9.3.2 Soft Start

The TPS61251 device has an internal charging circuit that controls the current during the output capacitor charging and prevents the converter from inrush current that exceeds the set current limit. For typical 100 µs the current is ramped to the set current limit. After reaching the current limit threshold the output capacitor is charged with a constant current until the programmed output voltage is reached. During the phase where V_IN > V_OUT the rectifying switch is controlled by the current limit circuit and works as a linear regulator in constant current mode. If then V_IN = V_OUT, the converter starts switching and boosting up the voltage to its nominal output voltage by still charging the capacitor with a constant current set by resistor R_ILIM. During constant current charging power dissipation in the TPS61251 device is increased resulting in a thermal rise or heating of the device. If the output capacitor is very large charging time can be long and thermal rise high. To prevent overheating of the device during the charge phase the current will be limited to a lower value when device temperature is high. Please refer to Thermal Regulation.