There’s no doubt that switching regulators are, in general, more efficient than linear regulators. By using a switching element that transitions between an off state and a low-resistance on state, switch-mode voltage regulators can dissipate small amounts of power even when there is a large difference between the input voltage and the regulated output voltage.
However, this certainly does not mean that switching regulators are always as efficient as we want them to be. Any amount of wasted power must come from somewhere, and in devices such as wireless sensors, smart watches, and wearables, it comes from the battery. The endless pursuit of longer battery life necessarily entails the endless pursuit of higher-efficiency power-management ICs.
Size vs. Efficiency
Circuit design involves a fundamental trade-off involving physical size and efficiency, and it just so happens that electronic devices are nowadays expected to be not only efficient but also very small. Conductors with a smaller cross-sectional area have higher resistance per unit length, and for a given amount of current, higher resistance translates to higher power dissipation.
In the specific context of switching regulators, we have to deal with inductors and a switching device, i.e., a field-effect transistor.
The basic configuration of a step-down DC/DC converter, also known as a buck converter. Note the transistor, which functions as an on/off switch, and the inductor, which is an energy-storage element that maintains consistent current flow to the load.
The relationship between converter efficiency and inductor size is not particularly straightforward, but in general a larger inductor provides lower DC resistance and therefore higher efficiency.
The details of MOSFET on-state resistance are also quite complicated, but in low-voltage applications the FET’s channel resistance is a major contribution to overall on-state resistance, and a physically wider channel leads to lower resistance.
This new DC/DC converter from STMicro is an attempt to overcome the opposition between form factor and energy efficiency. At 1.14 × 1.44 mm, it’s definitely small. It reaches efficiencies over 90% for an output voltage of 1.8 V, and at VOUT = 3.3 V you might even see 95% or 96%.
Plot taken from the ST1PS01 datasheet.
As usual, efficiency goes down as output current decreases, but I give the ST1PS01 credit for maintaining solid performance even at load currents below 1 mA:
Plot taken from the ST1PS01 datasheet.
I’m not an expert in the design of state-of-the-art switching regulators, but my impression is that if you want to create a device that is both very small and very efficient, you need to focus on the control circuitry.
Diagram taken from the ST1PS01 datasheet.
The ST1PS01 uses a hysteretic comparator that senses the inductor’s ripple current, and it automatically shifts between two types of regulation: pulse-frequency modulation (PFM) and pulse-width modulation (PWM).
It also adjusts the switching frequency based on the ripple current and the input voltage; the maximum switching frequency is 2 MHz. The relationship between switching frequency and efficiency is discussed in this article. The basic idea is that higher frequency means more switching, and more switching means more power dissipation. However, there’s a tough trade-off here, because higher switching frequency reduces output ripple and reduces the amount of board space required for the passive components.
I like parts that are versatile and can be incorporated into a variety of applications, and consequently I appreciate the ST1PS01’s convenient method of providing adjustable output voltage. You can choose from four different output voltages by changing the logic states applied to the D0 and D1 input pins, and you can choose from three different output-voltage ranges by selecting the appropriate version of the part. When all three versions of the part are considered, the output-voltage options extend from 1.8 V to 3.3 V.
The ST1PS01 supports input voltages from 1.8 V to 5.5 V. This is a pretty good range for low-voltage applications; it makes the device compatible with a variety of battery chemistries and ensures that regulation can be maintained as battery discharge results in progressively lower input voltage.
Do you think that the ST1PS01 would a good PMIC for your next wearable design? Let us know in the comments section below.