LM2577S-15/NOPB

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The LM2577S-15/NOPB is a monolithic integrated switching voltage regulator by Texas Instruments. This step-up, step-down regulator can be used in a range of applications. As a PMIC (Power Management Integrated Circuit) it is designed to convert an input voltage to a regulated output voltage. The part has many features of an application specific integrated circuit and is ideal for efficient DC to DC conversion for use in general purpose applications and automobiles.

Features

• Wide input voltage range from 6V to 50V
• Input current in excess of 3A sourced from input side
• Available in maximum 12V or 15V output voltage in step-up mode and 2.85V output voltage in step-down mode
• Output current up to 5A with available 1A or 3A current limit
• 88% typical efficiency
• Input/Output undervoltage lockout protection
• Soft start and output overvoltage protection
• Short circuit protection
• Low input/output ripple and fast transient response
• Thermal shut down protection
• Line and load regulation of 0.4%

Applications

• Uninterruptible power supplies
• Power supply for notebook computers and power tools
• Battery powered systems
• Automotive applications such as Redundant System Power Supplies (RSPS
• Chargers, Step-up and Step-down regulators
• DC to DC converters
• DC motor speed control
• Battery powered systems

Working Principle

The LM2577S-15/NOPB is a switching regulator with a power converter stage consisting of a high-side power MOSFET switch, inductor and output capacitor. The high-side switch is driven by an internal Pulse Width Modulation (PWM) regulator to provide output voltage regulation. The internal high-voltage cycle-by-cycle current limiting feature helps in protecting the device from overload. When the high-side switch current exceeds the limit, the device shuts down and resumes the device operation when the overload condition is removed. This current limiting helps in achieving the constant current response. The PWM regulator also provides soft-start, under-voltage lockout, overshoot undervoltage and temperature shutdown protection.

The internal oscillator controls the PWM regulator, providing a variable frequency range of 100kHz to 500kHz. This high frequency range allows the use of smaller inductor and capacitors, reducing the overall size of the conversion circuit. The output voltage is adjusted by the external feedback resistors which sets the output voltage to a desired level. The soft-start feature allows the device turn on slowly, reducing the shock of power-up on the connected load.

User Considerations

While designing the circuit, the user should consider the input/output voltage drop due to MOSFET voltage drop. It is recommended to use an output capacitor of 10µF or higher for stable operation. The output voltage ripple depends on the inductor, the output capacitance, current and frequency. Choosing the right size of the inductor according to the output current requirement is key to stability of the circuit. A low ESR ceramic capacitor is recommended at the output for noise filtering and a good electrical connection between the PCB and the switcher.

Parasitic power losses due to the wire resistance of leads should be taken into consideration. In order to operate the LM2577S-15/NOPB safely, the power dissipated inside the IC should not exceed the absolute maximum rating of junctions temperature. The user must take necessary measures to reduce the junction temperature. These measures may include the use of large copper area for the ground plane and adding a small heat sink to reduce the IC temperature.

Conclusion

The LM2577S-15/NOPB voltage regulator works by switching between a high-side power MOSFET switch, inductor and output capacitor to achieve its output voltage regulation. Its wide operating input voltage range, good line and load regulation, short-circuit protection, thermal shut down protection and low ripple output, makes it a good choice for converter applications. It is recommended to design the power converter circuit considering the input/output voltage drops, output voltage ripple, parasitic power losses and IC junction temperature, to ensure reliable and stable operation.

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