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

 

Part Number	Replacement	Explanation

Description

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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 temperature range.

MAX5974A, MAX5974B: Typical Application Circuit Enlarge+

Key Features

• 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

Applications/Uses

• 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