9.4.1 Power-Save Mode

Setting the MODE/DATA pin to low for both converters enables power-save mode. If the load current of a converter decreases, this converter enters power-save-mode operation automatically. The transition of a converter to power-save mode is independent from the operating condition of the other converter. During power-save mode, the converter operates with reduced switching frequency in PFM mode and with a minimum quiescent current to maintain high efficiency. The converter positions the output voltage in PFM mode to typically 1% above nominal V_OUTx. This voltage positioning feature minimizes voltage drops caused by a sudden load step.

In order to optimize the converter efficiency at light load, the device monitors average inductor current. The device changes from PWM mode to power-save mode if in PWM mode the inductor current falls below a certain threshold. The typical output current threshold, which one can calculate using Equation 1 for each converter, depends on V_IN.

Equation 1: Average output current threshold to enter PFM mode

Equation 1.

Equation 2: Average output current threshold to leave PFM mode

Equation 2.

To keep the output-voltage ripple in power-save mode low, a single threshold comparator (skip comparator) monitors the output voltage. As the output voltage falls below the skip-comparator threshold (skip comp) of 1% above nominal V_OUTx, the corresponding converter starts switching for a minimum time period of typically 1 μs and provides current to the load and the output capacitor. Therefore, the output voltage increases and the device maintains switching until the output voltage trips the skip comparator threshold (skip comp) again. At this moment, all switching activity stops and the quiescent current reduces to minimum. The output capacitor supplies the load until the output voltage has dropped below the threshold again. Hereupon, the device starts switching again.

The converter leaves power-save mode and enters PWM mode if the output current exceeds the I_{OUT_PFM_leave} current or if the output voltage falls below a second comparator threshold, called the skip-comparator-low (Skip Comp Low) threshold. This skip-comparator-low threshold is 2% below nominal V_OUTx and enables a fast transition from power-save mode to PWM mode during a load step.

Power-save mode typically reduces the quiescent current to 19 μA for one converter and 32 μA for both converters active. This single-skip comparator threshold method in power-save mode results in a very low output-voltage ripple. The ripple depends on the comparator delay and the size of the output capacitor. Increasing output capacitor values minimizes the output ripple. One can disable the power-save mode by setting the MODE/DATA pin to high. Both converters then operate in fixed PWM mode. Power-save mode enable or disable applies to both converters.