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L6751 датащи(PDF) 46 Page - STMicroelectronics |
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L6751 датащи(HTML) 46 Page - STMicroelectronics |
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46 / 59 page ![]() Main oscillator L6751 46/59 Doc ID 023992 Rev 1 10 Main oscillator The internal oscillator generates the triangular waveform for the PWM charging and discharging, with a constant current, on the internal capacitor. The switching frequency for each channel is internally fixed at 200 kHz (FSW) and at 230 kHz (FSSW): the resulting switching frequency at the load side for the multi-phase section results in being multiplied by N (number of configured phases). The current delivered to the oscillator is typically 20 μA and may be varied using an external resistor (ROSC, RSOSC) typically connected between the OSC/SOSC pins and GND. Since the OSC/SOSC pins are fixed at 1.02 V, the frequency is varied proportionally to the current sunk from the pin considering the internal gain of 10 KHz/ μA for the multi-phase section and of 11.5 kHz/ μA for the single-phase section, see Figure 14. Connecting ROSC/RSOSC to SGND, the frequency is increased (current is sunk from the pin), according to the following relationships: Equation 17 Equation 18 Connecting ROSC/RSOSC to a positive voltage Vbias, the frequency is reduced (current is injected into the pin), according to the following relationships: Equation 19 Equation 20 Figure 14. ROSC vs. FSW per phase (ROSC to GND - left; ROSC to 3.3 V - right) F SW 200kHz 1.02V R OSC kΩ () --------------------------- 10 kHz μA ----------- ⋅ + = F SSW 250kHz 1.02V R SOSC kΩ () ------------------------------- 11.5 kHz μA ----------- ⋅ + = F SW 200kHz Vbias 1.02V – R OSC kΩ () -------------------------------------- 10 kHz μA ----------- ⋅ – = F SSW 250kHz Vbias 1.02V – R SOSC kΩ () -------------------------------------- 11.5 kHz μA ----------- ⋅ – = 10 100 1000 200 300 400 500 600 700 800 900 1000 Multi Phase section SinglePhase section 100 1000 75 100 125 150 175 200 225 Multi Phase section SinglePhase section AM14820v1 |
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