ADI LTC7803 synchronous step-down controller
Switching frequency range: 100kHz to 3MHz
Supports forced continuous conduction and burst mode


Analog Devices' (ADI) LTC7803 synchronous step-down controller simplifies the design of high-performance power supplies. It integrates low-impedance gate drivers capable of switching N-channel MOSFETs, reducing overall converter cost and increasing efficiency. Its quiescent current is 5mA and it operates over an input/output voltage range of up to 40V.

The switching frequency can be set over a wide range of 100 kHz to 3 MHz. Further efficiency gains are possible using a sense resistor or DCR detection, and 100% duty cycle is provided. Spread-spectrum operation is also possible, and switching frequency can be adjusted within a ±15% range, making it easy to achieve EMC compliance and reducing EMI filter costs.

◇ Circuit diagram and functions

The LTC7803-based converter delivers 3.3V, 20A from a 5V to 38V input. Includes MOSFETs Q1~Q4, inductor L1, and input/output filter capacitors.

The MODE pin and jumper JP1 can be used to configure forced continuous conduction (FCM), pulse skipping, and "Burst Mode®" operation. Each mode has advantages and disadvantages. FCM provides low output ripple across the entire output current range, but has poor efficiency at light loads. Burst mode operation provides high efficiency at light loads and no-load, but has high output voltage ripple at light loads.

Another advantage of the LTC7803 is its ability to be synchronized to an external clock, avoiding interference problems caused by the interaction of different frequencies in the host system. Jumper JP2 for frequency setting allows selection of fixed frequency, synchronization to an external clock, or spread-spectrum operation.

Connecting the MODE pin to GND selects burst mode, and enabling synchronization (providing clock pulses to the PLLIN/Spread pin) causes the controller to operate in FCM mode. However, at light loads, pulse skip mode, which connects the MODE pin to INTVCC through a 100kΩ resistor, provides much better efficiency than FCM.

◇ Efficiency and spread spectrum operation

The above shows the converter efficiency for different input voltages in burst mode.

Above is a thermal image of the DC2834A demo board used to evaluate the LTC7803, along with a full load image of the DC2834A. A key advantage of the LTC7803 is its spread-spectrum operation. This significantly reduces the radiated and conducted noise of the converter, making it easier to achieve overall system-level compliance with EMI standards.

The above shows a comparison of two conducted EMI performances with and without spread spectrum operation against the CISPR 25 Class 5 limits.

The LTC7803 is a low-IQ synchronous step-down controller that simplifies the design of highly efficient power converters. It operates over a wide input/output voltage range and achieves optimal transient response. Features such as burst mode and spread-spectrum operation enhance efficiency and facilitate EMI compliance.



This article is a summary of the article titled "LTC7803 or Easy Design Path to Advanced Power Supply" written by Victor Khasiev, Senior Applications Engineer at Analog Devices (ADI).