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TPS60250RTE Fiches technique(PDF) 11 Page - Texas Instruments |
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TPS60250RTE Fiches technique(HTML) 11 Page - Texas Instruments |
11 / 24 page www.ti.com LED CURRENT SINKS (DM_, DS_) OPEN LAMP DETECTION CAPACITOR SELECTION SETTING THE LED CURRENT TPS60250 SLVS769 – APRIL 2007 APPLICATION INFORMATION (continued) The TPS60250 has constant current sinks which drive seven individual LED current paths. Each current sink regulates the LED current to a constant value determined by the I2C interface. The internal register addressing allows the LED main channels DM1~DM5 to be controlled independently from the LED sub channels DS1~DS2. All the LED channels sink up to 25mA of current except DM5 which has an 80-mA maximum current when configured as an auxiliary output. Using the I2C interface, the user may assign DM5 to the main display bank with up to 25-mA current or as an auxiliary output for torch or keypad light or low/weak camera flash with 80-mA current. DM5 has 64 dimming steps same as main and sub display banks when assigned to the main display. However, it has its own current programming register and enable control. When assigned as an auxiliary, DM5 has 4 dimming steps (full scale, 70%, 40%, 20%). These optimized current sinks minimize the voltage headroom required to drive each LED and maximize power efficiency by increasing the amount of time the controller stays in 1 × mode before transitioning to 1.5× mode. In system production it is often necessary to leave LED current paths open depending on the phone model. For example, one phone may use 2 LEDs to backlight the main display while another uses 4 LEDs. Rather than use two different ICs for these different phone applications, the TPS60250 may be used in both applications with no additional efficiency loss in the 2 LED applications. In traditional LED driver applications when an LED current path is open, the current sink voltage falls to ground and the current regulation circuitry drives the output to a maximum voltage in an attempt to regulate the current for the missing LED path. This severely reduces the system efficiency. The TPS60250 uses 7 internal comparators to detect when one or more open LED condition occurs and shut down prevent it from forcing the device to gear up the open current sink. The open lamp detection is enabled/disabled using the I2C interface. The TPS60250 is optimized to work with ceramic capacitors with a dielectric of X5R or better. The two flying capacitors must be the same value for proper operation. The 750-kHz switching frequency requires that the flying capacitor be less than 4.7 µF. Use of 1-µF ceramic capacitors for both chargepump flying capacitors is recommended. For good input voltage filtering, low ESR ceramic capacitors are recommended. A 1- µF ceramic input capacitor is sufficient for most of the applications. For better input voltage filtering this value can be increased. The output capacitor controls the amount of ripple on the output. Since small ripple is undetectable by the human eye, a 4.7- µF output capacitor works well. If better output filtering and lower ripple is desired, a larger output capacitor may be used. Figure 14. Dimming Steps for Sub, Main, and Keypad Backlight 11 Submit Documentation Feedback |
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