LP3383 datasheet(6/9 Pages) POWER

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Preliminary Datasheet

LP3383

LP3383 – 00 Version 1.0 Datasheet

OCT.-2012

www.lowpowersemi.com

Page 6 of 9

Applications Information

LED Current Control

The LP3383 regulates the LED current by setting

the current sense resistor (R1) connecting to

feedback and ground. The internal feedback

reference voltage is 300mV. The LED current can

be set from following equation easily.

ILED=300mV/R1

In order to have an accurate LED current, precision

resistors are preferred (1% is recommended). The

table for R1 selection is shown below.

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 LP3383 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℃, the chip also will

enter protection mode. The power MOSFET will be

turned off during protection mode to prevent

abnormal operation.

Inductor Selection

The recommended value of inductor for 2 to

8WLEDs applications are 2.2 to 22µH. Small size

and better efficiency are the major concerns for

portable device, such as LP3383 used for mobile

phone. The inductor should have low core loss at

1.3MHz and low DCR for better efficiency. To

avoid inductor saturation current rating should be

considered.

Capacitor Selection

Input ceramic capacitor of 2.2uF and output ceramic

capacitor of 1uF are recommended for the LP3383

applications for driving 8 series WLEDs. For better

voltage filtering, ceramic capacitors with low ESR

are recommended. X5R and X7R types are suitable

because of their wider voltage and temperature

ranges.

Dimming control

a. Using a PWM Signal to EN Pin

For controlling the LED brightness, the LP3383 can

perform the dimming control by applying a PWM

signal to EN pin. The internal soft-start and wide

range dimming frequency from 100Hz to 200KHz

can insignificantly reduce audio noise when

dimming. The average LED current is proportional

to the PWM signal duty cycle. The magnitude of the

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