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TPS60251RTW Fiches technique(PDF) 11 Page - Texas Instruments |
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TPS60251RTW Fiches technique(HTML) 11 Page - Texas Instruments |
11 / 28 page www.ti.com x1 Operating Area x1.5 Operating Area V I V I V O VA VF VDX CP WLEDDriver VB VC VHYS TPS60251 SLVS767 – APRIL 2007 APPLICATION INFORMATION (continued) Figure 13. Input Voltage Hysteresis Between ×1 and ×1.5 Mode As shown in Figure 13, there is input voltage hysteresis voltage between 1 × and 1.5× mode to ensure stable operation during mode transition. For the 1 cell Li-Ion battery input voltage range, the TPS60251 operates in 1 × mode when a fully charged battery is installed. Once the battery voltage drops below the VB level, which is the mode transition voltage from 1 × to 1.5×, the WLED driver operates in 1.5× mode. Once in 1.5× mode, the battery voltage must rise to the VC level in order to transition from 1.5× to 1×. This hysteresis ensures stable operation when there is some input voltage fluctuation at the 1 ×/1.5× mode transition. The WLED driver provides a typical 280mV hysteresis voltage (VHYS) that changes based on LED current, to prevent oscillating between modes. The transition voltage, VB, depends on VDX (the mode transition threshold voltage), VF (WLED forward voltage drop) and VA (the drop out voltage of the charge pump stage) and is calculated as follows: VB = VA + VF + VDX VA = ROUT1X× ILEDTOTAL Where ROUT1X is the 1× mode output impedance of the IC. See the Electrical Characteristics table for output impedance specifications. The TPS60251 switches to 1.5 × mode when the input voltage is below V B and remains in 1.5× mode as long as the input is lower than VC. 1.5× Mode is exited when the input voltage rises above VC. VC is calculated as: VC = VF + 470 mV The input voltage mode transition hysteresis voltage (VHYS) between 1× and 1.5× is calculated using the following equation. VHYS = VC– VB = 470 mV – VDX– VA, where VDX = 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. 11 Submit Documentation Feedback |
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