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IS31BL3233A Datasheet(PDF) 11 Page - Integrated Silicon Solution, Inc |
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IS31BL3233A Datasheet(HTML) 11 Page - Integrated Silicon Solution, Inc |
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11 / 13 page  IS31BL3233A Integrated Silicon Solution, Inc. – www.issi.com 11 Rev. 0C, 09/11/2014 LED SHORT PROTECTION When the IS31BL3233A is enabled, there is a 2.5mA (Typ.) LED sensing current through each current sink. It is used to detect whether either LED is shorted by generating a voltage drop through each LED. The IC internally compares the voltage difference between VOUT and each sink node (D1 and D2). If this difference is below a preset threshold (Typ. 1.7V), the IC will treat the respective LED as shorted and disable its Flash/Torch Mode current through this LED channel. However, the 2.5mA sensing current will be kept to generate the LED’s voltage drop. If the short circuit is removed during operation, the channel will automatically recover to the programmed current setting. LED OPEN PROTECTION In case of LED open, the open channel will control the loop first so that VOUT will reach OVP, then IS31BL3233A will automatically detect which channel’s LED is open and disable that channel. From that point, the other channel with properly operating LED will control the loop and VOUT will be regulated down to a normal operating voltage. This protection feature avoids unnecessary power consumption in the current sink by regulating the output voltage at the lowest level possible to maintain regulation for the active channel. Not only does this protect from open LEDs failures, but also allows only single flash LED operation with the unused channel floating or open. Open-circuit LED fault protection is reset when the IC is powered down and up again. INDUCTOR SELECTION The IS31BL3233A is designed to use a 1.0μH to 4.7μH inductor. To prevent core saturation, ensure that the inductor-saturation current rating exceeds the peak inductor current for the application. The worst-case peak inductor current can be calculated with the following formula: L t V V I V I MAX ON MIN IN MIN IN MAX LED MAX OUT L PEAK 2 8 . 0 ) ( ) ( ) ( ) ( ) ( ) ( (3) where 0.8 is the estimated efficiency of 80%. For example, for a 1.0A total (or 0.5A per channel) LED current, the peak inductor current for a 1.0uH inductor could be as high as (estimated 50% as the maximum duty ratio at the minimum input voltage, maximum LED forward voltage and maximum load current conditions): A V V A V I L PEAK 9 . 1 1 2 25 . 0 5 . 3 5 . 3 8 . 0 1 4 ) ( If the inductor value is smaller, the inductor peak current will increase. To maintain stable operations for the boost converter, the inductor peak current must be less than both the IS31BL3233A current limit threshold and the inductor saturation current rating. Manufacturer’s specifications of inductors list both the inductor DC current rating, which is a thermal limitation, and peak inductor current rating, which is determined by the saturation characteristics. Measurements at full load and high ambient temperature should be performed to ensure that the inductor does not saturate or overheat due to its parasitic resistance. Bench measurements are recommended to confirm actual inductor peak current IPEAK and to ensure that the inductor does not saturate at maximum LED current and minimum input supply voltage. CAPACITOR SELECTION For good input voltage filtering low ESR ceramic capacitors are recommended. At least a 2.2μF input capacitor is recommended for high current flash LEDs to improve transient behavior of the regulator and EMI behavior of the total power supply circuit. The input capacitor should be placed as close as possible to the input pin and the PGND pin of the IS31BL3233A. The output capacitance required depends on the required LED current. A 4.7μF ceramic capacitor works well in most situations, but a 1.0μF capacitor is acceptable for lower LED current conditions. PCB LAYOUT Due to the fast switching transitions and high-current paths, careful PC board layout is required. Connect AGND pin directly to the exposed paddle underneath the IC; connect the exposed paddle to the PCB ground plane. The output bypass capacitor should be placed as close to the IC as possible. Minimize trace lengths between the IC and the inductor, the input capacitor, and the output capacitor; keep these traces short, direct, and wide. The ground connections of CIN and COUT should be as close together as possible and connected to PGND. |
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