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ST1208 Fiches technique(PDF) 5 Page - Sitronix Technology Co., Ltd. |
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ST1208 Fiches technique(HTML) 5 Page - Sitronix Technology Co., Ltd. |
5 / 12 page ST1208 V1.1 5/12 2004/10/27 ! Function Description The ST1208 is a complete charge-pump buck-boost converter which is requiring only four small ceramic capacitors. In order to regulated constant current for uniform intensity, the ST1208 utilizes a proprietary 1x/1.5x fractional charge-pump topology to drive up three white LEDs. While two digital inputs control on/off and provide brightness control, an external resistor (RSET) programs the full-scale LED current. The ST1208 operates with a 1MHz fixed frequency. Output Regulation The ST1208 operates in 1x mode until just above dropout. Then the ST1208 switches to 1.5x charge pump mode to regulate the LED1 to 300mV and maintain constant LED brightness even at very low battery voltages. Using this topology, there is no LED brightness change from 1x to 1.5x. which guarantees no flicker on the display. The 1x mode produces almost no ripple, while the 1.5x mode regulates the output voltage by controlling the rate at which the charge pump capacitors are charged. In this way, the switching frequency remains constant for reduced input ripple and stable noise spectrum. Shutdown Mode When EN1 and EN2 are grounded, the ST1208 is in shutdown. The shutdown current is less than 1uA. ST1208 which is out of shutdown will be operated in 1x mode while the input voltage is higher than output voltage. Thermal Shutdown The ST1208 includes a thermal-limit circuit that shutdown the IC at about +160°C. Turn-on occurs after the IC cools by approximately 20°C. Setting the Output Current SET control the LED bias current. Current into LED1, LED2 and LED3 is a multiple of the current flowing out of SET. Set the output current as follows: ILEDx = K X ( 0.6 / RSET ) Where K = 160, 48 or 16 ( depending upon EN1 and EN2. see Table1 ), and RSET is the resistor connected between SET and GND. Soft Start The ST1208 includes soft-start circuitry to limit inrush current at turn-on. When starting up with an output voltage that is not near the input voltage, the output capacitor is charged directly from the input with a ramped current source until the output voltage is near the input voltage. Once this occurs, the charge pump determines if 1x or 1.5x mode is required. In the case of 1x mode, the soft start is terminated and normal operation begins. In the case of 1.5x mode, soft-start operates until the LED1 reaches regulation. |
Numéro de pièce similaire - ST1208 |
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Description similaire - ST1208 |
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