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SP7685ER-L-TR Datasheet(PDF) 5 Page - Exar Corporation |
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SP7685ER-L-TR Datasheet(HTML) 5 Page - Exar Corporation |
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5 / 13 page  5 Date: 6/20/06 Rev T SP7685, 1.2A Buck/BoostCharge Pump LED Driver © 2006 Sipex Corporation In the Torch mode, (Flash = GND) the Flash pin is set to logic low and the SP7685 FB pin regulates to 50mV output: VFB = 50mV (Torch Mode) When in Flash mode, (Flash = V IN), the FB regulation voltage is set by the resistor RSET connected between the RSET pin and SGND and the equation: VFB = RSET * 14μA / 5 (Flash Mode) Where 14μA is an internal regulated current and 5 is an internal factor used to scale the VSET voltage to the VFB voltage. Typical values of RSET are 140KΩ to 35KΩ for a range of VFB = 400mV to 100mV in Flash mode. The output current is then set in either Flash or Torch mode by the equation: I OUT = VFB / RSENSE FLASH TIMEOUT PROTECTION Due to the high currents typically available in Flash mode, it is necessary to protect the white LED from damage if left on too long. The SP7685 has a timeout in Flash mode of approximately 2.6 seconds after which it will shut down operation. Operation will not begin again in Flash mode until the Enable pin or Flash pin have been set Low and then High again. OVERTEMPERATURE PROTECTION When the temperature of the SP7685 rises above 145˚C, the overtemperature protec- THEORY OF OPERATION tion circuitry turns off the output switches to prevent damage to the device. If the temperature drops back down below 135 degrees Celsius, the part automatically recovers and executes a soft start cycle. OVERVOLTAGE PROTECTION The SP7685 has over voltage protec- tion. If the output voltage rises above the 5.5V threshold, the over voltage protection shuts off all of the output switches to prevent the output voltage from rising further. When the output decreases below 5.5V, the device re- sumes normal operation. OVERCURRENT PROTECTION The over current protection circuitry moni- tors the average current out of the V OUT pin. If the average output current exceeds approximately 1.6 Amps, then the overcurrent protection circuitry shuts off the output switches to protect the chip. BRIGHTNESS CONTROL USING PWM Dimming control can be achieved by ap- plying a PWM control signal to the EN pin. The brightness of the white LEDs is con- trolled by increasing and decreasing the duty cycle of the PWM signal. While the operating frequency range of the PWM control is from 60Hz to 700Hz, the recom- mended maximum brightness frequency range of the PWM signal is from 60Hz to 200Hz. A repetition rate of at least 60Hz is required to prevent flicker. |
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