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LDS8680 Datasheet(PDF) 8 Page - IXYS Corporation |
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LDS8680 Datasheet(HTML) 8 Page - IXYS Corporation |
8 / 12 page ![]() LDS8680 © 2009 IXYS Corp. 8 Doc. No. 8680_DS, Rev. N1.0 Characteristics subject to change without notice drop Vd across PCR at every channel in ON state. If this voltage falls below 150 mV (typical) at any one channel, (channel with LED with highest forward voltage), the Mode Control Block changes charge pump mode to the next multiplication ratio. Vd (LEDA/B) = VIN x M – VF – Rcp x IOUT, where Rcp is a Charge Pump Output Resistance at given mode, IOUT is sum of all LED currents, and M is a charge pump’ multiplication ratio. If the input voltage is insufficient or falls to a level where Vd ≤150 mV, and the regulated currents cannot be maintained, the low dropout PowerLite ™ Current Regulator switches the charge pump into 2-x mode (after a fixed delay time of about 800 μs). In 2-x mode, the charge pump’ output voltage is approximately equal to 2 times the input supply voltage (less any internal voltage losses). LED Current Setting The current in each of the two LED channels is programmable through the Smart OneWire ™ EN/SET digital control input. By pulsing this signal according to a shown above protocol, a set of internal registers can be addressed and written into allowing to configure each of LEDs with the desired current. See recommended commands at the Table “CURRENT SETTING COMMANDS” Unused LED Channels For applications with only one LED, unused LED can be disabled via the register internally and connect to VOUT. Protection Modes The LDS8680 has follow protection modes: 1. LED short to VOUT protection If LED pin is shorted to VOUT, LED burned out becomes as short circuit, or LED pin voltage is within from VOUT to (VOUT - 1.5V) range, LDS8680 recognizes this condition as “LED Short” and disables this channel. If LED pin voltage is less than (Vout – 1.5V), LDS8680 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 LDS8680 is enabled, input current may increase up to 1 A within 20 µs after enable and is limited to ~ 90 mA after that. 4. Over-Temperature Protection If the die temperature exceeds +160°C, the driver will enter shutdown mode. Device restores normal operation after die temperature falls below 135°C. 5. Input Voltage Under-Voltage Lockout If VIN falls below 2.3 V but above 1.7 V (typical value), LDS8680 will restart when input voltage rises above 2.3 V called lockout mode. If VIN continues fall below 1.7 V, LDS8680 device will enter shutdown mode and reset all data register. To restart device, set EN/SET pin logic low for more than 50 ms and program new ILED current value using Smart OneWire™ interface. 6. Open LED Definition When LED becomes an open circuit, device will enter into 2-x mode. Unless this unused LED channel was disabled via Smart OneWire ™ commands. 7. LED protection LDS8680 uses timeout ~250 ms to protect LEDs from thermal damage at currents above 200 mA. This function may be disabled in command mode. LED Selection If the power source is a Li-ion battery, LEDs with lowest forward voltages are recommended to achieve highest efficiency and extended operation on a single battery charge. LEDs with forward voltages (VF) ranging from 1.6 V to 4 V may be used. Charge pump operates in highest efficiency when VF voltage is close to VIN voltage multiplied by switching mode, i.e. VIN x 1 or VIN x 2 External Components The driver requires three external ceramic capacitors (C1, CIN, and COUT) X5R or X7R type. In 2-x charge pump mode, the input current ripple is very low, and an input bypass capacitor of 1µF is sufficient. In 1-x 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 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 TDFN exposed pad (PAD) must be connected to the |
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