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IS31LT3910-GRLS2-TR Datasheet(PDF) 7 Page - Integrated Silicon Solution, Inc |
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IS31LT3910-GRLS2-TR Datasheet(HTML) 7 Page - Integrated Silicon Solution, Inc |
7 / 10 page IS31LT3910 Integrated Silicon Solution, Inc. – www.issi.com 7 Rev. B, 05/12/2014 in Temperature Compensation section). To use the internal 240mV, the LD pin must be connected to PWMD pin. PWM DIMMING PWM Dimming can be achieved by driving the PWMD pin with a low frequency square wave signal. When the PWM signal is low, the GATE driver is disabled; and when the PWMD signal is high, the GATE driver is enabled. Since the PWMD signal does not turn off the other parts of the IC, the response of the IS31LT3910 to the PWMD signal is almost instantaneous. The rate of rise and fall of the LED current is thus determined solely by the rise and fall times of the inductor current. To disable PWM dimming function, leave the PWMD pin floating. TEMPERATURE COMPENSATION IS31LT3910 provides thermal protection for your LEDs. Refer to application circuit, adding a NTC themistor close to the LEDs string will realize the temperature compensation of LEDs current. If the temperature of the LEDs rises, the resistance of the NTC thermistor decreases until the voltage of the LD pin falls below 0.24V. Then the average current is controlled by the LD pin and the temperature compensation function starts. The formula is given as below: V R R V R NTC REF NTC 24 . 0 1 Assuming a 30% ripple in the inductor, the temperature compensated continuous current may be computed as: 15 . 1 ) ( 1 S NTC REF NTC OUT R R R V R I Make sure the value of R1 is more than 1kΩ. When the LD pin voltage reduces to less than 50mV the chip is shutdown. After the ambient temperature returns to a safe temperature, the current will return to the set value. Example: DC input voltage: VINDC =230V Output LED strings: VO =134.4V (42 LEDs in series, 3.2V for each one), IO_AVG =160mA (8 parallels LEDs, 20mA for each one) IC INPUT RESISTOR (RIN) AND HOLD CAPACITOR (CIN) k I V V R IN IN INDC IN 223 1 0 . 7 230 parallel two 430kΩ/0.5W resistor for lifetime consideration. CIN is chosen to be 10µF/40V capacitor TOFF TIME REGULATION CAPACITOR (COFF) AND TOFF TIME For high output voltage, low output current application, we need shorter tOFF time to obtain the smaller application inductor. For high output current application, it is suggested that the frequency is set to not more than 50kHz (typical 25kHz~30kHz). In IS31LT3910, ) 10 1 ( 10 51 . 0 6 _ pF C t OFF TIME OFF So, apply COFF =150pF, then tOFF =8.16µs. CURRENT SENSE RESISTOR (RCS) Design for low current ripple will also improve current accuracy, but it will require a large value of inductor. High current ripple allows a lower cost inductor. So we need to consider these two factors when selecting an inductor. A capacitor placed in parallel with the array of LEDs can be used to reduce the LED current ripple while keeping the same average current. A typical value is 1µF should be used. Since the output average IO_AVG = 160mA, Assume 50% current ripple, then mA mA I Ripple 40 160 % 50 2 1 2 1 mA I I Ripple PEAK O 200 2 1 I O_AVG _ 2 . 1 200 24 . 0 mA V R CS THE INDUCTOR (L1) CHOSEN The inductor value depends on the ripple current in the LEDs. tOFF =8.16µs mH I t V L Ripple OFF O 7 . 13 80 16 . 8 4 . 134 The inductor chosen should have a saturation current higher than the peak output current and a continuous current rating above the required mean output current. The DC resistance (DCR) of the inductor is also essential when choosing an inductor. Bigger DCR will lead to more heat. The value of the inductor will reduce as its temperature rises, leading to higher current ripple, which in turn, reduces the average output current. |
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