This application note introduces methods for tuning the adjustable compensation of the LTC®3886 to achieve optimal phase margin, bandwidth and transient response. The information in this note is applicable to all constant frequency peak current mode power system management controllers (PSMCs) in the ADI family with adjustable compensation. In typical designs using current-mode PSMCs, the device is adapted to the design requirements by manually choosing the capacitors and resistors for the compensation network. PSMCs with adjustable compensation contain internal circuitry that quickly and easily modify the compensation via PMBus, allowing the user to optimize a solution with greater ease. By eliminating the need to make external component changes on the PCB to change loop properties, adjustments to the circuit can be made dynamically to expedite the tuning process.
In addition to demonstrating the techniques by which optimal phase margin, bandwidth and transient response time is achieved, this note addresses the trade-offs that come along with adjusting the compensation. The goal is to familiarize the reader with the DC2155A demo board and its supporting software, to develop an understanding of compensation network adjustment and to provide a tuning procedure. The first part of this note examines the compensation design, provides a tuning sequence, explains the PSMC open-loop and closed-loop transfer function and offers some general design advice. The focus of the note then shifts to the effects of adjusting the compensation.