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LDS8724
40 WLED Driver with Boost Converter
FEATURES
High efficiency boost converter with the input voltage range from 2.7 to 5.5 V No external Schottky Required (Internal synchronous rectifier*) 250 mV current sense voltage Drives up to 5 LEDs per string with up to 42 LED in 6 strings at 25 mA 1 MHz Switching Frequency Efficiency greater than 83% PWM LED Dimming Control Mode Over-voltage, under-voltage, over-current, and over-temperature protection Low Shutdown Current (<1A) Available in 2 x 3 x 0.8 mm 8-pin TDFN package
resistance. Under-voltage protection disables the part when VIN voltage reaches 2.0 V The LDS8724 has thermal shutdown set at 150 C. Above this value , the boost converter stops switching. The part resumes normal operations when 0 temperature drops below 130 C. The over-current protection provides cycle-by-cycle current limit. The internal output over-voltage protection prevents damage in the case of a faulty LED disconnect. The PWM control ensures brightness adjustment with a frequency from 100 Hz up to 30 KHz. The EN/PWM logic input functions as a chip enable and LED current PWM contol pin. The device is available in 8-pin TDFN 2 x 3 mm package with a max height of 0.8 mm. Table 1 Matrix configuration for LDS8724 Number of strings 1-6 7, 8 9 10 11 12 Number of LED per string 5 5 5 4 4 3 String Current max, mA, 30 25 25 25 25 30 V IN
MIN, 0
APPLICATION
Color Display Backlight Portable Navigation and GPS Receivers Smart phones Digital Photo Frames Portable DVD Players, Notebooks
DESCRIPTION
The LDS8724 is a fixed frequency current mode boost converter with internal synchronous rectifier and cycle-by-cycle switch current limit specifically designed to drive matrix of LEDs in respect with the Table 1. Operation at 1 MHz allows use of small value low profile inductor (10 - 33 H) and 1 F 50 V ceramic capacitor. The use of integrated synchronous rectifier makes the efficiency dependent on only inductor DC
V
2.7 2.7 3.0 2.7 3.0 3.0
TYPICAL APPLICATION CIRCUIT
*) Patent pending (c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8724DS, Rev. N1.0
LDS8724
ABSOLUTE MAXIMUM RATINGS
Parameter V IN voltage V OUT, SW, LEDS, VHVB voltage EN/SET voltage Storage Temperature Range Junction Temperature Range 8-pin TDFN package Thermal Resistance Soldering Temperature Rating 6 40 V IN + 0.7V -65 to +160 -40 to +125 65 300 Unit V V V C C C/W C
RECOMMENDED OPERATING CONDITIONS
Parameter V IN Ambient Temperature Range Rating 2.7 to 5.5 -40 to +85 Unit V C
Typical application circuit with external components is shown on page 1.
ELECTRICAL OPERATING CHARACTERISTICS
(Over recommended operating conditions unless specified otherwise) V IN = 3.6V,TAMB = 25C, C IN = 1 F, COUT = 1 F, L = 22 H
Name Quiescent Current Shutdown Current LED Current Accuracy Oscillator Frequency Maximum Duty Cycle NMOSFET Switch On Resistance PMOSFET Switch On Resistance Switch Leakage Current Switch Current Limit Dropout/Current sense voltage PWM Frequency EN/PWM Input Resistance (pull down) Pin High Logic Level Low Thermal Shutdown Thermal Hysteresis Over-Voltage Protection Soft Start Time Delay Time to Shutdown
Conditions V IN = 3.6 V, VOUT open V IN = 3.6 V, ILED = 25 mA @ VOUT =36 V V IN = 5.5V, EN = 0V At factory preset value
Min
Typ 1 3
Max
Units mA
1 3 1.0 92 0.3 1.5 0.1 1.9 250 0.1 250 1.4 0.4 150 20 16 1.5 10 30
At factory preset value
A % MHz % A A mV kHz k V C V ms ms
EN = 0
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8724DS, Rev. N1.0
LDS8724
TYPICAL CHARACTERISTICS
Vin = 3.6V, IOUT = 75mA ( 3 strings with 4 LEDs at 25 mA per string, C IN = 1 COUT = 1 L = 22 H, TAMB = 25C unless otherwise specified F, F,
Power Efficiency vs. Input Voltage
LED Current per string vs. Input Voltage
LED Current vs. PWM Duty Cycle
LED Current Error vs. PWM Frequency
Soft Start Waveforms
Operating Waveforms
Ch 1 (yellow) - EN pin, Ch 2 (green) - SW node voltage Ch 3 (red) - Inductor current, Ch 4 (blue) - VOUT
Ch 1 (yellow) - SW node voltage, Ch 3 (red) - Inductor current
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8724DS, Rev. N1.0
LDS8724
TYPICAL CHARACTERISTICS
Vin = 3.6V, IOUT = 30mA (4 LEDs per string), CIN = 1 COUT = 1 L = 22 H, TAMB = 25C unless otherwise specified F, F,
Waveforms at PWM LED Current Regulation
Waveforms at PWM LED Current Regulation
Ch 1 (yellow) - PWM signal at 1 kHz 5% duty cycle C2 (green) - SW node voltage, Ch 3(red) - LED Current. Ch4 (blue) - VOUT
Ch 1 (yellow) - PWM signal at 1 kHz 98% duty cycle C2 (green) - SW node voltage, Ch 3(red) - LED Current. Ch4 (blue) - VOUT
Waveforms at PWM LED Current Regulation
Waveforms at PWM LED Current Regulation
Ch 1 (yellow) - PWM signal at 30 kHz 25% duty cycle C2 (green) - SW node voltage, Ch 3(red) - LED Current. Ch4 (blue) - VOUT
Ch 1 (yellow) - PWM signal at 30 kHz 98% duty cycle C2 (green) - SW node voltage, Ch 3(red) - LED Current. Ch4 (blue) - VOUT
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8724DS, Rev. N1.0
LDS8724
PIN DESCRIPTION
Pin # 1 2 3 4 5 6 7 8 PAD Function Input Voltage, connect to battery or power supply Analog Ground Power Ground Switch input; Connect inductor terminal to this pin Output voltage; Connect the LED string anode V OUT terminal to this pin. High Voltage Bypass; Connect a 470 pF capacitor HVB between this point and VOUT Current Sense input. Connect LED string cathode LEDS terminal to this pin EN/PWM Device Enable (active high) and Dimming Control PAD Connect to GND on the PCB Name VIN GND PGND SW
Top view: TDFN 8-lead 2 X 3 mm
PIN FUNCTION
V IN is the supply pin for the driver. A small 1 F ceramic bypass capacitor is required between the VIN pin and ground near the device. The operating input voltage range is from 2.7 V to 5.5 V. If the input supply voltage falls below the under-voltage threshold, switch is disabled, and the device enters shutdown mode. HVB is the internal high voltage reference point. Connect a 4700 pF capacitor between this point and V OUT. EN/PWM is the enable and PWM control logic input. Guaranteed levels of logic high and logic low are set at 1.4 V and 0.4 V respectively. When EN/PWM is initially taken high, the device becomes enabled and LED current sets to maximum programmed value.. To place the device into shutdown mode, the EN/PWM pin must be held low for more than 10 ms. V OUT is the driver output. Connect it to the LED anode. A ceramic bypass capacitor of 1 F requires between the VOUT pin and ground near the device. GND is the ground reference for the analog circuits. The pin must be connected to the ground plane on the PCB. Avoid high currents flowing trough traces connecting this pin with EN/PWM signal source. PGND is the current return for high current circuits. The pin must be connected to the ground plane on the PCB. Connect GND and PGND as close to the driver as possible. LEDS is the Current Sense pin that provides internal regulated current sink for LED string. Connect LED string cathode to this pin. This pin enter highimpedance zero current state whenever the device is in shutdown mode. PAD is the exposed pad underneath the package. For best thermal performance, the pad should be soldered to the PCB and connected to the ground plane
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8724DS, Rev. N1.0
LDS8724
BLOCK DIAGRAM
Figure 2. LDS8724 Functional Block Diagram
BASIC OPERATION
At power-up, EN/PWM pin should be logic LOW. The LDS8724 starts operating when EN/PWM pin is asserted logic high. If the input voltage is sufficient to regulate all LED currents, the device remains in operating mode. The low dropout Current regulator performs well at VOUT voltage up to 250 mV above summary LED forward voltage significantly increasing driver's efficiency. If the input voltage is insufficient or falls to a level where the regulated currents cannot be maintained, the Under-Voltage protection turns device off setting it in shutdown mode. The LDS8724 has soft start function that prevent high input current spike at device' wake-up. The EN/PWM pin should be held low for more than 10 ms to completely turn device in low current shutdown mode. LED Current Setting The maximum current sink value in the LEDS pin is factory preset at 30 mA. An external resisror RFB should be used to bypass current if more than one string is connected. RFB = 0.25 V / (I x M - 0.03), ohms, where 0.25 V is a feedback voltage, I - is current value per string in ampers, and M is number of strings connected in parallel. Power dissipated at R FB is equal W = 0.0625/R FB, watt. In case of three strings with 25 mA current per string R FB = 0.25/(0.025 x 3 - 0.03) = 5.6 , and dissipated power W = 0.0625/5.6 = 0.011 W. The average LED string current that determines LED brightness may be controlled applying PWM signal to the EN/PWM pin. The maximum PWM frequency is 30 kHz, while frequiencies below 100 Hz are not recommended to avoid visible LED flikering. Duty cycle that determines average LED string current may vary in the range from 5% to 100% at 1 kHz or from 20% to 100% at 30 kHz with high linearity current regulation. Protection Mode The output voltage VOUT is limited at about 36 V. This is to prevent the output pin from exceeding its absolute maximum rating if LED string is disconnected or any LED.in string burns out creating open circuitry. If the die temperature exceeds +150C, the driver will enter a thermal protection shutdown mode. When the device temperature drops by about 20C, the device will resume normal operation.
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8724DS, Rev. N1.0
LDS8724
If the input supply voltage falls below the undervoltage threshold, switch is disabled, and the device enters shutdown mode. LED Selection The number of the LEDs in string is limited by maximum output voltage that cannot exceed overvoltage protection level. We recommend using not more than 10 LEDs with VF 3.6 V in string if VIN voltage is above 3.0 V and not more that 8 LEDs if V IN may fall up to 2.7 V. External Components Selection The LDS8724 requires four external components only. The recommended input capacitor value is between 1.0 and 10uF, while the output capacitor selection is function of desired output ripple, loop stability, and inrush current. We recommend COUT = 1 F. The inductor should allow around 20% higher peak current than LDS8724 Switch Current Limit ILIM (see table Electrical Operating Characteristics on page 2). However, the maximum ripple current through inductor IR should not exceed L - is an inductance, H, and f - is a switching frequency, 700 kHz. Inductor should have minimum DC resistance to avoid driver's efficiency degradation. The equation for the output capacitor selection is:
( NVF d IN ) I OUT VV C OUT , where ( NV F d )V R f V
VR - is a ripple voltage at the output. For example: If V IN = 2.7 V, N = 4, VF = 3.3 V, Vd = 0.25 V, f = 1 MHz, IOUT = 90 mA, (3 strings in parallel x 30 mA) and ripple voltage VR = 0.1 V, C OUT = 0.56 F so 1 F is a good choice. We recommend COUT = 1 F to achieve better efficiency and driver's stability. Recommended Layout In active mode, the driver switches internally at a high frequency. We recommend minimize trace length to all external capacitors and inductor. The input and output ceramic capacitors (X5R or X7R type) should located as close to the device' pins as possible to prevent from EMI distribution A ground plane should cover the area under the driver IC as well as the bypass capacitors. Short connection to ground on capacitors CIN and COUT can be implemented with the use of multiple via. A copper area matching the TDFN exposed pad (PAD) must be connected to the ground plane underneath. The use of multiple via improves the package heat dissipation.
I F d NV V I R 2LIM LED I where , V IN
V F - is a LED forward voltage, V N - is number of LEDs per string Vd - is a current regulator voltage drop = 0.2 V, V IN - is an input voltage, V We recommend continuous conduction mode for inductor to achieve highest efficiency. That limits IR value as
2I F d NV V I R LED IN V
Inductor value L is a function of switching frequency, input and output voltage and is determined by following equation:
L
1 1 1 IR f NV V V V F d PM IN V IN

, where Figure 3. Recommended layout
V PM - is a voltage drop across synchronous rectifier (PMOSFET) = 1.5 ohms x ILED, (A), .
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8724DS, Rev. N1.0
LDS8724
PACKAGE DRAWING AND DIMENSIONS
8-PIN TDFN, 2mm x 3mm, 0.5mm PITCH
SYMBOL A A1 A2 b D D1 E E1 e L
MIN 0.700 0.180 2.950 1.750 1.950 1.550 0.350
NOM 0.750 0.000 0.203 Ref. 0.230 3.000 1.800 2.000 1.600 0.500 Bsc 0.400
MAX 0.800 0.050 0.280 3.050 1.850 2.050 1.650 0.450
Note: 1. All dimensions are in millimeters 2. Complies with JEDEC Standard MO-220
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8724DS, Rev. N1.0
LDS8724
ORDERING INFORMATION
Part Number LDS8724 008-T2 - 200
Notes: 1. 2. 3.
1)
Package TDFN - 8 2 x 3mm
2)
Package Marking 720
Maximum LED current is factory preset at 20 mA. Consult factory if other current values are required. Matte-Tin Plated Finish (RoHS-compliant) Quantity per reel is 2000
EXAMPLE OF ORDERING INFORMATION
Prefix LDS
Device # 8724
Suffix 008 T2 200
Product Number Optional Company ID Package
008: 2 x 3 TDFN
Tape & Reel T: Tape & Reel 2: 2000/Reel
LED Current
200 - 20 mA, 0.7 MHz 201 - 20 mA, 1.0 MHz
Notes: 1) All packages are RoHS-compliant (Lead-free, Halogen-free). 2) The standard lead finish is Matte-Tin. 3) The device used in the above example is a LDS8724 XXX-T2-200 (2x3 TDFN, Tape & Reel, 20 mA LED Current, 0.7 MHz switching frequency). 4) For additional package and current options, please contact your nearest IXYS Corp. Sales office.
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8724DS, Rev. N1.0
LDS8724
Warranty and Use IXYS CORP. MAKES NO WARRANTY, REPRESENTATION OR GUARANTEE, EXPRESS OR IMPLIED, REGARDING THE SUITABILITY OF ITS PRODUCTS FOR ANY PARTICULAR PURPOSE, NOR THAT THE USE OF ITS PRODUCTS WILL NOT INFRINGE ITS INTELLECTUAL PROPERTY RIGHTS OR THE RIGHTS OF THIRD PARTIES WITH RESPECT TO ANY PARTICULAR USE OR APPLICATION AND SPECIFICALLY DISCLAIMS ANY AND ALL LIABILITY ARISING OUT OF ANY SUCH USE OR APPLICATION, INCLUDING BUT NOT LIMITED TO, CONSEQUENTIAL OR INCIDENTAL DAMAGES. IXYS Corp. products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the IXYS Corp. product could create a situation where personal injury or death may occur. IXYS Corp. reserves the right to make changes to or discontinue any product or service described herein without notice. Products with data sheets labeled "Advance Information" or "Preliminary" and other products described herein may not be in production or offered for sale. IXYS Corp. advises customers to obtain the current version of the relevant product information before placing orders. Circuit diagrams illustrate typical semiconductor applications and may not be complete.
IXYS Corp. 1590 Buckeye Dr., Milpitas, CA 95035-7418 Phone: 408.457.9000 Fax: 408.496.0222 http://www.ixys.com
(c) 2010 IXYS Corp. Characteristics subject to change without notice
Document No: 8724DS Revision: N1.0 Issue date: 6/28/2010
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Doc. No. 8724DS, Rev. N1.0

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