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IS31LT3354 Datasheet(PDF) 9 Page - Integrated Silicon Solution, Inc |
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IS31LT3354 Datasheet(HTML) 9 Page - Integrated Silicon Solution, Inc |
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9 / 12 page  IS31LT3354 Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 9 Example: For VIN=12V, L=47μH, rL=0.64Ω, VLED=3.4V, Iavg =333mA and VD =0.36V TON = (47e-6 × 0.105)/(12 - 3.4 - 0.612) = 0.62μs TOFF = (47e-6 × 0.105)/(3.4 + 0.36 + 0.322)= 1.21μs This gives an operating frequency of 546kHz and a duty cycle of 0.34 Optimum performance will be achieved by setting the duty cycle close to 0.5 at the nominal supply voltage. This helps to equalize the undershoot and overshoot and improves temperature stability of the output current. Diode selection For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the maximum operating voltage and temperature. If alternative diodes are used, it is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. It is very important to consider the reverse leakage of the diode when operating above 85°C. Excess leakage will increase the power dissipation in the device. The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the switch. If a silicon diode is used, care should be taken to ensure that the total voltage appearing across the switch including supply ripple, does not exceed the specified maximum value. Reducing output ripple Peak to peak ripple current in the LED can be reduced, if required, by shunting a capacitor Cled across the LED(s) as shown below: GND ADJ VIN ISENSE GATE Rs L1 D1 SN3354 C2 5 4 1 2 3 AP2306 C1 0.1uF Cled 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 driving to the switch until the supply has risen above the startup threshold set internally which makes the power MOSFET’s 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 The IS31LT3354 utilizes an external MOSFET to switch the inductor current, and thus dissipates very little power. The thermal characteristics of the MOSFET dominate in typical application circuits for the IS31LT3354. Care should be taken to ensure a large copper ground plane and a good thermal conductivity between the MOSFET and the ground plane. Note that the switch power dissipation increases with increasing supply voltage. This is caused primarily by two things, the resulting increase in switching frequency and the higher voltage across the switch during the off time. This may result from the use of unsuitable coils, or excessive parasitic output capacitance on the switch output. IS31LT3354 |
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