LP3390 Datasheet, PDF (6/8 Pages) Lowpower Semiconductor inc – 1.2MHzFixed-Frequency PWM Operation

Preliminary Datasheet
LP3390
get different output current. The LED current can be
switching and the power N-MOSFET will be turned
calculated by the following equation.
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.
Constant Output Voltage Control
Thermal Considerations
The output voltage of the LP3390 can be adjusted by
For continuous operation, do not exceed absolute
the divider circuit on the FB pin. The output voltage
maximum operation junction temperature. The
can be calculated by the following Equations.
maximum power dissipation depends on the thermal
resistance of IC package, PCB layout, the rate of
surroundings airflow and temperature difference
Power Sequence
between junction to ambient. The maximum power
In order to assure the normal soft start function for
dissipation can be calculated by following formula :
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
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.
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
LP3390 Version 1.0 Datasheet
NOV.-2013 www.lowpowersemi.com
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