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L6995 データシート(PDF) 6 Page - STMicroelectronics |
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L6995 データシート(HTML) 6 Page - STMicroelectronics |
6 / 25 page L6995 6/25 1 DEVICE DESCRIPTION 1.1 Constant On Time PWM topology Figure 2. 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: Eq 1 where KOSC = 250ns and τ is the internal propagation delay time (typ. 70ns). The system imposes in steady state a minimum on time corresponding to VOSC = 2V. In fact if the VOSC voltage increases above 2V 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: Eq 2 where Eq 3 Eq 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<2V. Further the Eq2 shows how the system has a switching frequency ideally independent from the input voltage. The delay introduces a light dependence from VIN. A minimum off-time con- strain of about 580ns is introduced in order to assure the boot capacitor charge and to limit the switching fre- 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 OU T V IN --------------- 1 T ON ----------- α OS C α OUT -------------- 1 K OSC --------------- α OSC → f SWKOSC 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 + -------------------- == |
同様の部品番号 - L6995 |
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同様の説明 - L6995 |
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