ZHCSI15L June 2006 – May 2018 TPS65023 , TPS65023B
PRODUCTION DATA.
As the load current decreases, the converters enter the power save mode operation. During PSM, the converters operate in a burst mode (PFM mode) with a frequency between 750 kHz and 2.25 MHz, nominal for one burst cycle. However, the frequency between different burst cycles depends on the actual load current and is typically far less than the switching frequency with a minimum quiescent current to maintain high efficiency.
To optimize the converter efficiency at light load, the average current is monitored and if in PWM mode the inductor current remains below a certain threshold, then PSM is entered. The typical threshold to enter PSM is calculated as shown in Equation 1, Equation 2 and Equation 3.
During the PSM the output voltage is monitored with a comparator, and by maximum skip burst width. As the output voltage falls below the threshold, set to the nominal VO, the P-channel switch turns on and the converter effectively delivers a constant current defined in Equation 4, Equation 5 and Equation 6.
If the load is below the delivered current then the output voltage rises until the same threshold is crossed in the other direction. All switching activity ceases, reducing the quiescent current to a minimum until the output voltage has again dropped below the threshold. The power save mode is exited, and the converter returns to PWM mode if either of the following conditions are met:
These control methods reduce the quiescent current to typically 14 μA per converter, and the switching activity to a minimum, thus achieving the highest converter efficiency. Setting the comparator thresholds at the nominal output voltage at light-load current results in a low output voltage ripple. The ripple depends on the comparator delay and the size of the output capacitor. Increasing capacitor values makes the output ripple tend to zero. The PSM is disabled through the I2C interface to force the individual converters to stay in fixed frequency PWM mode.