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LDS8711
High Efficiency 10 LED Driver with No External Schottky
FEATURES
High efficiency boost converter with the input voltage range from 2.7 to 5.5 V 250 mV current sense voltage Drives one LED string with 10 LEDs in series up to 32 mA No external Schottky Required (Internal synchronous rectifier*) 0.7 MHz Switching Frequency Efficiency greater than 83% Smart OneWireTM Interface and PWM LED Brightness Control Modes Provide Constant ILED current or constant color temperature brightness control Small External Inductor (10 H - 33 H) Over voltage, under voltage, over temperature, over current protection Low Shutdown Current (<1A) Available in 2 x 3 x 0.8 mm 8-pin TDFN package either in PWM or in Smart OneWireTM Interface mode. The Smart OneWireTM Interface mode allows set LED current in the range from 0.125 to 32 mA in 256 steps of 0.125 mA each. In PWM mode, LDS8711 operates with either factory, or customer preset maximum LED current values. Series connection of the LEDs provides constant current and subsequently uniform brightness, eliminating the need for ballast resistors and factory calibration. Operation at 0.7 MHz allows use of small value inductor (10 - 33 H) and external ceramic capacitor (1 - 2.2 F). The use of integrated rectifier makes the efficiency dependent on only two external components parameters: inductor DCR and external capacitor ESR. The supply voltage down to 2.7 V for new Li-ion battery chemistries or two alkaline battery cells extends the battery life. Under-voltage protection disables the part when VIN voltage reaches 2.0 V 0 The LDS8711 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 in 8-pin TDFN 2 x 3 mm package with a max height of 0.8 mm.
APPLICATION
Color Display Backlight Portable Navigation and GPS Receivers Smart phones Digital Photo Frames Portable DVD Players Multimedia Phones
DESCRIPTION
The LDS8711 is a fixed frequency current mode boost converter with internal rectifier and cycle-bycycle switch current limit specifically designed to drive a string of up to 10 White LEDs. It operates
TYPICAL APPLICATION CIRCUIT
*) Patent pending
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8711DS, Rev. N1.0
LDS8711
ABSOLUTE MAXIMUM RATINGS
Parameter V IN voltage V OUT, SW, LEDS, Vh voltage EN/SET voltage LED Current Storage Temperature Range Junction Temperature Range 8-pin TDFN package Thermal Resistance Soldering Temperature Rating 6 40 V IN + 0.7V 32 -65 to +160 -40 to +125 65 300 Unit V V V mA 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) VIN = 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 1 0.7 92 0.3 1.5 0.1 1.3 250 0.1 250 1.4 0.4 150 20 36 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. 8711DS, Rev. N1.0
LDS8711
Smart OneWireTM TIMING SPECIFICATION Parameter tSETUP1 tSETUP2 tBO tB1 tCLO tLO tLO1 tOFF Name EN/PWM setup from shutdown to PWM mode EN/SET setup from shutdown to Smart OneWireTM Interface mode Bit "zero" positive pulse Bit "one" positive pulse Low time between command pulses or two command sets EN LOW Time before LED ON in PWM mode EN LOW Time before LED ON in OneWireTM interface mode EN/SET low time to shutdown Min 50 350 1 40 2 0.002 0.3 10 Typ 100 Max 200 25 200 200 8 5 Units s s s s s ms ms ms
25
Fig. 1. PWM Mode operation
Fig. 2 LED Current programming Table 1: Smart OneWireTM interface commands
Command Register address
Bit 15 Bit 14 Bit 13 Bit 12 Bit 11 Bit 10 Bit 9 Bit 8 Bit 7 Bit 6 Bit 5
Data
Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Command description
Current Setting
0
0
0
0
0
1
0
0
LED Current Binary Code = (I LED / 0.125 mA - 1) (bin), where 0.125 mA < ILED < 32 mA
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8711DS, Rev. N1.0
LDS8711
TYPICAL CHARACTERISTICS
Vin = 3.6V, IOUT = 30mA (10 LEDs per string), CIN = 1 COUT = 1 L = 22 H, TAMB = 25C unless otherwise specified F, F,
Power Efficiency vs. Input Voltage
LED Current 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. 8711DS, Rev. N1.0
LDS8711
TYPICAL CHARACTERISTICS
Vin = 3.6V, IOUT = 30mA (10 LEDs per string), C IN = 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) - V OUT
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) - V OUT
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. 8711DS, Rev. N1.0
LDS8711
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 to the LED string anode VOUT terminal to this pin. High Voltage Bypass; Connect a 0.01 F capacitor HVB between this point and V OUT 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 V IN GND PGND SW
Top view: TDFN 8-lead 2 X 3 mm
PIN FUNCTION
VIN 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 F capacitor between this point and VOUT . EN/PWM is the Enable/Smart OneWireTM inteface 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 wait for the first pulse that determines mode, at which device will operate. If first pulse is equal tSETUP1, LED current sets to factory programmed value and every next incoming pulse will be recognized as PWM signal. If first pulse is equal tSETUP2, device enters Smart OneWireTM interface mode and wait for command. Every next of 16 incoming pulses will be recognized as command. If device is not able to recognize command due error in transmition, it enters shutdown mode after 10 ms. 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 that is connected to the LED anode. A ceramic bypass capacitor is required 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. 8711DS, Rev. N1.0
LDS8711
BLOCK DIAGRAM
Figure 3. LDS8711 Functional Block Diagram
BASIC OPERATION
At power-up, EN/PWM pin should be logic LOW. The LDS8711 starts operating when EN/PWM pin is asserted logic high. The device becomes enabled and wait for the first pulse that determines mode, at which device will operate. If first pulse is equal t SETUP1, LED current sets to factory programmed value and every next incoming pulse will be rec ognized as PWM signal. If first pulse is equal tSETUP2, device enters Smart OneWireTM interface mode and wait for command. Every next of 16 incoming pulses will be recognized as command. After LDS8711 accepts command, the current sets in respect to programmed value, and device changes mode to PWM. If device is not able to recognize command due some error, it enters shutdown mode after 10 ms. 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 V OUT 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 LDS8711 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 value in the LED string may be factory preset in the range from 10 to 30 mA in 0.125 mA steps, or set by Smart OneWireTM interface in the range from 0.125 to 32 mA. 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 k Hz or from 20% to 100% at 30 kHz with high linearity current regulation. It gives possibility regulate brightness either in DC mode with changing color temperature, or in PWM mode with stable color temperature. To start LDS8711 in PWM Mode, the first positive pulse applied to EN/PWM pin after power-up should be equal tSETUP1 and followed by EN/PWM pin LOW
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Doc. No. 8711DS, Rev. N1.0
(c) 2010 IXYS Corp. Characteristics subject to change without notice
LDS8711
state equal tLO (see Figure 1). Every next incoming pulse will be recognized as PWM pulse, and maximum LED current will be set equal factory preset value. If PWM frequency is above 5 kHz and duty cycle is less than 50%, device will start in PWM mode automatically; however, at lower frequencies or 100% duty cycle, start procedure requires. To start LDS8711 in Smart OneWireTM Interface Mode, the first positive pulse applied to EN/PWM pin after power-up should be equal tSETUP2 and followed by 16 command pulses in respect to Smart OneWireTM interface command shown in the Table 1. Every Smart OneWireTM interface command to program LED current may be represented in binary code as 0000 0100 XXXX XXXX (MSB to LSB), where X is a bit value in respect with desired current value. Example: If desired LED current value is 22 mA, binary code is equal 22 mA/0.125 mA - 1 = 43 (Dec) = 1010 1111 (Bin) and Smart OneWireTM interface command to set this current is: 0000 0100 1010 1111 (MSB to LSB). This command should be send (MSB first) through EN/PWM pin as sequence of 16 pulses, each of which represents bit zero or bit one depend on pulse length that should be equal tBO or tB1 respectively (see Smart OneWireTMTIMING SPECIFICATION). After 16 pulse, EN/PWM pin should be LOW for tLO1 time to confirm end of the command and transfer to PWM mode. Every next incoming pulse will be recognized as PWM pulse, and it will turn LEDs on (see Figure 2). If LDS8711 is not able recognize command due some error in programming or transmit, it will go in shutdown mode after 10 ms timeout Once in PWM mode, LDS8711 is not able recognize Smart OneWireTM Interface commands and requires restart to reprogram LED current. Protection Mode The output voltage VOUT is limited at about 38 V maximum. 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.
th
LED Selection LEDs with forward voltages (VF) ranging from 1.3 V to 5.0 V may be used. However, number of the LEDs in string is limited by maximum output voltage that cannot exceed over-voltage 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 eight LEDs if VIN may fall up to 2.7 V. External Components The LDS8711 requires four external components only. The recommended input capacitor value is between 1 and 10 F, while the output capacitor selection is function of desired output ripple, loop stability, and inrush current. The inductor should have minimum Rdc resistance to increase driver's efficiency. Recommended inductor values are from 10 to 33 H. The inductor ripple current IR is a function of switching frequency, inductor value, and duty cycle and is determined by the following equation:
1 IR 1 1 Lf NV V V V F d PM IN V IN

,
where V F - is a LED forward voltage, Vd - is a current regulator voltage drop, V PM - is a voltage drop across PMOSFET, V IN - is an input voltage, L is - inductance, and f - is a switching frequency, 700 kHz. Then, the switch cycle-by-cycle current limit is equal:
I (NV F d ) I R V I LIM OUT , V IN 2
where is expected efficiency. The selected inductor should allow around 20% higher peak current to avoid saturation. The criterion for the output capacitor selection is:
( NV F d IN )I OUT VV COUT ( NVF d )V R f V
For example: If V IN = 2.7 V, N = 10, VF = 3.6 V, Vd = 0.25 V, f = 0.7 MHz, IOUT = 30 mA, and ripple voltage VR = 0.05 V, C OUT = 0.77 F so 1.0 F is a good choice.
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8711DS, Rev. N1.0
LDS8711
We recommend C OUT in the range 1.0 - 2.2 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 C OUT 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.
Figure 4. Recommended layout
(c) 2010 IXYS Corp. Characteristics subject to change without notice
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Doc. No. 8711DS, Rev. N1.0
LDS8711
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. 8711DS, Rev. N1.0
LDS8711
ORDERING INFORMATION
Part Number LDS8711 008-T2 - 300
Notes: 1. 2. 3.
1)
Package TDFN - 8 2 x 3mm
2)
Package Marking 711
Maximum LED current is factory preset at 20, 25, or 30 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 # 8711
Suffix 008 T2 300
Product Number Optional Company ID Package
008: 2 x 3 TDFN
Tape & Reel T: Tape & Reel 2: 2000/Reel
LED Current
300 - 30 mA 250 - 25 mA 200 - 20 mA
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 LDS8711 XXX-T2-300 (2x3 TDFN, Tape & Reel, 30 mA LED Current). 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. 8711DS, Rev. N1.0
LDS8711
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: 8711DS Revision: N1.0 Issue date: 6/28/2010
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Doc. No. 8711DS, Rev. N1.0

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