5.4 Buck1 Hysteretic Control Mode (HCM) / B1HCEN Bit

If Buck1 is set in AutoPFM mode while the input voltage (e.g. a discharging battery) is decreasing and eventually pushing Buck1 to 100% duty cycle (bypass mode), the operational no-load quiescent current of the MCP16503 shows some increase due to the augmented switching activity of Buck1.

This is intrinsic to the AutoPFM architecture. The peak in the quiescent current is in the 1 mA range, and it may or may not be detrimental to the overall system efficiency and/or battery life, depending mostly on the minimum loading of Buck1.

If the increase in quiescent current when approaching bypass mode on Buck1 is an important factor for the application, the user can optionally activate a different mode of light-load high-efficiency operation (called Hysteretic Control Mode, HCM) where the output voltage is controlled in a hysteretic fashion between the nominal output voltage and +2.9% of it. This is done by setting bit B1HCEN (Buck1 Hysteretic Control Enable) in register SYS-CFG through I²C. This control method reduces significantly the average switching activity of Buck1, especially in proximity of the bypass operation, at the expense of an increase of the output ripple (typically 2.9% of the nominal output voltage).

The user should therefore carefully evaluate the need for activation of HCM for Buck1 and balance the increase in output ripple against the real benefit achieved in prolonging battery life. If the minimum loading on Buck1 is always significantly higher than 1 mA, activation of HCM is typically not needed.

The relevant EC Table parameter that defines the upper voltage regulation threshold (typically +3.45% of the nominal output voltage) is the Hysteretic Control Mode Upper Regulation Threshold.

Even if bit B1HCEN is set, HCM mode will only be activated when the input-to-output voltage differential decreases below a certain value. This is done to prevent fast inductor charging, which in turn may cause a poorer control of the effective upper regulation voltage.

The relevant EC Table parameter that defines the input voltage threshold (falling input voltage) below which HCM is enabled is the Hysteretic Control Mode Enable Threshold, and it is also expressed as a percentage of the nominal output voltage value (typically 2.9).