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LDS8724008T2-200 Datasheet(PDF) 7 Page - IXYS Corporation |
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LDS8724008T2-200 Datasheet(HTML) 7 Page - IXYS Corporation |
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7 / 10 page  LDS8724 © 2010 IXYS Corp. 7 Doc. No. 8724DS, Rev. N1.0 Characteristics subject to change without notice If the input supply voltage falls below the under- voltage threshold, switch is disabled, and the device enters shutdown mode. LED Selection The 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 8 LEDs if VIN may fall up to 2.7 V. External Components Selection The LDS8724 requires four external components only. The recommended input capacitor value is between 1.0 and 10uF, while the output capacitor selection is function of desired output ripple, loop stability, and inrush current. We recommend COUT = 1 µF. The inductor should allow around 20% higher peak current than LDS8724 Switch Current Limit ILIM (see table Electrical Operating Characteristics on page 2). However, the maximum ripple current through inductor IR should not exceed IN d F LED LIM R V V NV I I I 2 , where VF - is a LED forward voltage, V N – is number of LEDs per string Vd - is a current regulator voltage drop = 0.2 V, VIN - is an input voltage, V We recommend continuous conduction mode for inductor to achieve highest efficiency. That limits IR value as IN d F LED R V V NV I I 2 Inductor value L is a function of switching frequency, input and output voltage and is determined by following equation: IN IN PM d F R V V V V NV f I L 1 1 1 , where VPM - is a voltage drop across synchronous rectifier (PMOSFET) = 1.5 ohms x ILED, (A), L - is an inductance, H, and f - is a switching frequency, 700 kHz. Inductor should have minimum DC resistance to avoid driver’s efficiency degradation. The equation for the output capacitor selection is: f V V NV I V V NV C R d F OUT IN d F OUT ) ( ) ( , where VR – is a ripple voltage at the output. For example: If VIN = 2.7 V, N = 4, VF = 3.3 V, Vd = 0.25 V, f = 1 MHz, IOUT = 90 mA, (3 strings in parallel x 30 mA) and ripple voltage VR = 0.1 V, COUT = 0.56 µF so 1 µF is a good choice. We recommend COUT = 1 µF to achieve better efficiency and driver’s stability. Recommended Layout In active mode, the driver switches internally at a high frequency. We recommend minimize trace length to all external capacitors and inductor. The input and output ceramic capacitors (X5R or X7R type) should located as close to the device’ pins as possible to prevent from EMI distribution A ground plane should cover the area under the driver IC as well as the bypass capacitors. Short connection to ground on capacitors CIN and COUT can be implemented with the use of multiple via. A copper area matching the TDFN exposed pad (PAD) must be connected to the ground plane underneath. The use of multiple via improves the package heat dissipation. Figure 3. Recommended layout . |
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