SSZT293 june 2020 TPS562200 , TPS562201 , TPS562208 , TPS562209 , TPS562210A , TPS562219A , TPS562231 , TPS563200 , TPS563201 , TPS563208 , TPS563209 , TPS563210A , TPS563219A , TPS563231 , TPS563240 , TPS563249 , TPS62912 , TPS62913 , TPSM82913
Jim Perkins, Dan Tooth
Your new design needs to fit twice as much into half the space and cost nothing – sound familiar? You selected the smallest point-of-load regulator and generated the tightest layout you could with the most cost-effective passive components. So far so good. But then you look at the output ripple on your critical rails and it’s not what you expected. What’s going on?
Let’s start by understanding what makes up the output ripple on a buck DC/DC regulator. It is a composite waveform. Traditionally only the three dominant elements shown in Figure 1 have been considered:
However, what you measure has spikes on the edges and a higher square-wave content that changes polarity when you reverse the inductor shown in Figure 2:
What has caused these undesirable components? And more importantly, what can you do about it?
When you selected your inductor, the self-resonant frequency (SRF) was above your regulator switching frequency, so all was good. Let’s re-look at that – the inductor has an SRF because it has a parallel parasitic capacitance. Applying the fast edge of the switching voltage to the parasitic capacitance generates a large current spike through the capacitor, which in turn generates a large voltage spike across the ESL of the output capacitor:
To reduce this spike:
Let’s say that you selected a cost-effective, unshielded inductor. The magnetic field from an unshielded (or resin-shielded inductor) can spread beyond the physical body of the component. The simulation plots in Figure 3 show the field for an unshielded open drum inductor and a fully shielded molded inductor.
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