Maxim（美信）—MAX20079AATP/VY PMIC芯片详解

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PMIC是Power Management Integrated Circuit的缩写，中文是电源管理集成电路。主要特点是集成度比较高，将传统的多路分立式电源集成封装到一颗芯片内，使得多电源应用场景效率更高以及体积更小，方便PCB板的布局。

此次架构中Maxim的MAX20079AATP/VY作为一级电源使用，下面将详细介绍此芯片。

1. 描述

MAX20079是一款集成了高侧和低侧开关的小型汽车级同步降压转换器。该器件可在+3V至+36V的输入电压范围内提供高达3.5A的电流，而空载时静态电流仅3.5µA。MAX20079在+6V至+18V的正常工作输入范围内以±2%的精度提供输出电压。该转换器具有65ns的超短导通时间能力，能够提供较大的输入至输出转换比。电压质量可以通过观察PGOOD信号来监测。MAX20079可以在99%占空比低压差下运行，非常适合汽车和工业应用。该IC提供固定输出电压为3.3V和5V的标准部件。此外，MAX20079可使用外部电阻分压器配置为3V至12V的输出电压。频率在内部固定为2.1MHz，因此所需外部元件数量少，同时能够减少输出纹波，并确保没有AM干扰。还提供400kHz选项以尽量降低开关损耗和提高效率。MAX20079会在轻负载条件下自动进入跳跃模式，空载时静态电流低至3.5µA。它提供引脚使能的展频调制能力，以充分减少调制频率导致的电磁辐射。

MAX20079采用小型4mm x 4mm 20引脚SW-TQFN封装，使用的外部元件非常少。智能封装布局造就了EMI性能出色的超低噪声解决方案。

2. 特性

Synchronous DC-DC Converter with Integrated FETs

—3.5A Output-Current Capability

—3.5μA Quiescent Current in Standby Mode

Small Solution Size Saves Space

—65ns Minimum On-Time

—2.1MHz or 400kHz Fixed Operating Frequency Options

—Programmable 3V to 12V Output Voltage or Fixed 5V/3.3V Options Available

—Fixed 3.5ms Internal Soft-Start

—Innovative Current-Mode-Control Architecture Minimizes Total Board Space and BOM Count

PGOOD Output and High-Voltage EN Input Simplify Power Sequencing
Protection Features and Operating Range Ideal for Automotive Applications

—3V to 36V Operating V IN Range

—40V Load-Dump Protection

—99% Duty-Cycle Operation with Low Dropout

—-40°C to +125°C Automotive Temperature Range

—AEC-Q100 Qualified

3. 应用

Automotive
Industrial
High Voltage DC-DC Converters

4. 框架图

5. 典型电路

6. 引脚定义

PIN	NAME	FUNCTION
1	EN	High-Voltage-Compatible Enable Input. If this pin is low, the part is off.
2	BST	Bootstrap pin for HS driver. It is recommended to use 0.1μF from BST to LX.
4,5,11	SUP	Supply Input. Connect a 4.7μF ceramic capacitor from SUP to ground.
8,9	LX	Buck Switching Node. Connect inductor between LX and OUT. See the Inductor Selection section. If the part is off, this node is high impedance.
13	FB	Feedback pin. Connect a resistor-divider from the buck output to FB to ground for external adjustment of the output voltage. Connect FB to BIAS for internal fixed voltages.
15	BIAS	5V Internal BIAS supply. Connect a 1μF (minimum) ceramic capacitor to ground.
17	SYNC	Sync Input. If connected to ground or left floating, skip-mode operation is enabled under light loads. If connected to BIAS, forced PWM mode is enabled. This pin has a 1MΩ internal pulldown.
18	PGOOD	Open-Drain Reset Output. External pullup required.
3,6,16,19	NC	No Connect
20	SPS	Spread-Spectrum Enable. Connect logic-high to enable spread spectrum of internal oscillator, or logic-low to disable spread spectrum. This pin has a 1MΩ internal pulldown.
7,10	PGND	Power Ground.
12	AGND	Analog Ground.
14	OUT	Buck Regulator Output-Voltage-Sense Input. Bypass OUT to PGND with ceramic capacitors.

7. PCB Layout

Careful PCB layout is critical to achieve low switching-power losses and clean, stable operation. Use a multilayer board whenever possible for better noise immunity. The package for MAX20079 offers a unique symmetrical design, which helps cancel the magnetic field generated in the opposite direction. Adhere the following guidelines to ensure a low-noise PCB layout:

Place two high-frequency ceramic capacitors (C IN ) on two SUP pins, on opposite sides of the IC and close to the device. High-frequency AC current flows on the loop formed by the input capacitor and the half-bridge MOSFETs internal to the device. A small loop would reduce the radiating effect of high switching currents and improve EMI functionality. Two capacitors placed on opposite sides create current loops in the opposite direction, which cancels the magnetic field to reduce radiated EMI.
Solder the exposed pad to a large copper-plane area under the device. To effectively use this copper area as a heat exchanger between the PCB and ambient environment, expose the copper area on the top and bottom. Add a few small vias (or one large via) on the copper pad for efficient heat transfer.
Connect PGND and AGND pins directly to the exposed pad under the IC. This ensures the shortest connection path between AGND and PGND.
Keep the power traces and load connections short. This practice is essential for high efficiency. Use a thick copper PCB to enhance full-load efficiency and power-dissipation capability.
Using internal PCB layers as ground planes helps to improve the EMI functionality, as ground planes act as a shield against radiated noise. Spread multiple vias around the board, especially near the ground connections, for better overall ground connection.
Keep the bias capacitor (C BIAS ) close to the device to reduce the bias current loop. This helps to reduce noise on the bias for smooth operation.
Place output capacitors (C OUT ) symmetrically on the opposite sides of the inductor. This further reduces the radiated noise.