A V A I L A B L E Maxim > App Notes > Amplifier and Comparator Circuits Power-Supply Circuits Keywords: powered device, PD, power over Ethernet, PoE, IEEE 802.3af, IEEE 802.3at, Class 2 PD, Class 3 PD, IP phones, IP cameras, security cameras, WAP, wireless access point, point of sales, POS, thin client, Ethernet repeater, active clamped forward, flyback Feb 10, 2011 APPLICATION NOTE 4984 High-Efficiency, Low Cost Flyback Converter Uses No Optocoupler Feedback Abstract: This reference design is for a highly efficient, flyback, 12V, Class 3 powered device (PD). The design features the MAX5969B as its controller. The design also uses the MAX5974A, which controls current-mode PWM converters and provides frequency foldback for PoE applications. Using these devices, this reference design is IEEE® 802.3af/at compliant and is a high-performance, compact, and cost efficient solution for a Class 3 PD. The design can also be customized to 12V/2A Class 4 PD. General Description This reference design is for a highly efficient, flyback, 12V/1A Class 3 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. Specifications 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) Design Features Inductive feedback: no optocoupler or shunt-regulator, so costs are lowered Self-driving input voltage: achieves synchronous rectifier, provides high efficiency and good load regulation Frequency foldback: 150kHz switching frequency when load is about 1W, provides high efficiency at light loads Page 1 of 4

Top view of the reference design. Bottom view of the reference design. Performance Data LINE REGULATION vs. INPUT VOLTAGE POUT = 1.2W_12V POUT = 12W_12V ) V ( E G A T L O V T U P T U O 12.200 12.150 12.100 12.050 12.000 11.950 11.900 11.850 35.00 40.00 50.00 45.00 INPUT VOLTAGE (V) 55.00 60.00 ) % ( I Y C N E C F F E I EFFICIENCY vs. OUTPUT CURRENT 48V_12V AT 0.1A-1A 100.00 90.00 80.00 70.00 60.00 50.00 40.00 30.00 20.00 10.00 0.00 0.000 0.200 0.400 12V OUTPUT CURRENT (A) 0.600 0.800 1.000 Transient Response VIN = 36V VOUT = 12V/0.6A–0.9A–0.6A CH1: 200mV/div, 12V output voltage CH4: 500mA/div, output current Time base: 200µs/div Page 2 of 4 Page 2 of 4 Page 2 of 4

2 1 2 D C 1 TX1+P1 TX1-P2 TX2+P3 TX2-P6 TX3+P4 TX3-P5 TX4+P7 TX4-P8 1 2 2 2 TD1+ TC1 TD1- VC1 TD2+ TC2 TD2- VC2 TD3+ TC3 TD3- VC3 TD4+ CT TD4- VC4 1 G 2 G 1 Y 2 Y 7 1 8 1 9 1 0 2 J1 11 12 10 13 4 6 5 14 3 1 2 15 8 7 9 16 T U O A T A D VDD VDD 2 C A 2 C A D1 C A 1 D2 C A 1 3 + V - V 4 3 + V - V 4 R7 C3 R6 VSS C14 VSS N I R E W O P & A T A D R15 R16 RTN B VDD VDD R1 D3 C2 VSS VSS A U1 MAX5969B VDD DET N.C. N.C. CLS 2EC PG WAD VSS P E RTN 1 2 3 4 5 10 CLS PG 9 8 7 6 P E R4 RTN CLS 1 2 3 Title MAX5969B MAX5974A FLYBACK 12V CLASS3 PD Size Letter :etaD Number Revision fo teehS Drawn By: 4 B A 3 4 D D4 T1 10 R12 9 8 7 N2 4 3 2 1 5 6 7 8 12V C5 C10 C12 C GND R5 C1 R14 D10 R2 VC R38 R3 R11 FB VDD 4 5 2 3 RTN C6 + C7 RTN D7 567 8 N3 R19 1234 R8 PG R9 U2 6 1 SS DT 5 1 4 1 3 1 DCLMP EN VC AUXDRV DITHER/SYNC NDRV MAX5974A PGND D11 C4 D6 12 11 10 RTN R17 3 4 RT FFB COMP FB SGND CSSC C24 CS 9 R26 5 R10 6 FB 7 8 C16 C15 RTN R18 RTN R25 R21 C18 RTN Page 3 of 4

IEEE is a registered service mark of the Institute of Electrical and Electronics Engineers, Inc. Related Parts MAX5969B IEEE 802.3af/at-Compliant, Powered Device Interface Controllers with Integrated Power MOSFET MAX5974A Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers Automatic Updates Would you like to be automatically notified when new application notes are published in your areas of interest? Sign up for EE-Mail™. Application note 4984: www.maxim-ic.com/an4984 More information For technical support: www.maxim-ic.com/support For samples: www.maxim-ic.com/samples Other questions and comments: www.maxim-ic.com/contact AN4984, AN 4984, APP4984, Appnote4984, Appnote 4984 Copyright © by Maxim Integrated Products Additional legal notices: www.maxim-ic.com/legal Page 4 of 4