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LP3382 Datasheet(PDF) 6 Page - Lowpower Semiconductor inc

Part No. LP3382
Description  1.2MHzFixed-Frequency PWM Operation
Download  9 Pages
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Manufacturer  POWER [Lowpower Semiconductor inc]
Direct Link  http://www.lowpowersemi.com
Logo POWER - Lowpower Semiconductor inc

LP3382 Datasheet(HTML) 6 Page - Lowpower Semiconductor inc

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Preliminary Datasheet
LP3382
LP3382 – 01 Version 1.0 Datasheet
OCT.-2011
www.lowpowersemi.com
Page 6 of 9
Applications Information
LED Current Control
The LP3382 regulates the LED current by setting
the current sense resistor (R1) connecting to
feedback and ground. The internal feedback
reference voltage is 250mV. The LED current can
be set from following equation easily.
ILED=250mV/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 LP3382 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 4.7 to 22µH. Small size
and better efficiency are the major concerns for
portable device, such as LP3382 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 LP3382
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 LP3382 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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