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, ΔiF.
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