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LP3390 Datasheet(PDF) 6 Page - Lowpower Semiconductor inc |
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LP3390 Datasheet(HTML) 6 Page - Lowpower Semiconductor inc |
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6 / 8 page  Preliminary Datasheet LP3390 LP3390 Version 1.0 Datasheet NOV.-2013 www.lowpowersemi.com Page 6 of 8 get different output current. The LED current can be calculated by the following equation. Constant Output Voltage Control The output voltage of the LP3390 can be adjusted by the divider circuit on the FB pin. The output voltage can be calculated by the following Equations. Power Sequence In order to assure the normal soft start function for suppressing the inrush current the input voltage should be ready before EN pulls high. Soft-Start The function of soft-start is made for suppressing the inrush current to an acceptable value at the beginning of power on. The LP3390 provides a built-in soft-start function by clamping the output voltage of error amplifier so that the duty cycle of the PWM will be increased gradually in the soft-start period. Current Limiting The current flow through inductor as charging period is detected by a current sensing circuit. As the value comes across the current limiting threshold, the N-MOSFET will be turned off so that the inductor will be forced to leave charging stage and enter discharging stage. Therefore, the inductor current will not increase over the current limiting threshold. OVP/UVLO/OTP The Over Voltage Protection is detected by a junction breakdown detecting circuit. Once VOUT goes over the detecting voltage, LX pin stops switching and the power N-MOSFET will be turned off. Then, the VOUT will be clamped to be near VOVP. As the output voltage is higher than a specified value or input voltage is lower than a specified value, the chip will enter protection mode to prevent abnormal function. As the die temperature is higher then 160°C, the chip also will enter protection mode. The power MOSFET will be turned off during protection mode to prevent abnormal operation. Thermal Considerations For continuous operation, do not exceed absolute maximum operation junction temperature. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow and temperature difference between junction to ambient. The maximum power dissipation can be calculated by following formula : Where TJ(MAX) is the maximum operation junction temperature, TA is the ambient temperature and the qJA is the junction to ambient thermal resistance. For the recommended operating conditions specification of LP3390, the maximum junction temperature of the die is 125°C. The junction to ambient thermal resistance Qja is layout dependent. |
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Similar Description - LP3390 |
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