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L4993D application field and working principle

The L4993D is a monolithic integrated circuit specifically designed for off-line switched mode power supplies (SMPS)and line-powered applications. It features many integrated functions, such as overvoltage, overcurrent and power-on resetprotection, on/off control circuit, primary-side sensing function, etc. It also offers low standby consumption and highoperating efficiency.

Application Field

The L4993D is a highly versatile power solution designed to operate in both low-power linear and high-power switched modeapplications. The device is suitable for use in consumer devices, lighting, industrial equipment, medical equipment,domestic appliances, etc. It can also be used in the following scenarios:

• DC/DC converters: The L4993D offers three different versions for AC/DC, DC/DC and DC/AC applications.
• Main/auxiliary/charging supplies: This device is well adapted to current off-line primary/auxiliary/charging power supplies forcharging batteries, charging mobile phones, etc.
• Telecommunication equipment: It is ideal for use in the primary power applications and their respective charging circuits intelecommunication equipment.
• Industrial equipment: This device is suitable for use in industrial equipment supplying energy to platforms, sensor nodes,etc.
• Automotive applications: The L4993D can be adapted to automotive applications requiring high power solutions, such asfuel pumps, HVAC and infotainment.

Working Principle

The L4993D is based on a linear regulator structure. It offers integrated protection and various engineering features. Due to theintricacies of power supplies, the L4993D is best suited to speaking in broad strokes. The basic operation of the deviceconsists in monitoring and controlling the power supply output by using Voltage and Current Feedback Loops as bellow:

Voltage Feedback Loop (VFL)

This loop controls the output voltage and is composed of a voltage divider connected to the output and a comparatormonitoring the output voltage and providing a constant voltage reference. If the output voltage drops below the reference, thecompensating current at the error amplifier output increases, causing the output voltage to go up until the reference is achieved. The same occurs when the output voltage is higher than the reference.

Current Feedback Loop (CFL)

This loop is used for an added protection of the power supply and serves to ensure that the allowed maximum current is not exceeded.It consists of an optocoupler monitoring the current flowing through the output power transistor and a comparator, which comparesthe current sensed with a reference current. If the sensed current is higher than the reference, the compensating current at theerror amplifier output decreases, causing the output voltage to go down until the allowed current is reached.

Primary-Side Sensing

The L4993D also incorporates a primary-side sensing feature that allows the device to monitor and control the current in theprimary winding of the power supply transformer, thus providing enhanced protection and extending the life of the device. Itworks in a similar way to the CFL (Current Feedback Loop), where an optocoupler monitors the current in the primary winding anda comparator compares it to a reference. If the sensed current is higher than the reference, the compensating current at theerror amplifier output decreases, causing the output voltage to go down until the allowed current is reached.

Fault Detection

The L4993D is equipped with a fault detection circuit that monitors the output voltage, the current in both the primary and thesecondary windings, and the primary-side sensing signals. If any of these signals exceed the defined limits, the device will shutdown the output and then restart after a programmed delay.

Power-on Reset

The L4993D includes a power-on reset (POR) circuit that monitors the input voltage of the power supply. If the voltage dropsbelow a predefined level, the device will shut down and restart after a certain delay. This ensures the correct operation of thepower supply in the event of power line brownouts or other transient events.

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