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.
MAX5974系列主要特性:
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/MAX5974C应用:
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
图1.MAX5974A/B方框图
图2.MAX5974C/D方框图
图3.MAX5974C/D典型应用电路图
图4.MAX5974A/B典型应用电路图
图5.MAX5974C/D典型应用电路图
MAX5974A/B参考设计
This reference design is for a highly efficient, flyback, 12V/1A Class 2 powered device (PD) that is IEEE 802.3af/3at complaint and cost effective. The reference design can be customized to a 12V/2A Class 4 PD.
The design features the MAX5969B and MAX5974A. The MAX5969B controller is fully compliant with the IEEE 802.3af/at standard in a power-over-Ethernet (PoE) system.
The device can also get power from a wall adapter (WAD). The MAX5974A controls wide-input-voltage, active-clamped, inductive feedback, current-mode PWM converters and provides frequency foldback for PoE applications. Using these devices, this reference design is IEEE 802.3af/at compliant. It is also a high-performance, compact, and economical solution for a Class 3 PD or a Class 4 PD.
MAX5974A/B参考设计主要特性:
The 12V/1A PD meets the following specifications:
Input voltage: 36V to 57V
WAD input voltage: 36V to 57V (10V to 57V with a different transformer)
VOUT1: 12V/1A (up to 2A by optimizing the switching frequency)
Output ripple: ±2.5% (can be reduced by adding more output ceramic capacitors or disabling the frequency foldback, FFB)
Load transient VP-P: ±3% (25% step-load)
Line and load regulation: ±2%
Switching frequency: 300kHz (150kHz in FFB mode)
Total efficiency with a load of 2A at 3V and 48V input: 90.3%, including input LAN transformer and diode bridge (93.3%, not including input LAN transformer and diode bridge)
图6.MAX5974A/B参考设计外形图
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