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SP7611AEC6/TR Datasheet(PDF) 8 Page - Sipex Corporation |
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SP7611AEC6/TR Datasheet(HTML) 8 Page - Sipex Corporation |
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8 / 14 page  Apr20-07 RevS SP7611A/7612A/7614A Low Dropout Linear LED driver 2007 Sipex Corporation Page 8 APPLICATION INFORMATION Setting the LED Current The LED current is controlled by ICONTROL and RSET according to the following formula: I LED = Gain X (VCONTROL - VCTRL) / RSET Gain depends on mode of operation VCONTROL is the voltage going into RSET VCTRL is the voltage at the pin of the device The voltage VCTRL can be determined using the I SET vs Control voltage graph located in the typical performance characteristics section. As an example, to set the LED current for an SP761x the procedure is: Take the LED current and divide it by the Gain to get ISET = ILED / Gain. Look up the VCTRL pin voltage using ISET on the graph “Ctrl Current vs. Control Voltage”. Once VCTRL is known, use the following formula to find RSET. The gain is not linear, the LED current vs. Cathode voltage graph needs to be used to determine gain. Typically, the gain is ~870 for SP7614A and ~435 for SP7611A/12A when the part is operating in the linear region. RSET = (VCONTROL - VCTRL) / ISET RSET Example For an SP7614A with VCONTROL of 3V and LED current of 20mA. I SET = 20mA/870 (870 is the typical gain in linear operation) = 22uA. Using the graph on page 5 in the typical performance section, the VCTRL voltage on the control pin will be about 1.1V, then RSET will be determined by: RSET = (3V - 1.1V) / 22uA RSET = 86K PWM FUNCTION The LED's brightness can also be adjusted by driving either the ENABLE or the CTRL pin with a PWM signal. The driving signal frequency should be greater than 100Hz to avoid flickering, increasing to more than 1MHz, if necessary. Temperature Considerations LEDs are very sensitive to temperature. In most cases the maximum allowed junction temperature is 100ºC. The case of overtemperature due to power dissipation is de-scribed by the following: TJ = TA + JA x I x VF where TJ is the LED junction temperature, TA is the ambient temperature, JA is the junction to ambient thermal resistance, I is the LED current and VF is the LED forward voltage. When the temperature rises and the cathode voltage increases, SP761X reduces the current through LEDs. Refer to "LED Current vs. LED Cathode Voltage" graph under the Typical Performance Characteristics section. Efficiency The system efficiency, defined as the ratio between the LED's power and the input supplied power can be calculated as follows: Efficiency = (VIN - VCATHODE) / VIN The lower the VCATHODE , the higher the system efficiency. Efficiency can be further improved using a higher VIN with more LEDs as shown in example 3. The SP761X driver's low dropout architecture can significantly improve the efficiency compared to using simple ballast resistors. Application Information The ultra-low voltage drop across the SP761X series of LED drivers allows the devices to drive white, blue, and other color LEDs in a wide range of input voltages. The driver can be used in many |
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