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AAT1271 Datasheet(PDF) 9 Page - Advanced Analogic Technologies |
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AAT1271 Datasheet(HTML) 9 Page - Advanced Analogic Technologies |
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9 / 19 page  AAT1271 1.5A Step-Up Current Regulator for Flash LEDs SwitchRegTM PRODUCT DATASHEET 1271.2008.08.1.1 9 www .analogictech.com AAT1271 1.5A Step-Up Current Regulator for Flash LEDs SwitchRegTM PRODUCT DATASHEET 1271.2008.08.1.1 9 www .analogictech.com Functional Description The AAT1271 is a boost converter with a current regu- lated output designed to drive high current white LEDs used in camera flash applications. The AAT1271 has two channels to accurately regulate the current flow through two separate white LEDs. There are two basic modes of operation in the AAT1271; a Flash mode controlled by the FLEN pin and the movie mode controlled through the AS2Cwire interface. Flash Mode A flash pulse is initiated by strobing the FLEN input pin low-to-high, which initiates a flash pulse and also starts the internal timer. The maximum flash current in the AAT1271 is set by an external resistor, RSET, which sets the flash current and the maximum movie-mode current reduced by a factor of 2. The flash timer will terminate the flash current regardless of the status of the FLEN pin. This can be either used as a simple flash timing pulse or can be used as a safety timer in the event of a control logic malfunction to prevent the LED from over-heating. The maximum flash time is determined by an external timing capacitor connected to the CT pin. The flash dura- tion can be set from 50ms up to a maximum of 1s. The AS2Cwire interface allows further adjustment of the flash timer duration. This allows the flash timer duration to be reduced in 16 linear steps from the maximum time set by the timing capacitor. If the safety timer is not needed in the application, it can be disabled by connecting the CT pin directly to AGND. The AAT1271 has two LED current sources which share the output current equally. For a single white LED appli- cation, the two current sources can be connected togeth- er to apply full output current into the LED. In two LED applications, each diode can be connected to its corre- sponding current source (FLOUTA or FLOUTB) and the output current will be shared. In applications where only one LED is connected to either FLOUTA or FLOUTB, the unused current sink must be directly connected to OUT, thereby disabling that channel. In mobile GSM systems where the phone remains in constant contact with the base station by regular com- munication, a FLINH pin is provided to prevent both the camera flash and PA transmission pulses from occurring simultaneously. This avoids potential dips to the Li-ion battery voltage below the system’s undervoltage lockout threshold (UVLO). During a flash event, strobing the FLINH pin low-to-high reduces the LED current to the default movie-mode current level for the duration of FLINH. Strobing FLINH high-to-low instructs the AAT1271 to revert the flash LED current to its maximum level, assuming that the FLEN pin is still active (HIGH) and the flash timer has not expired. Movie Mode The movie mode current level, channel enable, and the flash to maximum movie mode current ratio are set using the AAT1271’s AS2Cwire interface at the EN/SET pin. The movie-mode current level can be adjusted in 16 steps using a logarithmic scale where each code is 1dB below the previous code. Channel current outputs (FLOUTA and FLOUTB) in movie-mode can be enabled or Functional Block Diagram SW PGND FLOUTA AGND Control RSET IN CT FLOUTB FLGND OUT EN/SET FLEN FLINH GND 200kΩ |
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