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IS31LT3360-SDLS3 Datasheet(PDF) 10 Page - Integrated Silicon Solution, Inc |
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IS31LT3360-SDLS3 Datasheet(HTML) 10 Page - Integrated Silicon Solution, Inc |
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10 / 15 page  IS31LT3360 Integrated Silicon Solution, Inc. – www.issi.com 10 Rev. C, 12/22/2013 APPLICATION INFORMATION SETTING NOMINAL AVERAGE OUTPUT CURRENT WITH EXTERNAL RESISTOR RS The nominal average output current in the LED(s) is determined by the value of the external current sense resistor (RS) connected between VIN and ISENSE pins and in is given by Equation (1): S NOM OUT R I 1 . 0 _ (1) Note that RS=0.083Ω is the minimum allowed value of sense resistor under these conditions to maintain switch current below the specified maximum value. It is possible to use different values of RS if the ADJ pin is driven from an external voltage. The table below gives values of nominal average output current for several preferred values of current setting resistor (RS) in the typical application circuit Figure 1: RS (Ω) Nominal Average Output Current (mA) 0.083 1200 0.15 667 0.3 333 The above values assume that the ADJ pin is floating and at a nominal voltage of VREF =1.2V. Rs need to be chosen 1% accuracy resistor with enough power tolerance and good temperature characteristic to ensure stable output current. OUTPUT CURRENT ADJUSTMENT BY EXTERNAL DC CONTROL VOLTAGE The ADJ pin can be driven by an external DC voltage (VADJ), as shown in Figure 16, to adjust the output current to a value above or below the nominal average value defined by RS. Figure 16 Dimming by External DC Voltage The nominal average output current in this case is given by Equation (2): S ADJ DC OUT R V I 083 . 0 _ (2) For 0.3V< VADJ <1.2V. Note that 100% brightness setting corresponds to VADJ = VREF. When driving the ADJ pin above 1.2V, the current will be clamped to 100% brightness automatically. The input impedance of the ADJ pin is 500kΩ (Typ.). OUTPUT CURRENT ADJUSTMENT BY PWM CONTROL Directly Driving ADJ Input A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the ADJ pin, as shown in Figure 17, to adjust the output current to a value below the nominal average value set by resistor RS, the signal range is from 0V~5V.The logic “HIGH” is higher than 1.2V, the logic “LOW” is lower than 0.2V.The PWM signal must have the driving ability to drive internal 500kΩ pull-up resistor. Figure 17 PWM Dimming Control Via ADJ Pin Driving The ADJ Input From A Microcontroller Another possibility is to drive the chip from the open drain output of a microcontroller. The Figure 18 below shows one method of doing this: Figure 18 Dimming By MCU The diode and resistor suppress possible high amplitude negative spikes on the ADJ input resulting from the drain-source capacitance of the FET. Negative spikes at the input to the chip should be avoided as they may cause errors in output current or erratic device operation. SHUTDOWN MODE Taking the ADJ pin to a voltage below 0.2V will turn off the output and supply current will fall to a low standby level of 120μA nominal. INHERENT OPEN-CIRCUIT LED PROTECTION If the connection to the LED(s) is open-circuited, the coil is isolated from the LX pin of the chip, so the chip will not be damaged, unlike in many boost converters, where the back EMF may damage the internal switch by forcing the drain above its breakdown voltage. CAPACITOR SELECTION A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in |
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