Maxim公司的MAX5974A/MAX5974B/ MAX5974C/MAX5974D是寬輸入電壓的電流模式正向電源轉換器，主要用于PoE的受電設備（PD）。MAX5974A/MAX5974C適合通用或電信輸入電壓，而MAX5974B/ MAX5974D輸入電壓低到10.5V，器件內(nèi)部提供1%的誤差放大器，可編開關頻率從100kHz到600kHz，主要用在PoE IEEE® 802.3af/at受電設備，大功率PD(Beyond the 802.3af/at Standard)，有源箝位的正向DC/DC轉換器，IP電話，無線接入點，安檢照相機。本文介紹MAX5974A/B/C/D主要特性，方塊圖，典型應用電路圖，以及PoE應用參考設計指標和電路圖。
The MAX5974_ provide control for wide-input-voltage, active-clamped, current-mode PWM, forward converters in Power-over-Ethernet (PoE) powered device (PD) applications.
The MAX5974A/MAX5974C are well-suited for universal or telecom input range, while the MAX5974B/ MAX5974D also accommodate low input voltage down to 10.5V.
The devices include several features to enhance supply efficiency. The AUX driver recycles magnetizing current instead of wasting it in a dissipative clamp circuit. Programmable dead time between the AUX and main driver allows for zero-voltage switching (ZVS). Under lightload conditions, the devices reduce the switching frequency(frequency foldback) to reduce switching losses.
The MAX5974A/MAX5974B feature unique circuitry to achieve output regulation without using an optocoupler, while the MAX5974C/MAX5974D utilize the traditional optocoupler feedback method. An internal error amplifier with a 1% reference is very useful in nonisolated design, eliminating the need for an external shunt regulator.
The devices feature a unique feed-forward maximum duty-cycle clamp that makes the maximum clamp voltage during transient conditions independent of the line voltage, allowing the use of a power MOSFET with lower breakdown voltage. The programmable frequency dithering feature provides low-EMI, spread-spectrum operation.
The MAX5974_ are available in 16-pin TQFN-EP packages and are rated for operation over the -40℃ to +85℃ temperature range.
MAX5974A/B/C/D主要特性：
Peak Current-Mode Control, Active-Clamped Forward PWM Controller
Regulation Without Optocoupler (MAX5974A/ MAX5974B)
Internal 1% Error Amplifier
100kHz to 600kHz Programmable Q8% Switching Frequency, Synchronization Up to 1.2MHz
Programmable Frequency Dithering for Low-EMI, Spread-Spectrum Operation
Programmable Dead Time, PWM Soft-Start, Current Slope Compensation
Programmable Feed-Forward Maximum Duty- Cycle Clamp, 80% Maximum Limit
Frequency Foldback for High-Efficiency Light- Load Operation
Internal Bootstrap UVLO with Large Hysteresis
100μA (typ) Startup Supply Current
Fast Cycle-by-Cycle Peak Current-Limit, 35ns Typical Propagation Delay
115ns Current-Sense Internal Leading-Edge Blanking
Output Short-Circuit Protection with Hiccup Mode
Reverse Current Limit to Prevent Transformer Saturation Due to Reverse Current
3mm x 3mm, Lead-Free, 16-Pin TQFN-EP
MAX5974A/B/C/D應用：
PoE IEEE® 802.3af/at Powered Devices
High-Power PD (Beyond the 802.3af/at Standard)
Active-Clamped Forward DC-DC Converters
IP Phones
Wireless Access Nodes
Security Cameras
圖1。MAX5974A/MAX5974B方框圖
圖2。MAX5974C/MAX5974D方框圖
圖3。MAX5974C/MAX5974D典型應用電路圖
圖4。MAX5974A/MAX5974B典型應用電路圖
圖5。MAX5974C/MAX5974D典型應用電路圖（2）
PoE應用參考設計
Active-Clamped Forward, Dual-Output PD Provides High-Performance Solution for PoE Applications
This reference design is for a highly efficient, active-clamped forward, dual 3.3V/8.8Voutput powered device (PD). The design features the MAX5969A and MAX5900 as its controllers. The design also uses the MAX5974C, which controls current-mode PWM converters and provides zero-voltage switching (ZVS) and frequency foldback to enhance supply efficiency.
Using these devices, this RD is IEEE® 802.3at compliant and is a high-performance, compact, and cost-efficient solution for PoE and nonstandard high-power PDs. The design can also support the auxiliary input voltage to provide maximum 55W output power.
The design features the MAX5969A, MAX5900, and MAX5974C. The MAX5969A is compliant with the IEEE 802.3at standard in a power-over-Ethernet (PoE) system. The device can also get power from a wall adapter (WAD). The MAX5900 hot-swap controller smoothly enables a nonstandard high power from a nonstandard Power Sourcing Equipment (PSE). The MAX5974C provides control for wide-input-voltage, ZVS active-clamped, current-mode PWM converters and frequency foldback for PoE and high-power applications. Using these devices, this reference design is IEEE 802.3at compliant. It is also a high-performance, compact, and cost-effective solution for a nonstandard high power PD.
PoE應用參考設計指標：
The 3.3V/3.35A and 8.8V/4.6A PD is designed to meet the following specifications:
Input voltage: 42V to 57V
Nonstandard PSE: 37V to 57V
WAD input voltage: 30V up to 57V
VOUT1: 3.3V/3.35A
VOUT2: 8.8V/4.6A
Output ripples: ±1%
Line and load regulation: 3.3V = ±0.2%, 8.8V = ±2%
Total efficiency with loads of 3A at 3.3V, 4.2A at 8.8V, and 48V input: 90.4% (including input LAN transformer and MOSFET bridge)
圖6。PoE應用參考設計外形圖
圖7。PoE應用參考設計電路圖（1）
圖8。PoE應用參考設計電路圖（2）
詳情請見：
http://pdfserv.maxim-ic.com/en/an/AN5043.pdf
