12 Integrated Capacitors

The input current loop is a high di/dt loop that affects EMI at higher frequency ranges. Devices that integrate a high-frequency input decoupling capacitor effectively reduce the high di/dt inductive loop area and help further reduce EMI.

Figure 12-1 provides an example of a device with integrated capacitors. These integrated capacitors are soldered directly onto the internal leadframe of the device, which minimizes the parasitic inductance on the input loop.

The lab EMI measurements shown in Figure 12-2 demonstrate that, without an input ferrite bead, a device with an integrated capacitor provides approximately 8 dBμV of margin compared to a device without an integrated capacitor. When comparing a device with an integrated capacitor to a device without an integrated capacitor – but with an input ferrite bead – the device with the integrated capacitor provides an approximate 2- to 3-dBμV improvement.

The LMQ61460-Q1 and LM62440-Q1 step-down converters use this device package construction method. Integrating the input decoupling capacitors inside the device provides a solution that is resistant to EMI and easy to lay out.

As shown in Figure 12-3, a device with integrated input decoupling capacitors lowers EMI and offers more margin without the need for slew-rate control compared to a device using only slew-rate control and no integrated capacitors. By combining the HotRod package with integrated capacitors, the LMQ61460-Q1 aims to attenuate high-frequency noise at the input path. Any additional device features to help attenuate EMI noise such as true slew-rate control add to a more reliable EMI design.