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LP3359
High performance, Current Source For Parallel White-LED Driver
General Description
The LP3359 is a parallel white-LED driver with four matched current outputs. It can supply a total output current of 100mA over an input voltage range of 3.0V to 5.5V. The amount of constant current sourced to the outputs is user selectable using one external sense resistor. LP3359 typically draws 0.01A when placed in shutdown, and 180A when operating in the no-load condition. If any of the outputs are not used, leave the pin(s) unconnected. Brightness can be controlled by PWM techniques or by adding a DC voltage. A PWM signal can be applied to the EN/PWM pin to vary the perceived brightness of the LED. The LP3359 uses an active-high enable level. The LP3359 is available in DFN-8 and TSSOP-8 package.
Features
Regulated IOUT With 0.3% Matching Between Constant Current Outputs Drives One, Two, Three or Four White LED's with No Ballast Resistors 3.0V to 5.5V Input Voltage Up to 100mA Output Current Active-High Enable Very Small Solution Size Very Low Shutdown Current (0.01A typical) Available in DFN-8 ,TSSOP-8 Package RoHS Compliant and 100% Lead(Pb)-Free
Applications
Cellular and Smart mobile phone PDA/DSC LCD Display
Ordering Information
LP3359 - F: PB-Free Package MS: MSOP-8 QV: DFN-8
Typical Application Circuit
LP3359
Pin Configurations
LP3359-datasheet
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LP3359
Functional Pin Description
Pin Number 1 2 3 4 5-8 Pin Name Vin GND EN/PWM Iset LEDx Pin Function Input Voltage Power Ground. Active-High Enable Input - There is no internal pull-down resistor. Current Set Input-The resistor value tied between this pin and ground sets the output current. Current Source Outputs 1- 4 - Connect directly to LED's
Function Block Diagram
Fig1.
LP3359 Block Diagram
Absolute Maximum Ratings
VIN ------------------------------------------------------------------------------------------------------------------0.3V to 6V max EN/PWM-----------------------------------------------------------------------------------------0.3V to (VIN+0.3V) w/ 6 max Continuous Power Dissipation ---------------------------------------------------------------------- Internally Limited DFN-8L , 3 x3,JA -------------------------------------------------------------------------------------------------------- 50C/W TSSOP-8L , JA ----------------------------------------------------------------------------------------------------------- 70C/W Junction Temperature (TJ ) ---------------------------------------------------------------------------------------------150C Storage Temperature Range ------------------------------------------------------------------------------ -65C to 150C Lead Temp (Soldering, 5sec) ------------------------------------------------------------------------------------------ 260C ESD Rating Human Body Model -----------------------------------------------------------------------------------------2kV
Operating Conditions
Input Voltage (VIN) -------------------------------------------------------------------------------------------------- 3V to 5.5V Ambient Temperature (TA) ---------------------------------------------------------------------------------- -40C to 85C
LP3359-datasheet
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Electrical Characteristics
(Limits in standard typeface and typical values apply for TA=25C. Limits in boldface type apply over the operating junction temperature range (-40C~+85C). Unless otherwise specified: VIN=5V, VLEDX=3.6V, RSET=7.5k, V(EN/PWM)=VIN.)
Symbol
ILEDx ILEDX-MATCH
Description
LED Current Current Matching Between Any Two Outputs
Conditions
3.0V VIN 5.5V 2.0V VLEDX (VIN-0.4V) RSET=7.5k
Min
14.3
Typ
15.8
Max
17.3
Units
mA %
0.3 1.06 (-8%)
1 1.3 (+8%)
VSET
ISET Pin Voltage Output Current to Current
1.18 100
V
ILEDX/ISET VHR
Set Ratio Current Source Headroom Voltage ILED=95% x ILED(nom), RSET= 4.7K (ILED(nom) approx. 25mA) ILED=95% x ILED(nom), RSET= 12K (ILED(nom) approx. 10mA) ILED=0mA, RSET=Open
320 130
440 220
mV
175 325 0.01
285
uA
IQ
Quiescent Supply Current ILED=0mA, RSET=7.5k
ISHUT-DOWN
Shutdown Supply Current
EN/PWM=0
1
V
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LP3359
Typical Operating Characteristics
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LP3359
Applications Information
Enable/Shutdown When the voltage on the active-high-logic enable pin is low, the LP3359 will be in shutdown. While disabled, the LP3359 typically draws 0.01A. There is no internal pull-up or pull-down on the PWM pin of the LP3359, Do not let PWM pin floating. Output Current Capability The LP3359 is capable of providing up to 25mA of current to each of the four outputs given an input voltage of 3.0V to 5.5V. The outputs have a typical current matching of 0.3% between adjacent sources. An external resistor can be used to set the output current, as approximated with the following the equation: Brightness Control
(1)Using a PWM Signal to EN/PWM Pin
should be avoided.
In order for the output currents to be regulated properly, sufficient headroom voltage (VHR) must be present. The headroom voltage refers to the minimum amount of voltage that must be present across the current source in order to ensure the desired current is realizable. To ensure the desired current is obtained, apply the following equations to find the minimum input voltage required:
Brightness control can be implemented by pulsing a signal at the PWM pin. The RSET value should be selected using the RSET equation. LED brightness is proportional to the duty cycle (D) of the PWM signal. For linear brightness control over the full duty cycle adjustment range, the PWM frequency (f) should be limited to accommodate the turn-on time (TON = 20s) of the de- vice.
VLEDX is the diode forward voltage, and VHR is defined by the following equation:
ILEDX is the desired diode current, and kHR, typically 15mV/mA in the LP3359, is a proportionality constant that represents the ON-resistance of the internal current mirror transistors. For worst-case design calculations, using a kHR of 20mV/mA is recommended. (Worst-case recommendation accounts for parameter shifts from part-to-part variation and applies over the full operating temperature range). Changes in headroom voltage from one output to the next, possible with LED forward voltage mismatch, will result in different output currents and LED brightness mismatch. Thus, operating the LP3359 with insufficient headroom voltage across all current sources
If the PWM frequency is much less than 100Hz, flicker may be seen in the LEDs. For the LP3359, zero duty cycle will turn off the LEDs and a 50% duty cycle will result in an average ILED being half of the programmed LED current. For example, if RSET is set to program 15mA, a 50% duty cycle will result in an average ILED of 7.5mA, LED being half the programmed LED current. RSET should be chosen not to exceed the maximum current delivery capability of the device. (2)Using a DC Voltage Added to RSET Using an analog input voltage VADJ via a resister RADJ connects to the RSET pin can also be used to achieve setting LED current. Figure 3 shows this application circuit. For this application the LED's current can be derived from the following Equation. Figure 4 and table 2 shows the relation between VADJ and ILED of a typical application example, where the VADJ from 0 to 2.5V, RSET equals 11.5k and RADJ
5
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LP3359
equals 12.5k. the LED process variations due to the constant current outputs of the LP3359. Parallel LEDx Outputs for Increased Current Driver Outputs LED1 through LED4 may be connected together in any combination to drive higher currents through fewer LEDs. For example in Figure 5, outputs LED1 and LED2 are connected together to drive one LED while LED3 and LED4 are connected together to drive a second LED.
Figure4. LED current setting example which using a DC voltage to Rset
Figure 5. Two Parallel Connected LEDs With this configuration, two parallel current sources of equal value provide current to each LED. RSET should therefore be chosen so that the current through each output is programmed to 50% of the desired current through the parallel connected LEDs. For example, if 30mA is the desired drive current for 2 parallel connected LEDs, RSET should be selected so that the current through each of the outputs is 15mA. Other combinations of parallel outputs may be implemented in similar fashions, such as in Figure 6. 1.2 10.1 2.5 0
VADJ(V) ILED(mA) VADJ(V) ILED(mA)
0 19.7 1.4 8.5
0.2 18.1 1.6 6.9
0.4 16.5 1.8 5.3
0.6 14.9 2 3.7
0.8 13.3 2.2 2.1
1 11.7 2.4 0.5
LED Selection The LP3359 is designed to drive white-LEDs with a typical forward voltage of 3.0V to 4.0V. The maximum LED forward voltage that the LP3359 can accommo -date is highly dependant upon VIN and ILEDX (See the section on Output Current Capability for more information on finding maximum VLEDX.) For applications that demand color and brightness matching, care must be taken to select LEDs from the same chromaticity group. Forward current matching is assured over
Figure 6. One Parallel Connected LED
LP3359-datasheet
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LP3359
Connecting outputs in parallel does not affect internal operation of the LP3359 and has no impact on the Electrical Characteristics and limits previously presented. The available diode output current, maximum diode voltage, and all other specifications provided in the Electrical Characteristics table apply to parallel output configurations, just as they do to the standard 4-LED application circuit. Power Consumption It is recommended that power consumed by the circuit (VIN x IIN) be evaluated rather than power efficiency. Figure 7 shows the power consumption of the LP3359 Typical Application Circuit.
Where T is the maximum junction temperature, T is the ambient temperature, and JA is the junction -to-ambient thermal resistance of the specified package. The LP3359 come in the DFN-8 package that has a junction-to-ambient thermal resistance (JA)equal to 50/W.This value of JA is highly dependant upon the layout of the PC board. The actual power dissipated by the LP3359 follows the equation:
Where N equals the number of active outputs, VLEDX is the LEDX LED forward voltage, and ILEDX is the current supplied to the LEDX diode by the LP3359. Power dissipation must be less than that allowed by the package. Please refer to the Absolute Maximum Rating of the LP3359. Input Capacitor Selection The LP3359 is designed to run off of a fixed input voltage. Depending on the stability and condition of this voltage rail, it may be necessary to add a small input capacitor to help filter out any noise that may be present on the line. In the event that filtering is needed, surface mount multi-layer ceramic capacitors are recommended. These capacitors are small and inexpensive. A capacitance of 0.1F is typically sufficient.
Figure 7. 4LEDs, LED VF=2.7V, ILED=15mA Power Dissipation
The maximum allowable power dissipation that this package is capable of handling can be determined as follows:
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Applications Information
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