AC-DC and DC-DC Peak-Current-Mode Converters with Integrated Step-Down Regulator

Description

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The MAX17497A/MAX17497B include both a current-mode fixed-frequency flyback converter and a synchronous step-down regulator. They contain all the control circuitry required to design wide input-voltage nonisolated power supplies to supply multiple output rails for smart meters, industrial control, and other similar applications. The MAX17497A has its rising/falling undervoltage lockout (UVLO) thresholds optimized for universal offline (85V AC to 265V AC) applications, while the MAX17497B supports undervoltage lockout (UVLO) thresholds suitable to low-voltage DC-DC applications. Both devices also include a 3.3V fixed-output synchronous step-down regulator that delivers up to 600mA load current.

The switching frequency of the MAX17497A flyback converter is 250kHz, while the MAX17497B flyback/ boost converter is 500kHz. The internally compensated synchronous step-down regulator switches at 1MHz on both versions. These frequencies allow the use of tiny magnetic and filter components resulting in compact, cost-effective power supplies. An EN/UVLO input allows the user to start the power supply precisely at the desired input voltage, while also functioning as an on/off pin. The OVI pin enables implementation of an input overvoltage-protection scheme that ensures the converter shuts down when the DC input voltage exceeds the desired maximum value.

Programmable current limit allows proper sizing and protection of the primary switching FET. The devices support a maximum duty cycle greater than 92% and provides programmable slope compensation to allow optimization of control-loop performance. The devices provide an open-drain RESETN pin that serves as a power-good indicator and enters the high-impedance state to indicate that the flyback/boost converter and 3.3V step-down regulator outputs are in regulation. An SSF pin allows programmable soft-start time for the flyback/boost converter, while an internal digital soft-start is employed for the 3.3V step-down regulator to limit inrush current. Hiccup mode overcurrent protection and thermal shutdown are provided to minimize dissipation under overcurrent and overtemperature fault conditions. The devices are available in a space-saving 16-pin (3mm x 3mm) TQFN package with 0.5mm lead spacing.

Key Features

• Reduced Component Count and Board Space
  • Flyback/Boost with Integrated Internally Compensated Step-Down Regulator
  • No Current-Sense Resistor
  • Space-Saving 16-Pin (3mm x 3mm) TQFN Package
• Minimal Radio Interference
  • 250kHz Switching in Offline Version Minimizes Interference with Radio Receivers in Smart Meter Applications
• Reduced Inrush Current
  • Programmable Flyback/Boost Soft-Start
  • Internal Digital Soft-Start for Step-Down Regulator
• Reduced Power Dissipation Under Fault
  • Hiccup Mode Overcurrent Protection
  • Thermal Shutdown with Hysteresis
• Robust Protection Features
  • Flyback/Boost Programmable Current Limit
  • Input Overvoltage Protection
• Optimized Loop Performance
  • Programmable Slope Compensation for Flyback/Boost Maximizes Obtainable Phase Margin
• High Efficiency
  • Low R_DSON, 150mΩ, 65V-Rated Internal nMOSFET
  • 3.3V Step-Down Regulator Efficiency Greater Than 90%
• Optional Spread Spectrum

Applications/Uses

• AC/DC Power Supplies for Smart Meter Applications
• Universal-Input Offline AC-DC Power Supplies
• Wide-Range DC Input Flyback/Boost Industrial Power Supplies