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6 UVLO and Thermal Shutdown If VIN drops below 2V, the UVLO circuit inhibits switching. Once VIN rises above 2.1V, the UVLO clears, and the soft-start sequence activates. Thermal-overload protection limits total power dissipation in the device. When the junction temperature exceeds TJ= +160°C, a thermal sensor forces the device into shutdown, allowing the die to cool. The thermal sensor turns the device on again after the junction temperature cools by 15°C, resulting in a pulsed output during continuous overload conditions. Following a thermal-shutdown condition, the soft-start sequence begins. DESIGN PROCEDURE Inductor Selection The peak-to-peak ripple is limited to 30% of the maximum output current. This places the peak current far enough from the minimum overcurrent trip level to ensure reliable operation while providing enough current ripples for the current mode converter to operate stably. In this case, for 2A maximum output current, the maximum inductor ripple current is

667 mA. The inductor size is estimated as following equation: LIDEAL=(VIN(MAX)-VOUT)/IRIPPLE*DMIN*(1/FOSC) Therefore, for VOUT=1.8V, The inductor values is calculated to be L = 0.60μH. Choose 1μH And for VOUT =1.2V, The inductor values is calculated to be L = 0.469μH. Choose 0.47μH The resulting ripple is IRIPPLE =(VIN(MAX)-VOUT)/LACTUAL*DMIN*(1/FOSC) When, VOUT=1.8V, IRIPPLE = 403mA VOUT=1.2V, IRIPPLE = 665mA Output Capacitor Selection For most applications a nominal 10μF or 22μF capacitor is suitable. The internal compensation is designed for a fixed corner frequency that is equal to FC=

1 2?π??????L = 50Khz For example, for VOUT=1.8V, L=1μH, COUT=10μF, for VOUT =1.2V, L=0.47μH, COUT=22μF Setting Output Voltage Output voltages are set by external resistors. The FB_ threshold is 0.6V. RTOP = RBOTTOM x [(VOUT / 0.6) - 1] Guidelines for Input Capacitor and Output Capacitor The input capacitor in a DC-to-DC converter reduces current peaks drawn from the battery or other input power source and reduces switching noise in the controller. The impedance of the input capacitor at the switching frequency should be less than that of the input source so high-frequency switching currents do not pass through the input source. The output capacitor keeps output ripple small and ensures control-loop stability. The output capacitor must also have low impedance at the switching frequency. Ceramic, polymer, and tantalum capacitors are suitable, with ceramic exhibiting the lowest ESR and high-frequency impedance. Output ripple with a ceramic output capacitor is approximately as follows: VRIPPLE = IL(PEAK)[1 / (2π x fOSC x COUT)] If the capacitor has significant ESR, the output ripple component due to capacitor ESR is as follows: VRIPPLE(ESR) = IL(PEAK) x ESR 深圳市科........