Simplified Block Diagram
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- High Integration Saves Space and BOM Cost
- Efficient, Integrated DC-DC Converter (with Integrated Switches)
- Built-In Output-Voltage Monitoring
- Protects Against Overload, Output Short Circuit, Output Overvoltage, and Overtemperature
- Integrated TVS Diode Withstands Cable Discharge Event (CDE)
- Internal LDO Regulator with Up to 100mA Load
- IEEE 802.3af-Compliant Features Reduce Design Time
- PoE Class 1/Class 2 Classification Set with Single Resistor
- Intelligent Maintain Power Signature (MPS) Complies with IEEE 802.3af Specification
- Simplified Wall Adapter Interface
- Pass 2kV, 200m CAT-6 Cable Discharge Event
- High Efficiency During Light Loads Reduces Power Consumption
- Sleep and Ultra-Low-Power Mode
- Frequency Foldback for High-Efficiency Light-Load Operation
- Back-Bias Capability to Optimize the Efficiency
- Robust Performance
- 8.7V to 60V Wide Input Voltage Range
- Hiccup-Mode Runaway Current Limit
- 49mA (typ) Inrush Current Limit
- Open-Drain (Active-Low RESET) Output
- Easy to Design With
- 3.2V to 14V Programmable Output Voltage Range
- Internal Compensation
- Fixed 430kHz Switching Frequency
- IEEE 802.3af-Powered Devices
- IP Phones
- IP Security Cameras
- Wireless Access Nodes
The MAX5991A/MAX5991B provide a complete power-supply solution as IEEE® 802.3af-compliant Class 1/Class 2 Powered Devices (PDs) in a Power-over-Ethernet (PoE) system. The devices integrate the PD interface with an efficient DC-DC converter, offering a low external part count PD solution. The devices also include a low-dropout regulator, MPS, sleep, and ultra-low power modes.
The PD interface provides a detection signature and a Class 1/Class 2 classification signature with a single external resistor. The PD interface also provides an isolation power MOSFET, a 60mA (max) inrush current limit, and a 321mA (typ) operating current limit.
The integrated step-down DC-DC converter uses a peak current-mode control scheme and provides an easy-to-implement architecture with a fast transient response. The step-down converter operates in a wide input voltage range from 8.7V to 60V and supports up to 6.49W of input power at 1.3A load. The DC-DC converter operates at a fixed 430kHz switching frequency, with an efficiency-boosting frequency foldback that reduces the switching frequency by half at light loads.
The devices feature an input undervoltage-lockout (UVLO) with wide hysteresis and long deglitch time to compensate for twisted-pair cable resistive drop and to assure glitch-free transition during power-on/-off conditions. The devices also feature overtemperature shutdown, short-circuit protection, output overvoltage protection, and hiccup current limit for enhanced performance and reliability.
All devices are available in a 20-pin, 4mm × 4mm, TQFN power package and operate over the -40°C to +85°C temperature range.