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TPS60255RTER Datasheet(PDF) 9 Page - Texas Instruments |
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TPS60255RTER Datasheet(HTML) 9 Page - Texas Instruments |
9 / 25 page www.ti.com V A V F V DX TPS60255 Vin Gear Down Gear Up Vout Vin 1.5 Mode Operating Area ´ 1 Mode Operating Area ´ VB VC Vhys TPS60255 SLVS825 – MARCH 2008 Figure 10. Input Voltage Hysteresis Between 1 × and 1.5× Modes As shown in Figure 10, there is input hysteresis voltage between 1 × and 1.5× modes to ensure stable operation during mode transition. For the single-cell Li-ion battery input voltage range, the TPS60255 operates in 1 × mode when a fully charged battery is installed. Once the battery voltage drops below the VB level, the WLED driver operates in the 1.5 × mode. Once in 1.5× mode with the same LED current condition, the battery voltage must rise to the VC level in order to transition from 1.5× to 1× mode. This hysteresis ensures stable operation when there is some input voltage fluctuation at the 1 ×/1.5× mode transition. The input transition voltage (VB) depends on the drop out voltage of the charge pump stage (VA), WLED forward voltage (VF), and the mode transition threshold voltage (VTH_GU). The input transition voltage is calculated as: VB = VA + VF + VTH_GU VA = RO_1X × IO where RO_1X is the 1 × mode output impedance and I O is the total output current. See the ELECTRICAL CHARACTERISTICS table for output impedance specifications. The TPS60255 switches to 1.5 × mode when the input voltage is below V B and remains in the 1.5× mode as long as the input is lower than VC. When the input voltage rises above VC, 1.5× Mode is exited. VC is calculated as: VC = VF + 550mV The input voltage mode transition hysteresis voltage (VHYS) between the 1× and 1.5× modes is calculated as: VHYS = VC – VB = 550 mV – VTH_GU – VA, where VTH_GU = 100 mV Note that VA is the key factor in determining VHYS and is dependant on the 1× mode charge pump output impedance and WLED current. Example: If we choose LWE67C (Osram) and set Iled = 15.3 mA, then VF = 3.55 V according to the LED characteristics curve. Total load current, IO = 15.3 mA × 6 = 91.8 mA VB = RO1X * IO + VTH_GU + VF = 1*0.0918 + 0.1 + 3.55 = 3.748 (Gear up voltage) Copyright © 2008, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Link(s): TPS60255 |
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