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IS31LT3350-V2STLS2-TR Datasheet(PDF) 10 Page - Integrated Silicon Solution, Inc |
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IS31LT3350-V2STLS2-TR Datasheet(HTML) 10 Page - Integrated Silicon Solution, Inc |
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10 / 12 page  IS31LT3350 Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 10/18/2011 10 A value of 1μF will reduce nominal ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of LED voltage. Operation at low supply voltage The internal regulator disables the drive to the switch until the supply has risen above the startup threshold set internally which makes power MOSFET on-resistance small enough. Above this threshold, the chip will start to operate. However, with the supply voltage below the specified minimum value, the switch duty cycle will be high and the chip power dissipation will be at a maximum. Care should be taken to avoid operating the chip under such conditions in the application, in order to minimize the risk of exceeding the maximum allowed die temperature. (See next section on thermal considerations). Note that when driving loads of two or more LEDs, the forward drop will normally be sufficient to prevent the chip from switching below approximately 6V. This will minimize the risk of damage to the chip. Thermal considerations When operating the chip at high ambient temperatures, or when driving maximum load current, care must be taken to avoid exceeding the package power dissipation limits. Two packages are available for different application because of thermal consideration. SOT23-5 is only available for 350mA current output application. The SOT89-5 package’s thermal resistance is much lower so that it can handle 700mA current and suitable for 700mA application. Note that the chip power dissipation will most often be a maximum at minimum supply voltage. It will also increase if the efficiency of the circuit is low. This may result from the use of unsuitable coils, or excessive parasitic output capacitance on the switch output. Layout considerations LX pin The LX pin of the chip is a fast switching node, so PCB traces should be kept as short as possible. To minimize ground 'bounce', the ground pin of the chip should be soldered directly to the ground plane. Coil and decoupling capacitors It is particularly important to mount the coil and the input decoupling capacitor close to the chip to minimize parasitic resistance and inductance, which will degrade efficiency. It is also important to take account of any trace resistance in series with current sense resistor RS. ADJ pin The ADJ pin is a high impedance input, so when left floating, PCB traces to this pin should be as short as possible to reduce noise pickup. ADJ pin can also be connected to a voltage between 1.2V~5V. In this case, the internal circuit will clamp the output current at the value which is set by ADJ=1.2V. High voltage traces Avoid running any high voltage traces close to the ADJ pin, to reduce the risk of leakage due to board contamination. Any such leakage may raise the ADJ pin voltage and cause excessive output current. A ground ring placed around the ADJ pin will minimize changes in output current under these conditions. GND ADJ VIN ISENSE LX L1 D1 C1 IS31LT3350 LED Cled VIN Rs |
Similar Part No. - IS31LT3350-V2STLS2-TR |
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Similar Description - IS31LT3350-V2STLS2-TR |
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