Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers

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 light-load 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°C to +85°C and -40°C to +125°C temperature ranges.

Applications

* Active-Clamped Forward DC-DC Converters * 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, Active-Clamped Forward PWM Controller

* Regulation Without Optocoupler (MAX5974A/MAX5974B)

* Internal 1% Error Amplifier

* 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

* Reverse Current Limit to Prevent Transformer Saturation Due to Reverse Current

* Internal 18V Zener Clamp on Supply Input

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