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L6995DTR bảng dữ liệu(PDF) 10 Page - STMicroelectronics |
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10 / 25 page L6995 10/25 be supposed linear function of the time. Eq 8 so the output voltage is given by: Eq 9 calling Vout as the Vout final value, the output charging time can be estimated as: Eq 10 the minimum CSS value is given imposing this condition: Eq 11 Tout =Tuvp 1.5 Current limit The current limit comparator senses the inductor current through the low side MOSFET RDSON drop and com- pares this value with the ILIM pin voltage value. While the current is above the current limit value, the control inhibits the one-shot start. To properly set the current limit threshold, it should be noted that this is a valley current limit. Average current depends on the inductor value, VIN VOUT and switching frequency. The average output current in current limit is given by: Eq 12 Thus, to set the current threshold, choose RILIM according to the following equation: Eq 13 Where KC is the current limit factor (Typ 1µA). In current limit the system keeps the current constant until the output voltage meets the undervolatge threshold. The system is capable to sink current, but it has not a negative current limit. The system accuracy is function of the exactness of the resistance connected to ILIM pin and the low side MOS- FET RDSON accuracy. Moreover the voltage on ILIM pin must range between 10mV and 2V to ensure the sys- tem linearity. Figure 6. Current limit schematic I L t,CSS () R ilim /Rds on K C I SS t ⋅⋅ ⋅ () V SS C SS ⋅ ∆ () ---------------------------------------------------------------------------- = V out t,CSS () Qt,C SS () C out ------------------------- R ilim /Rds on K C I SS t 2 ⋅⋅ ⋅ () C ou t V SS C SS 2 ⋅⋅ ∆ ⋅ () ------------------------------------------------------------------------------- == I out CSS () V out C ou t V SS C SS 2 ⋅⋅ ∆ ⋅⋅ () R i lim /Rdson K C I SS ⋅⋅ () --------------------------------------------------------------------------------------- 0.5 = I OUT CL I max valley I ∆ 2 ----- + = I max valley R ILim Rds on ----------------- K C ⋅ = Positive current limit R ILIM 5µA Current Comparator To logic PGN D PHASE To inductor LS |
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Mô tả tương tự - L6995DTR |
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