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LDS8711 Datasheet(PDF) 8 Page - IXYS Corporation |
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LDS8711 Datasheet(HTML) 8 Page - IXYS Corporation |
8 / 12 page ![]() LDS8711 © 2010 IXYS Corp. 8 Doc. No. 8711DS, Rev. N1.0 Characteristics subject to change without notice 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 OneWire ™ 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 OneWire ™ interface command shown in the Table 1. Every Smart OneWire ™ 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 OneWire ™ 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 OneWire ™TIMING SPECIFICATION). After 16 th pulse, EN/PWM pin should be LOW for t LO1 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 OneWire ™ 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 +150°C, the driver will enter a thermal protection shutdown mode. When the device temperature drops by about 20°C, the device will resume normal operation. 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 µF, 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 µH. The inductor ripple current IR is a function of switching frequency, inductor value, and duty cycle and is determined by the following equation: IN IN PM d F R V V V V NV Lf I 1 1 1 , where VF - is a LED forward voltage, Vd - is a current regulator voltage drop, VPM - is a voltage drop across PMOSFET, VIN - 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: 2 ) ( R IN d F OUT LIM I V V NV I I , 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: f V V NV I V V NV C R d F OUT IN d F OUT ) ( ) ( For example: If VIN = 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, COUT = 0.77 µF so 1.0 µF is a good choice. |
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