DC/DC switching regulators

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Welcome to our low power DC-DC mini lab video. The subject of today is light load efficiency. For that, we will look at the evaluation module of the TPS62135 and we will monitor the output voltage ripple, as well as inductor current in both operation modes, pulse width modulation as well as pulse frequency modulation. This device operates very well down to load current of 1 milliampere at high efficiency levels, with its device quiescent current just 17 microampere.

In order to understand what we mean with light load efficiency, let's start with the basics. Ah. But before we start the step, for more information, please look at ti.com/lowiq.

Now let's have a look at the efficiency curve of the TPS62135 operating from 12 volt input to 1.8 volt output. And the device is in forced PWM operation. And as we reduce the load current, you can see the efficiency rolls off. And at the load current of 100 milliampere, the efficiency drops down to 45%. This translates into a power loss of 220 milliwatt, which gives us a conversion loss current of 18 milliampere.

Now our light load efficiency comes in to gain. The red curve now represents the 62135 operating in pulse width modulation at nominal current. And as you reduce the load current, we reduce the switching frequency and operate with pulse frequency modulation, keeping the efficiency up. And at 100 milliampere load current, we achieve above 80% efficiency. That reduces our conversion loss current from 80 milliampere down to 3 milliampere. And we do this by reducing the switching frequency, reducing the conversion losses, and minimizing the device quiescent current to 17 microampere.

OK. Let's look at the bench and see the different operation modes. Here we have our evaluation module. We measure the output voltage across the output pins using high frequency decoupling capacitor. And we have soldered a wire in series to the inductor to measure with the current probe the inductor current. So here we have the scope, with the yellow, the top trace, the output voltage ripple, 20 millivolt per division, full bandwidth resolution. And the inductor ripple current is 500 milliampere per division.

Now I'm going to reduce the load current. And we can see that the inductor ripple current goes down and down until we approach zero. And this is the [INAUDIBLE] where we start now pulse frequency modulation. And you can see we reduce the switching frequency.

As I increase the load current again, the pulses move closer together. And now we move into pulse width modulation again. And very important is that you don't see any spiking in the output voltage. The output voltage should be still very stable during this operation. And as I further reduce to load current, the pulses move farther apart, until a minimum switching frequency of around 35, 36 kilohertz.

We could see that by reducing the switching frequency and device quiescent current, we can achieve high efficiency levels. But what happens as we further reduce the load current? The red curve shows the efficiency curve of the 62135 operating in pulse width modulation at heavy load. And once the load current gets reduced, we operate with pulse frequency modulation, with using the switching frequency.

Now you can see that we operate at very high efficiency level down to a load current of roughly 1 milliampere with an efficiency level of 73%. Since there are applications out there that operate at even lower load currents than 1 milliampere, like metering variable or Internet of Things applications, we need to have a device that operates at high efficiency levels even at load currents loader than 1 milliampere.

Now for that, we look at the efficiency curve of the 62743. And here we can see now that this device operates at 90% efficiency down to 1 milliampere. And even at 2 decades lower, we still achieved an efficiency level of 83%. And this we do with the device quiescent current of 360 nanoampere. And this enables a no load quiescent we measure on the bench of typically 450, 460 nanoampere.

Now, this is very important, for instance if you run off a coin cell, because with an 8 microampere load current, your current you draw from your battery is reduced by almost 30%. And that really extends your lifetime of your coin cell battery.

Thanks for watching. For more information, please visit us at ti.com/lowiq.