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L6995S Scheda tecnica(PDF) 10 Page - STMicroelectronics |
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L6995S Scheda tecnica(HTML) 10 Page - STMicroelectronics |
10 / 26 page Obsolete Product(s) - Obsolete Product(s) L6995S 10/26 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 4.5 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 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 9. Current limit schematic I L t,CSS () R ilim /Rdson K C I SS t ⋅⋅ ⋅ () V SS C SS ⋅ ∆ () ------------------------------------------------------------------- = V out t,CSS () Qt,C SS () C out ------------------------- R ilim/Rdson K C I SS t 2 ⋅⋅ ⋅ () C out V SS C SS 2 ⋅⋅ ∆ ⋅ () ---------------------------------------------------------------------- == I out CSS () V out C out V SS C SS 2 ⋅⋅ ∆ ⋅⋅ () R ilim /Rdson K C I SS ⋅⋅ () ---------------------------------------------------------------------------- 0.5 = I OUT CL I max valley I ∆ 2 ----- + = I max valley R ILim Rds on ----------------- I Lim 5.2 ---------- ⋅ = Positive current limit R ILIM 5µA Current Comparator To logic PGN D PHASE To inductor LS |
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