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.

Figure 12-1 Integrated capacitors in the LMQ61460-Q1.

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.

Figure 12-2 EMI comparisons between the LM61460-Q1 and LMQ61460-Q1 automotive buck converters.

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.

Figure 12-3 EMI comparisons between the LM61460-Q1 with slew-rate control and the LMQ61460-Q1.