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L6997STR bảng dữ liệu(PDF) 6 Page - STMicroelectronics |
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6 / 30 page L6997S 6/30 4 DEVICE DESCRIPTION 4.1 Constant On Time PWM topology Figure 5. Loop block schematic diagram The device implements a Constant On Time control scheme, where the Ton is the high side MOSFET on time duration forced by the one-shot generator. The On Time is directly proportional to VSENSE pin voltage and in- verse to OSC pin voltage as in Eq1: (1) where KOSC = 180ns and τ is the internal propagation delay time (typ. 40ns). The system imposes in steady state a minimum On Time corresponding to VOSC = 1V. In fact if the VOSC voltage increases above 1V the cor- responding Ton will not decrease. Connecting the OSC pin to a voltage partition from VIN to GND, it allows a steady-state switching frequency FSW independent of VIN. It results: (2) where (3) (4) The above equations allow setting the frequency divider ratio αOSC once output voltage has been set; note that such equations hold only if VOSC<1V. Further the Eq2 shows how the system has a switching frequen- cy ideally independent from the input voltage. The delay introduces a light dependence from VIN. A mini- mum Off-Time constraint of about 500ns is introduced in order to assure the boot capacitor charge and to Q Vsense R3 R4 R2 R1 R S Vout Vin HGATE LGATE Q OSC FB Vref HS LS DS PWM comparator FFSR One-shot generator - + T ON K OSC V SENSE V OSC ---------------------- τ + = f SW V OUT V IN --------------- 1 T ON ----------- α OSC α OUT --------------- 1 K OSC --------------- α OSC → f SW KOSC αOUT == = α OSC V OSC V IN --------------- R 2 R 2 R 1 + -------------------- == α OUT V FB V OUT --------------- R 4 R 3 R 4 + -------------------- == |
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