LDOs Bring Serenity to an Otherwise Hectic World
By WenLong Zhu - SLL Business Unit
The world may seem hectic at times, but stability is crucial to sensitive electronic devices. Imagine a duck making its way across a millpond; it might look serene above the waterline, but underwater its feet are working away in a rhythmic frenzy of paddling. The harder the duck paddles, the quicker it becomes exhausted and needs to stop. The analogy with electronic circuits continues - the harder a regulator needs to work in order to maintain a stable voltage and current supply, the more energy it consumes. The key here is to use a regulator that makes the best use of the available energy at all times.
An increasing number of devices now operate from battery sources; IoT is replete with them. While this might seem to reduce the need for voltage regulators, in practice it doesn’t, because batteries are fickle devices. They are advertised as delivering 1.5V, but in reality, they deliver 1.8V for a while before quickly falling off to somewhere around 1.4V. To a smart sensor detecting changes in the micro-voltage region, these variations in supply can easily manifest as false alarms or worse, missed triggers.
Low-dropout voltage regulators, or LDOs, provide the necessary stability in these and many other applications. The term ‘dropout’ refers to the minimum difference between the input and the output voltages that the device can sustain before it ceases to function. By its nature, the LDO needs to experience a voltage drop internally, which means the supply always needs to be higher than the regulated output; it requires that wiggle room in order to maintain the output. It’s the electronic equivalent of using a stabilization feature on a camcorder, which works by windowing the scene and moving that window around the total field of view in opposition to the camera’s movement. The same feature is available in post-production editing tools, and the cost of using it is a smaller field of view. If the camera shakes a lot, the stabilized image will be small in comparison to the captured scene, but if the camera operator’s hand is steady, the stabilized image will be closer in size to the original scene.
Similarly, the smaller the dropout voltage, the wider the operating range of the regulator, which ultimately means more of the battery’s charge can be used, leading to a longer-lasting user experience. This is where the latest iteration of LDO technology comes in.
In terms of an equivalent circuit, an LDO can be seen as a resistor network, because it typically uses an internal MOSFET and the load to split the supply voltage into two components: the regulated output and anything that is leftover. As every Electronics engineer is aware, when using a resistor network as a voltage splitter, passing even a modest amount of current is wasteful, and so linear regulators have a reputation for being inefficient. Clearly, this is not conducive to any application that needs to make the best use of every available Watt, and for that reason conventional LDOs are rarely used in ultra-low powered applications. A low dropout voltage regulator designed for this kind of application (on the other hand), has been architected to minimize internal losses, which means by nature it also maximizes efficiency. This makes this class of low dropout regulator ideal for many applications, including battery-powered devices or those that must remain on permanently, such as home automation, climate control or smart meters.
As an example consider the AP7381 series of ultra-low quiescent current, low dropout voltage regulator ICs. They integrate a voltage reference and error amplifier, as well as implement output voltage and current limiting. The series also includes versions that feature an ultra-low quiescent current, and are only 2.5µA when zero load, making them ideal for use in various USB and portable devices.
The AP7381 series is offered in fixed output voltages of 2.8, 3.3, 5.0 and 7.0V, and can operate from up to 40V input. With a load current of 50mA the dropout voltage is just 500mV, but even at 150mA output current at a regulated output voltage of 5.0V, the dropout voltage is typically only 1200mV.
Bringing stability and serenity to a chaotic world can be exhausting, but the right solution can be found without being wasteful. Just look at the duck!