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LDS8868 Datasheet(PDF) 11 Page - IXYS Corporation |
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LDS8868 Datasheet(HTML) 11 Page - IXYS Corporation |
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11 / 14 page  LDS8868 © 2009 IXYS Corp. 11 Doc. No. 8868_DS, Rev. N2.1 Characteristics subject to change without notice 1.5V), LDS8868 restores LED current at this particular channel to programmed value. 2. VOUT Over-Voltage Protection The charge pump’ output voltage VOUT automatically limits at about 6.2 V maximum. This is to prevent the output pin from exceeding its absolute maximum rating. 3. VOUT Short Circuit Protection If VOUT is shorted to ground before LDS8868 is enabled, input current may increase up to 200 – 300 mA within 20 µs after enable and is limited to 35 – 40 mA after that. 4. Over-Temperature Protection If the die temperature exceeds +150°C, the driver will enter shutdown mode. The LDS8868 requires restart after die temperature falls below 130°C. 5. Input Voltage Under-Voltage Lockout If VIN falls below 2.2 V (typical value), LDS8868 enters shutdown mode and all registers data are cleared. Device requires restart when input voltage rises above 2.3 V. To restart device, set EN/SET pin logic low, turn VIN off/on, set EN/SET pin logic high, and program ILED using 1-wire interface. 6. Low VIN or High LED VF Voltage Detection If, in 2x mode, VIN is too low to maintain regulated LED current for given LED VF, or LED becomes an open circuit, or if any LED at active channels is disconnected, LDS8868 starts subsequently changing modes (2x – 1x –1.5x – 2x -…) in an attempt to compensate insufficient voltage. As a result, average current at all other channels that are ON may fall below regulated level. LED Selection LEDs with forward voltages (VF) ranging from 1.6 V to 3.6 V may be used. Charge pumps operate in highest efficiency when VF voltage is close to VIN voltage multiplied by switching mode, i.e. VIN x 1, VIN x 1.5, and so on. If the power source is a Li-ion battery, LEDs with VF = 2.7V - 3.3V are recommended to achieve highest efficiency performance and extended operation on a single battery charge. External Components The driver requires two external 1 µF ceramic capa- citors (CIN and COUT) and two 0.22 µF ceramic capa- citors (C1 and C2) X5R or X7R type. Capacitors C1 and C2 may be increased up to 1 µF to improve charge pump efficiency by 3%. In all charge pump modes, the input current ripple is very low, and an input bypass capacitor of 1µF is sufficient. In 1x mode, the device operates in linear mode and does not introduce switching noise back onto the supply. Recommended Layout In charge pump mode, the driver switches internally at a high frequency. It is recommended to minimize trace length to all four capacitors. 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 TQFN exposed pad (TAB) must be connected to the ground plane underneath. The use of multiple via improves the package heat dissipation. Figure 4. Recommended layout |
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