Preliminary Datasheet
LP3311S
High Performance, Constant Current Switching
Regulator For 8PCS White LED In Series
General Description
The LP3311S is a 1MHz PWM boost switching regulator
designed for constant current white LED driver applications.
The LP3311S can drive a string of up to 8 white LED in
series, ensuring uniform brightness and eliminating several
ballast resistors. The LP3311S implements a constant
frequency 1MHz PWM control scheme. The high frequency
PWM operation also saves board space by reducing
external component size. To improve efficiency, the
feedback voltage is set to 200 mV, which reduces the
power dissipation in the current setting resistor. Highly
integration and internal compensation network minimizes
as 5 external component counts. Optimized operation
frequency can meet the requirement of small LC filters
value and low operation current with high efficiency.
Features
High Efficiency: 90%
1MHz Fixed-Frequency PWM Operation
Maximum Output Voltage up to 29V
Operating Range : 2.7V to 5.5V
Shutdown Supply Current:<1uA
Available in SOT23-6 Package
Built-in 29V Over Voltage Protection
Minimize the External Component
RoHS Compliant and 100% Lead
(Pb)-Free
Applications
WLED Backlight driver
OLED Backlight driver
PDA
MID/PTV
Ordering Information
LP3311S
F: Pb-Free
Package:
B6: SOT23-6
Typical Application Circuit
Vin
10uF
4.7uH
1uF
Vin
SW
Pin Configurations
EN
OVP
IN
OVP
EN
6
5
4
GND
FB
LP3311S
R1
1
2
3
SW
GND
FB
SOT-23-6
LP3311s – 02 Version 2.0 Datasheet
May.-2013
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Page 1 of 7
Preliminary Datasheet
LP3311S
Functional pin description
SOT-23-6
1
2
Pin Name
SW
GND
Pin Function
Switch Pin. Connect this Pin to inductor and catch diode. Minimize the
track area to reduce EMI.
Ground Pin
Feedback Reference Voltage Pin. Series connect a resistor between
3
FB
WLED and ground as a current sense. Sense the current feedback
voltage to set the current rating.
Chip Enable (Active High). Voltage sensing input to trigger the function of
4
EN
over voltage protection. Note that this pin is high impedance. There
should be a pull low 100kΩ resistor connected to GND when the control
signal is floating.
5
6
OVP
Vin
Over voltage pin.
Supply Input Voltage Pin. Bypass 1uF capacitor to GND to reduce the
input noise.
Function Block Diagram
IN
OUT
SW
200 mV
Soft
start
OTA
PWM_COMP.
Over
voltage
R
Q
S
Driver
FB
PWM
∑
A1
RS
4mS
DELAY
SLOPE
EN
ENABLE
1MHz
OBCILLATOR
GND
LP3311s – 02 Version 2.0 Datasheet
May.-2013
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Preliminary Datasheet
LP3311S
Absolute Maximum Ratings
Supply Input Voltage---------------------------------------------------------------------------------------------------−0.3V to 6.0V
SW
Input Voltage --------------------------------------------------------------------------------------------------------−0.3V to 43V
The Other Pins ----------------------------------------------------------------------------------------------------------−0.3V to 5.5V
Power Dissipation, P
D
@ T
A
= 25°C TSOT-23-5/6-----------------------------------------------------------------------0.455W
Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------------- 260°C
Operation Temperature Range ---------------------------------------------------------------------------------
−
40°C to 80°C
Storage Temperature Range ----------------------------------------------------------------------------------------
−
65°C to 150°C
Electrical Characteristics
Parameter
Symbol
Test Condition
Min
Typ.
Max
Units
System Supply Input
Operation voltage Range
Under Voltage Lock Out
Supply Current
Shut Down Current
Line Regulation
Oscillator
Operation Frequency
Maximum Duty Cycle
Feedback Voltage
MOSFET
On Resistance of MOSFET
SW Current Limit
Protection
OVP Threshold
OVP Sink Current
Shut Down Voltage
Enable on Voltage
VEN
VEN
0.4
1.4
VOVP
29
5
V
μA
V
V
RDS(ON)
ILM
0.5
1
Ω
A
LP3311S
FOSC
0.8
93
185
200
215
1
1.2
MHz
%
mV
VDD
VDD
IDD
IDD
FB=0.2V, Switch
VEN < 0.4V
VIN : 3.0~4.3V
2.7
2.2
2.4
430
0.1
2
5.5
2.6
550
1
V
V
uA
uA
%
LP3311s – 02 Version 2.0 Datasheet
May.-2013
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Page 3 of 7
Preliminary Datasheet
LP3311S
Typical Operating Characteristics
LP3311s – 02 Version 2.0 Datasheet
May.-2013
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Preliminary Datasheet
LP3311S
Applications Information
LED Current Control
The LP3311S regulates the LED current by setting the
current sense resistor (R1) connecting to feedback
and ground. The internal feedback reference voltage is
200mV. The LED current can be set from following
equation easily.
ILED=200mV/R1
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended). The
table for R1 selection is shown below.
Inductor Selection
The recommended value of inductor for 2 to 10
WLEDs applications are 4.7 to 22µH. Small size and
better efficiency are the major concerns for portable
device, such as LP3311S used for mobile phone. The
inductor should have low core loss at 1MHz and low
DCR for better efficiency. To avoid inductor saturation
current rating should be considered.
Vin
10uF
4.7uH
1uF
GND
FB
LP3311S
PWM
Vin
SW
Vin
10uF
4.7uH
1uF
EN
OVP
10R
b. Using a DC Voltage
Using a variable DC voltage to adjust the brightness is
a popular method in some applications. The dimming
control using a DC voltage circuit is shown in Figure 4.
According to the Superposition Theorem, as the DC
voltage increases, the voltage contributed to VFB
increases and the voltage drop on R2 decreases, i.e.
the LED current decreases. For example, if the VDC
range is from 0V to 2.8V, the selection of resistors in
Figure 4 sets dimming control of LED current from
20mA to 0mA.
Dimming control
a. Using a PWM Signal to EN Pin
For controlling the LED brightness, the LP3311S 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 1MHz can
Vin
SW
EN
OVP
GND
FB
LP3311S
5K
65K
insignificantly reduce audio noise when dimming. The
average LED current is proportional to the PWM signal
duty cycle. The magnitude of the PWM signal should
be higher than the maximum enable voltage of EN pin,
in order to let the dimming control perform correctly.
c. Using a Filtered PWM signal
VDC
10R
Another common application is using a filtered PWM
signal as an adjustable DC voltage for LED dimming
control. A filtered PWM signal acts as the DC voltage
to regulate the output current. The recommended
LP3311s – 02 Version 2.0 Datasheet
May.-2013
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Page 5 of 7
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