Simplify buck regulator EMI design with built-in capacitors
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Step-down regulators are versatile components known for their efficiency. However, deploying them in products that must meet the electromagnetic interference (EMI) requirements of groups such as the International Special Committee on Radio Perturbation (CISPR) can present challenges.
The high switching frequencies that drive the high performance of regulators can exacerbate emissions. Fortunately, regulators with built-in capacitors and employing spread-spectrum switching techniques can help you easily meet EMI requirements.
Figure 1 shows a simplified buck regulator with input capacitance (CIN) representing both low and high frequency capacitors. When a metal-oxide semiconductor field-effect transistor (MOSFET) Q1 turns on, high transient currents (di/dt) flow in the current loop of the high-frequency capacitor.
1. Capacitors CIN etcSTARTUP experience high transient currents when the high-side MOSFET Q1 switches.
The key factors in limiting EMI are to keep the areas of these current loops experiencing high di/dt as small as possible while also minimizing the area of high transient voltage (dv/dt) nodes. To help with both of these factors, you should place the input capacitors as close to the buck regulator IC as possible.
Layout constraints, however, often prevent the optimal placement of capacitors. To overcome layout limitations, you can use buck regulators such as Texas Instruments’ LMQ66430-Q1 and LMQ61460-Q1, which deliver 3 A at 36 V and 6 A at 36 V, respectively, and are both qualified to the AEC-Q100 quality. for automotive applications.
Each buck regulator incorporates high frequency input capacitors inside the regulator housing. This integration reduces the need for external capacitors and results in the smallest input capacitor current loop area possible, thus minimizing parasitic inductance in the loop and reducing the amount of power emitted.
Additionally, both offer improved conditioning to minimize switch node ringing. The LMQ61460-Q1, for example, comes in TI’s HotRod flip-chip-on-leadframe package, which has no internal jumper wires, eliminating a significant source of input loop inductance.
Also, note the start capacitor (CSTARTUP) in Figure 1. Its job is to provide a load to the high side gate driver when Q1 is on. (Internal circuitry refreshes the charge of this capacitor when Q1 is off.) CSTARTUP establishes another high di/dt current loop whose area should be minimized. The TI LMQ66430-Q1 integrates this capacitor inside its package, limiting the area of the high di/dt current loop and associated high dv/dt voltage node while reducing the need for external components.
Slew rate control
A common way to minimize EMI in buck regulator designs is to use slew rate control. With the LMQ61460-Q1, for example, an external start resistor (RSTARTUP) controls the power of the high-side FET driver when powering up. A value of 100 Ω for RSTARTUP translates to a switch node rise time of approximately 2.7 ns, virtually eliminating overshoot.
Although slew rate control reduces emissions, it comes at the expense of lower efficiency. However, by using a buck regulator with built-in input capacitors, you may be able to achieve the desired EMI performance without resorting to slew rate control.as the example of Figure 2.
2. The left plot shows a buck regulator with no built-in capacitors but using maximum slew rate control, while the right plot shows a device with built-in capacitors but no slew rate control.
The plot on the left in Figure 2 shows a buck regulator with no built-in capacitors but uses maximum slew rate control, while the plot on the right shows a device with built-in capacitors but has no slew rate control. The device with built-in capacitors achieves a 4-5 dBmV improvement in transmit performance over the device with maximum slew rate control. And the device’s superior performance with built-in capacitors doesn’t sacrifice efficiency.
Spread spectrum techniques
In addition to minimizing areas of high di/dt current loops and high di/dt voltage nodes, you can take other steps to minimize EMI. A switching device such as a buck regulator generates EMI at its switching frequency and at harmonics of that frequency.
One way to mitigate these EMIs is to use a spread spectrum technique to dither the switching frequency. It can attenuate the fundamental and mix the harmonics into a smoothed average waveform whose values remain within the limits set by the relevant EMI standards.
Common spread-spectrum implementations use triangular or pseudo-random modulation. Triangle modulation works well at the fundamental frequency, allowing you to reduce the size and cost of your EMI filter, but is less effective at higher frequencies. Additionally, many triangle modulation schemes have a triangle wave frequency in the range of 4 to 15 kHz. This can create an audible tone if coupled to a nearby audio circuit or even a ceramic capacitor resonating at the same frequency.
Pseudo-random modulation, on the other hand, may not attenuate the fundamental as evenly, thus limiting possible size reductions for your filter. However, it maintains its high frequency performance and has no audible tone issue.
To benefit from the advantages of both approaches while limiting the disadvantages, you can choose a switching regulator that employs TI’s patented Dual-Random Spread Spectrum (DRSS) technology, like the LMQ66430-Q1. The DRSS approach starts with triangular modulation (Figure 3, top). Then it dithers the frequency of the triangular waveform randomly (Figure 3, middle) to solve the audible tone problem, casting the tone into white noise.
3. TI’s DRSS technology starts with triangle modulation, randomly dithers the frequency of the triangle, and adds pseudo-random modulation on top of the dithered triangle modulation.
Finally, to address the suboptimal high-frequency performance of triangle modulation, DRSS adds pseudo-random modulation in addition to dithered triangle modulation. (Figure 3, bottom). In addition to casting the audible tone into white noise, DRSS goes a step further to actively reduce this noise.
High transient currents are the primary drivers of buck regulator EMI, and it is critical to minimize areas of high di/dt current loops and high dv/dt voltage nodes. Integrating high frequency capacitors inside the regulators minimizes these areas as much as possible to help meet EMI compliance requirements.
Choosing a device with TI’s proprietary DRSS technology can further reduce unwanted emissions. Finally, buck regulators with built-in capacitors also help reduce BOMs and reduce power supply size.