Current-Mode PWM Controllers for Low-Cost Flyback Supplies

The MAX5974E/MAX5974F provide control for wide-input-voltage, current-mode PWM, nonisolated, synchronous flyback converters in Power-over-Ethernet (PoE) powered device (PD) applications. The MAX5974E is well-suited for universal or telecom input range, while the MAX5974F also accommodates low input voltage down to 8.4V.

The devices include several features to enhance supply efficiency, including a synchronous MOSFET driver output. The AUX driver drives an output synchronous rectifier to lower conduction losses. Programmable dead time between the AUX and main driver allows for zero-voltage switching (ZVS). Under light load conditions, the devices reduce the switching frequency (frequency foldback) to reduce switching losses.

An internal error amplifier with a 1% reference eliminates the need for an external shunt regulator and optocoupler.

The programmable frequency dithering feature provides low-EMI, spread-spectrum operation.

The MAX5974E/MAX5974F are available in 16-pin TQFN-EP packages and are rated for operation over the -40°C to +85°C temperature range.

Applications

* High-Power PD (Beyond the 802.3af/at Standard) * IP Phones * PoE IEEE® 802.3af/at Powered Devices * Security Cameras * Wireless Access Nodes

* Peak Current-Mode Control, Synchronous Flyback PWM Controller

* Synchronous MOSFET Driver Output

* Internal 1% Error Amplifier Eliminates Shunt Regulator and Optocoupler

* 100kHz to 600kHz Programmable ±8% 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

* Internal 18V Zero Clamp on Supply Input

* 3mm x 3mm, Lead-Free, 16-Pin TQFN-EP