very good transient performance (higher bandwidth) and very low ripple at the output voltage. It also generates very low EMI. So, linear regulators are good choice for supplying RF components. The only bad thing about linear regulator is its efficiency. Efficiency reduces drastically as the conversion ratio (output voltage/input voltage) decreases (i.e. for fixed 5V input efficiency reduces from 80% to 50% as the output voltage changes from 4V to 2.5V).

linear regulator with parasitic
Diagram of linear regulator with parasitic

For higher conversion ratio, linear regulators are better choice than switching regulators. But as the conversion ration decreases, people tend to use switching regulators if the other above mentioned requirements are not so stringent. Low dropout regulator (LDO) is a special variant of linear regulator can be used when the conversion ratio is very high. It is also useful for declining input voltage (i.e. discharging battery).

Design guidelines for linear regulators

A linear regulator must be selected based on the load current, input and output voltage requirements. As the transistor is operated in linear region, there will be certain amount of power dissipation on the pass transistor. So, thermal design needs to be done so that the die temperature should not exceed 75 % of the maximum rated limit. The over current limit should be set such a way that linear regulator should not cross its safe operating area (SOA).

Other performance indices for linear regulators are stability and transient response, line regulation and load regulation, power supply rejection ratio (PSRR) and efficiency.

  • Stability and transient response is decided by the open loop frequency response of the linear regulator. The load capacitance in conjunction with output load adds a pole and the ESR of the output capacitor in conjunction with output capacitance adds a zero to the open loop transfer function. Internal pass transistor and error amplifier can be modelled combined as a first order system. The open loop frequency response of the overall is largely depends upon the selection of output capacitor. Very low or high value of ESR makes the system unstable. So, firstly an optimum ESR value needs to be chosen for proper operation.  Secondly instead of using simple resistive divider for voltage feedback, resistive capacitive divider network would be better choice and helps to tune the open loop frequency response to achieve stability and better transient performance for wide range of operating load.
  • Line regulation signifies the change in output voltage due to input supply voltage variation. Similarly, load regulation signifies the change in output voltage due to load variation. These data are available on the datasheet and should be well under the system requirements.
  • PSRR measures the AC coupling between the input supply voltage on output voltage. Low frequency input supply voltage ripple couples with bandgap reference generator and error amplifier. High frequency ripple finds path through capacitive coupling between drain and source (or collector and emitter) of the pass transistor. The bandwidth of the system limits the regulation at high frequencies too. Feedforward power supply noise rejection scheme can be adapted to make the system immune against power supply noise.
  • The efficiency of the linear regulator depends upon the dropout voltage. It’s good to use it when the dropout voltage is less.

Regulations with switching regulator

These guidelines go well for linear regulators however the story is a bit different for switching regulators. Switching mode regulator, transistors are operated in switching mode instead of linear mode. This helps to achieve higher efficiency, lower power dissipation and higher power density (small size) than linear voltage regulators. But nothing comes for free. There is performance reduction in terms of transient response, output voltage ripple and EMI generation.

Power supply design: closed loop voltage controlled buck regulator wth parasitic
Diagram of closed loop voltage controlled buck regulator wth parasitic

There are different topologies available for switching regulators. Most fundamentals are buck topology, boost topology and buck-boost topology. MOSFET is mostly used for lower voltage application. Synchronous switching is used mostly because of higher efficiency. Here a buck topology is considered. But all the things are still valid with different topologies also.

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