Why so many different Power Supply Voltages?

Why so many different Power Supply Voltages?

Looking back at the previous log on the CPU schematics, the FunKey device clearly needs a sophisticated power supply in order to fulfill the CPU power requirements. They are recalled below, along with the maximum current requirements found in the Allwinner V3s reference design (page 3):

  • +3.3V / 1.2A for the I/O power supply
  • +3.3V_AO / 30 mA for the Always-On power supply (RTC timer)
  • +3.0V / 200 mA for the analog power supply
  • +1.8V / 1A for the DDR2 DRAM power supply
  • +1.25V / 1.6 A for the core power supply

But why in the first place are so many different power supply voltages required?

Power Efficiency

A first answer is: for better power efficiency.

As P = U x I (Electrical power is the product of voltage level by current intensity), you can reduce power by decreasing the required current or reducing the operating voltage. Assuming you already do your best to reduce the required current, you can still reduce power by reducing voltage.

Reducing Power Supply Voltage

Voltage Drop

But how far can you go? Over long distance, you have the voltage drop from the conductor linear resistance, but this effect can be neglected for small boards. 

Noise Margin

You have inductive and capacitive coupling between conductive wires and planes too, but within a PCB, these coupling only have a limited direct effect on voltage. However, these coupling play a role in that they will pick up external electromagnetic noise from the surroundings and inject it into the circuit.

And with digital circuits, a critical limit when lowering the operating voltage is the “noise margin” or difference in absolute voltage levels between a logical ‘0’  and logical ‘1’, which determines the maximum amplitude of spurious voltage spikes that a conductor can pick up that will trigger an erroneous logic level change.

This phenomenon mostly depends on the circuit scale: a long-distance circuit between boards will require higher voltages (typically +12V or +24V) to limit this effect, whereas a circuit between boards a few meters apart or using through-hole chips on the same board wile require a lower voltage (typically +5V like the old Arduinos). Using SMT chips will allow even smaller boards and lower voltages (+3.3V is today typical), and with wires running on the same silicon die, it is possible to go down to +1.2V, given the current technological limits.

Voltage Swing

There are other reasons why you should try to minimize voltages: the core CPU for example needs to run as fast as possible, and lowering its operating voltage will shorten the signal rise and fall duration as the voltage swing is reduced.

Other Power Supply Considerations

Besides reducing the operating voltage, there are other considerations that may push to multiply the number of power supplies in a design:

Quiescent Current

As for power supply used for standby operation providing small currents,  a very-low leakage current (“quiescent current”) is required as it can no longer be neglected compared to the current required by the light load and even more importantly because this current consumption is permanent.

Ripple Voltage

For sensitive circuits such as ADCs (Analog to Digital Converters) or PLLs (Phase-Locked Loops) which rely on comparing very small voltage differences, a “clean” power supply featuring very low ripple voltage amplitude is required to achieve a good resolution and/or accuracy. This characteristic is only possible to obtain using LDOs and not SMPS, and the figure to pay attention to is then the PSRR (Power Supply Rejection Ratio) or how much a variation in the input voltage will affect the output voltage: the higher, the better! A value > 50 dB is a good starting point.

Application to the FunKey Design

Based on these considerations, it is now clear that each V3s power supply voltage has a good reason to exist:

  • +3.3V / 1.2A is used for powering the I/Os to connect between chips on the board. Given the required current, a SMPS is required for reaching a good efficiency
  • +3.3V_AO / 30 mA for the Always-On power supply (RTC timer) requires a low quiescent-current, so an LDO is used
  • +3.0V / 200 mA for the analog power supply also requires an LDO, this time to minimize the ripple voltage
  • +1.8V / 1A for the DDR2 DRAM power supply: this strange voltage level is typical for DDR2 DRAM memory chips, and is the result of driving the large memory array inside the chip
  • +1.25V / 1.6 A for powering the CPU core to minimize the voltage swing and increase the possible CPU frequency. Given the required current, a SMPS is required for reaching a good efficiency, too
Michel Stempin

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