LP3390 datasheet(6/8 Pages) POWER

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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