SNVT013 January 2024 LMQ66430 , LMQ66430-Q1 , LMR66430 , LMR66430-Q1
A step-down converter pulses on the totem-pole, power, and field-effect transistors (FETs) to generate a pulse-width modulation (PWM) signal (Figure 1-1). The PWM signal is filtered to produce the output voltage (VOUT). The input capacitors (CIN) support the AC current required during the high-side FETs on-time, D. These AC waveforms create a hurdle in passing noise-compliance standards, such as CISPR 25 Class 5.
High-ambient, step-down converter operation is achieved with low power dissipation. FET power dissipation is minimized by rapid switching. The input current has high energy harmonics as a consequence of the rapid switching. The 2.2MHz operation is then often challenging when more stringent noise standards need to be met.
The FET rapid turn-on, turn-off leads to an inductive ring which is seen in the switch waveform (Figure 1-2). Converter data sheets show that input capacitors must be close to VIN and GND pins. This makes sure that the input loop inductance is low enough to reduce the inductive ring amplitude.
Figure 1-2 demonstrates the switch node ring. The reference trace (grey)
has the input capacitor placed 70 mils closer to the VIN pin of the
device, in comparison to the overlaid purple trace. This results in approximately
a
3 Vp-p ring amplitude increase, impacting
radiated noise which often includes this frequency range.
LMQ66430-Q1 takes noise reduction a step further. This device implements integrated (VIN) capacitors directly on the device lead frame, fully encapsulated. This reduces the input loop inductance and the area. The reduced loop inductance and area impacts the magnetically coupled, broadband (harmonic) noise. These are directly proportional, so noise generation is reduced. LMR66430-Q1 is the non-integrated-capacitor version of LMQ66430-Q1. Figure 1-3 highlights the difference in conducted noise, with a sweep from 30MHz to 108MHz. Additionally, Figure 1-4, Figure 1-5, and Figure 1-6 demonstrate the difference in radiated noise. These sweeps were conducted on an identical PCB, under the same power conditions, and setup.
Input capacitors are often a multilayer, ceramic chip capacitor (MLCC). Over time, these capacitors can fail due to shock (that is, vibration, temperature, or mechanical). Standard MLCCs tend to fail-short, which can lead to system thermal instance.
LMQ66430-Q1 has integrated MLCC, input capacitors causing the same concerns for designers. This device utilizes 0201, AEC-Q200, integrated capacitors. The two VIN capacitors are placed in series between pin 3 and pin 6 (Figure 1-7). Having two capacitors in series reduces the risk of a fail-short to ground, because both capacitors have to fail. Additionally, having the capacitors orientated in a L-shape arrangement, reduces overall stress. Lifetime stress is reduced as stress is greatest when applied on the same axis. Lastly, the integrated, 0201 capacitors are a soft-termination type. Soft-termination, MLCCs are less prone to cracking. The LMQ64430-Q1 capacitor integration technique leads to a respectable temperature cycle report as shown in Table 1.
LMQ66430-Q1 Temperature Cycling Results | |
---|---|
Test Standard | IPC-9701 |
# of passing cycles | > 6080 (without failure, in-progress) |
Automotive noise requirements are becoming more concerning in designs. Capacitor integration is important for low noise, reducing noise with loop inductance reduction and area reduction. This provides simple layout and design for achieving CISPR 25 Class 5 compliance. Automotive safety also remains a concern for designers. Having an input capacitor short the battery terminals together can lead to a thermal instance. Designers are considering capacitor selection, placement, and series-use to reduce thermal instance probability. LMQ66430-Q1 or LMR66430-Q1 can be considered for low-noise, safety-critical designs as these devices are intended to achieve these typical, automotive, step-down requirements more easily.
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