ZHCSI59I July 2009 – May 2018 TPS65070 , TPS65072 , TPS65073 , TPS650731 , TPS650732
UNLESS OTHERWISE NOTED, this document contains PRODUCTION DATA.
The power save mode is enabled by default. If the load current decreases, the converter will enter power save mode operation automatically. During power save mode the converter skips switching and operates with reduced frequency in PFM mode and with a minimum quiescent current to maintain high efficiency. The converter will position the output voltage typically +1% above the nominal output voltage. This voltage positioning feature minimizes voltage drops caused by a sudden load step.
The transition from PWM mode to PFM mode occurs once the inductor current in the low side MOSFET switch becomes 0.
During the power save mode the output voltage is monitored with a PFM comparator. As the output voltage falls below the PFM comparator threshold of VOUTnominal +1%, the device starts a PFM pulse. For this the high side MOSFET switch will turn on and the inductor current ramps up. Then it will be turned off and the low side MOSFET switch will be turned on until the inductor current becomes 0.
The converter effectively delivers a current to the output capacitor and the load. If the load is below the delivered current the output voltage will rise. If the output voltage is equal or higher than the PFM comparator threshold, the device stops switching and enters a sleep mode with typical 19-µA current consumption.
In case the output voltage is still below the PFM comparator threshold, further PFM current pulses will be generated until the PFM comparator threshold is reached. The converter starts switching again once the output voltage drops below the PFM comparator threshold.
With a single threshold comparator, the output voltage ripple during PFM mode operation can be kept very small. The ripple voltage depends on the PFM comparator delay, the size of the output capacitor and the inductor value. Increasing output capacitor values and/or inductor values will minimize the output ripple.
The PFM mode is left and PWM mode entered in case the output current can not longer be supported in PFM mode or if the output voltage falls below a second threshold, called PFM comparator low threshold. This PFM comparator low threshold is set to –1% below nominal Vout, and enables a fast transition from power save mode to PWM Mode during a load step. In power save mode the quiescent current is reduced typically to 19 µA.
The power save mode can be disabled through the I2C interface for each of the step-down converters independent from each other. If power save mode is disabled, the converter will then operate in fixed PWM mode.