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Current-Mode PWM Controllers with Frequency Dithering for EMI-Sensitive Power Supplies

High Efficiency at Light Loads Meets Green Requirements of Next-Generation IP Phones, WLAN, and IEEE 802.3af/at Powered Devices

Product Details

Key Features

Simplified Block Diagram

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Key Features

  • Peak Current-Mode Control, Forward/Flyback PWM Controllers
  • Internal 1% Error Amplifier
  • 100kHz to 600kHz Programmable ±8% Switching Frequency
  • Switching Frequency Synchronization Up to 1.2MHz
  • Programmable Frequency Dithering for Low-EMI Spread-Spectrum Operation
  • 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
  • 3mm x 3mm, Lead-Free, 16-Pin TQFN

Applications/Uses

  • Flyback/Forward DC-DC Converters
  • IP Phones
  • PoE IEEE 802.3af/at Powered Devices
  • Power Devices in PoE/Power-over-MDI
  • Security Cameras
  • Wireless Access Nodes

Description

The MAX5975_ current-mode PWM controllers contain all the control circuitry required for the design of wide-input-voltage forward and flyback power supplies in Power-Over-Ethernet (PoE) IEEE® 802.3af/at powered devices. The MAX5975A is well-suited for universal input (rectified 85V AC to 265V AC) or telecom (-36V DC to -72V DC) power supplies. The MAX5975B is available for low-voltage supplies (12V to 24V) such as wall adapters.

The devices are suitable for both isolated and nonisolated designs. Because the devices have an internal error amplifier with a 1% accurate reference, they can be used in nonisolated power supplies without the need for an external shunt regulator.

An enable input (EN) is used to shut down the devices. Programmable soft-start eliminates output voltage overshoot. The MAX5975A has an internal bootstrap UVLO with large hysteresis that requires 20V for startup, while the MAX5975B requires 10V for startup.

The switching frequency for the ICs is programmable from 100kHz to 600kHz with an external resistor. For EMI-sensitive design, use the programmable frequency dithering feature for low-EMI spread-spectrum operation. The duty cycle is also programmable up to the 80% maximum duty-cycle limit. These devices are available in 16-lead TQFN packages and are rated for operation over the -40°C to +85°C temperature range.

Simplified Block Diagram

MAX5975A, MAX5975B: Typical Application Circuit MAX5975A, MAX5975B: Typical Application Circuit Zoom icon

Technical Docs

Support & Training

Search our knowledge base for answers to your technical questions.

Filtered Search

Our dedicated team of Applications Engineers are also available to answer your technical questions. Visit our support portal .