How to improve the performance of car power supplies

When using high frequency switching regulators, good automotive power PCB layout can provide cleaner output and simplify debugging during EMI testing. This article takes the MAX16903/MAX16904 switching regulator design as an example to introduce how to layout for optimal performance and reduce radiation.

introduction

When using high-frequency switching regulators such as the MAX16903/MAX16904, good automotive power PCB layout can provide cleaner output and simplify debugging during electromagnetic interference (EMI) testing. This article takes the design of the MAX16903/MAX16904 switching regulator as an example to introduce the layout principles for optimizing system performance.

General rules for wiring

Keep the loop area formed by input capacitor C3, inductor L1 and output capacitor C2 to a minimum.

Place the BIAS output capacitor (C4) as close as possible to pin 13 (BIAS) and pin 14 (GND). There should be no via holes between the pins and the capacitor. This is the analog power input of the IC, and any inductance on the negative line will introduce noise into the BIAS power supply, thereby increasing the jitter of the LX output.

Use the shortest possible leads.

Optimize AC-DC current path

To minimize electromagnetic radiation, the layout of the MAX16903/MAX16904 peripheral components is critical. The path of the current jump is called the AC path and occurs during switch ON/OFF operation. After the switch is turned on/off (ON/OFF), the current in the current path is a DC path.

communication path

The switching current path of the MAX16903 synchronous DC-DC converter requires three passive components (C2, C3, L1). These three components have a great impact on electromagnetic radiation and device performance. Figure 1 and Figure 2 show the current paths of the ON and OFF cycles; Figure 3 illustrates the difference between the two current paths with the highest di/dt. Priority should be given to routing C3, followed by routing L1 and C2.

Figure 1. OUT2 current path in PMOS conduction state

Figure 2. OUT2 current path in DMOS on state

Figure 3. Differences in OUT2 AC paths

Bootstrap communication path

The MAX16903/MAX16904 DC-DC converter uses a high-side DMOS transistor and requires a voltage higher than 5V at the LX pin (DMOS source). In order to generate this voltage, a bootstrap capacitor needs to be connected between the LX/BST pins (Figure 4). During DMOS OFF, the 5V BIAS regulator charges the bootstrap capacitor C1; the BIAS output also powers the error amplifier. Therefore, it is necessary to maintain a clean (low-noise) BIAS as much as possible to avoid negative effects on the error amplifier. Keep the inductance as small as possible between C4 and MAX16903/MAX16904. C4 should be as close as possible to pin 14 (GND) and pin 13. (BIAS) placement, do not add vias.

Figure 4. Bootstrap Capacitor AC Path