General Description
This reference design is for a highly effcient, fyback 3.3V and 5V Class 2 powered device (PD) that is IEEE 802.3af/at complaint and cost-effective. The reference design can be customized to a 3.3V/2A and 5V/1A Class 3 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 provides both control for wide-input-voltage, active-clamped, inductive feedback, current-mode PWM converters and 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 cost-effective solution for a Class 2 PD or a Class 3 PD.
Specifications:
- The 3.3V/1A, 5V/0.8A PD is designed to meet the following specifcations:
- Input voltage: 36V to 57V
- WAD input voltage: 36V up to 57V (note: the voltage can be 10V to 57V with a dierent transformer.)
- VOUT1 : 3.3V/1A
- VOUT2 : 5V/0.8A
- Output ripples: ±2%
- Load transient V P-P : ±3% (50% step-load)
- Line and load regulation: ±2%
- Total efficiency with load of 1A at 3.3V, 0.8A at 5V, and 48V input: 84.5% (including input LAN transformer and diode bridge)
Top view of the reference design
Bottom view of the reference design
Transient Response at 36V Input Voltage:
- VIN = 36V, IOUT1 = 1A, IOUT2 = 0.4A–0.8A–0.4A
- Ch1: 100mV/div, 5V output voltage
- Ch2: 100mV/div, 3.3V output voltage
- Ch4: 0.5A/div, output current
- Time base: 400µs/div
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