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AAT2842IBJ-EE-T1 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT2842IBJ-EE-T1 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
11 / 21 page AAT2842 High-Current Charge Pump with S2Cwire™ Control and Dual LDO for Backlight and Flash 2842.2006.10.1.0 11 The backlight and flash LED currents are controlled by a combination of an external programming resistor from BSET (for backlight) or FSET (for flash) to AGND and the backlight or flash serial S2Cwire interface BENS or FENS. The program- ming resistor sets the maximum LED current for each channel, and the serial S2Cwire interface con- trols the LED current relative to the maximum. To drive backlight LEDs with optimal absolute accuracy and channel-to-channel matching, the maximum output current is set to 30mA with a 280k Ω resistor connected at the BSET pin of the AAT2842. Depending upon the application and the white LEDs chosen, the preferred method to decrease the maximum BL output current is to use a 280k Ω resistor for RBSET and then set the desired current output using the product's S2Cwire inter- face, as shown in Table 1. Using Backlight LED Outputs for Low-Current LED Applications The AAT2842's backlight current outputs can be pro- grammed to drive lower current LEDs, such as those used for keypad applications. For best low-current accuracy and matching, the preferred method is to use a 280k Ω resistor for RBSET and then set the desired current output using the product's S2Cwire interface, as shown in Table 1. If any one of the current sinks is not used, connect the unused current sink to OUT. The current con- troller monitors the current sink voltage and, if it is connected to OUT, then it is assumed that the cur- rent sink is not used or that the LED is shorted, and the controller turns off that current sink. S2Cwire Serial Interface The S2Cwire serial interface records rising edges of the EN/SET pin and decodes them into 16 different states. The S2Cwire interface has flexible timing; data can be clocked-in at speeds greater than 1MHz or much slower, such as 15kHz. After data is submitted, EN/SET is held high to latch the data. Once EN/SET has been held in the logic high state for time TLAT, the programmed current becomes active and the internal data register is reset to zero. For subsequent current level programming, the number of rising edges corresponding to the desired code must be entered on the EN/SET pin. The AAT2842 features separate control interfaces for the backlight and flash current control. The backlight current features 16 current steps, each as a percentage of the maximum backlight current set by the BSET resistance. The flash has 16 current level settings, again as a percentage of the maxi- mum flash current set by the FSET resistance (see Tables 1 and 2). Initiating a flash current also initi- ates the flash timer which is programmed via an external capacitor CT. Calculate the flash time T by the following equation: where T is in seconds and CT is in µF. For example, for a 0.1µF capacitor: To disable the flash timer, connect CT to AGND. Table 1: Backlight Current Register: BL1-BL4 (RBSET = 280kΩ Ω). Data BL% of BSET 1 100 282 370 459 5 49.2 6 41.0 7 33.0 8 28.2 9 23.5 10 18.7 11 15.4 12 12.3 13 7.7 14 4.1 15 2.4 16 0.2 T = 10 × 0.1µF = 1s T = 10 × CT |
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