Chris Richardson
National Semiconductor

The constant on-time (COT) control method used by the LM3402 and LM3404 constant-current buck regulators provides a balance between control over switching frequency and fast transient response. Normally, this "quasi-hysteretic" control senses the input voltage and adjusts the on-time tON of the power MOSFET as needed to keep fSW constant. Investigating a little more deeply reveals that tON is in fact proportional to the current flowing into the RON pin. The addition of a single, general purpose PNP transistor forces tON to be proportional to (VIN-VO) and provides two benefits that are particularly useful to LED drivers: improved tolerance of the average LED current, IF, and constant LED ripple current.

This paper discusses the benefits of constant ripple and designing for constant ripple in a COT converter. It explains how, armed with the ability to control both LED ripple current and switching frequency, the LED lighting designer can make his/her own trade-offs between solution size, cost, and quality based on the needs of the application.