Synchronous Step-Down Switching Regulator


The LT3690 is an adjustable frequency monolithic buck switching regulator that accepts input voltages up to 36V. A high efficiency 90mΩ switch is included on the device along with the boost diode and the necessary oscillator, control, and logic circuitry. The internal synchronous power switch of 30mΩ increases efficiency and eliminates the need for an external Schottky catch diode.
Current mode topology is used for fast transient response and good loop stability. Shutdown reduces input supply current to less than 1μA. The low ripple Burst Mode maintains high efficiency at low output currents while keeping output ripple below 15mV in typical applications.
The LT3690 features robust operation and is easily configurable. Using a resistor divider on the UVLO pin provides a programmable under voltage lockout. A power good flag signals when VOUT reaches 90% of the programmed output voltage. Protection circuitry senses the current in the power switches to protect the LT3690 against short-circuit conditions. Frequency fold back and thermal shutdown provide additional protection. The LT3690 is available in a 4mm × 6mm QFN package with exposed pads for low thermal resistance.
Synchronous Step-Down Switching Regulator


  • Wide Input Range:
    – Operation from 3.9V to 36V
    – Overvoltage Lockout Protects Circuits Through 60V Transients
  • 4A Maximum Output Current
  • Integrated 30mΩ N-Channel Synchronous Switch
  • Low Ripple (<15mVP-P) Burst Mode® Operation:
    IQ = 70μA at 12VIN to 3.3VOUT
  • Programmable Input Undervoltage Lockout
  • 0.8V Feedback Reference Voltage
  • Output Voltage: 0.8V to 20V
  • Programmable and Synchronizable Oscillator
    (170kHz to 1.5MHz)
  • Soft-Startup and Output Voltage Tracking
  • Short-Circuit Robust
  • Power Good Flag
  • Small Thermally Enhanced 4mm × 6mm QFN Package

Demonstration circuit 1520A is a 36V, 4A synchronous Buck Regulator with 70μA Quiescent Current featuring the LT3690. The demo board is designed for 3.3V output from a 4.5V to 36V input with transient up to 60V. The wide input range of the LT3690 allows a variety of input sources, such as automotive batteries and industrial supplies. The switching frequency can be programmed either via oscillator resistor or external clock over a 170kHz to 1.5MHz range.

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