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L6996 Scheda tecnica(PDF) 10 Page - STMicroelectronics |
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L6996 Scheda tecnica(HTML) 10 Page - STMicroelectronics |
10 / 26 page L6996 10/26 1 DEVICE DESCRIPTION 1.1 Constant On Time PWM Topology Figure 12. Loop block schematic diagram This device implements a Constant On Time control, where the Ton is the on time duration forced by a one-shot circuit. The controller calculates the one-shot time directly proportional to the VCS- pin voltage and inversely to the OSC pin voltage as in Eq 1: Eq 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 = 2V. In fact if the VOSC voltage increases above 2V the cor- responding Ton will not decrease. Connecting OSC pin to a voltage partition from VIN to GND, it allows steady- state switching frequency FSW independent of VIN. It results: Eq 2 where Eq 3 The above equations allow setting the frequency divider ratio aOSC once output voltage has been set; note that such equations hold only if VOSC<2.A minimum off-time constrain of about 500nS is introduced in order to as- sure the boot capacitor charge and to limit switching frequency after a load transient as well as to mask PWM comparator output against switching noise and spikes. The system has not an internal clock, because this is a hysteretic controller, so the turn on pulse will start if three Q Vprog LS HS VFB+ DS CS- VID0-4 Rsense R1 R S Vout Vin HGATE LGATE Q R2 PWM comparator VFB- FFSR One-shot generator OSC - + + - DAC T ON K OSC V CS- V OS C --------------- τ + = F SW V OUT V IN --------------- 1 T ON ----------- ⋅α OSC → F SW K OSC ⋅ == α OSC V OS C V IN --------------- R 2 R 2 R 1 + -------------------- == |
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