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L6711 데이터시트(PDF) 38 Page - STMicroelectronics |
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L6711 데이터시트(HTML) 38 Page - STMicroelectronics |
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38 / 50 page ![]() Oscillator L6711 38/50 15 Oscillator The internal oscillator generates the triangular waveform for the PWM charging and discharging with a constant current an internal capacitor. The switching frequency for each channel, FSW, is internally fixed at 150kHz so that the resulting switching frequency at the load side results in being tripled. The current delivered to the oscillator is typically 25 µA (corresponding to the free running frequency Fsw=150kHz) and it may be varied using an external resistor (ROSC) connected between the OSC pin and SGND or VCC (or a fixed voltage greater than 1.24V). Since the OSC pin is fixed at 1.24V, the frequency is varied proportionally to the current sunk (forced) from (into) the pin considering the internal gain of 6KHz/ µA. In particular connecting ROSC to SGND the frequency is increased (current is sunk from the pin), while connecting ROSC to VCC=12V the frequency is reduced (current is forced into the pin), according to the following relationships: Maximum programmable switching frequency depends on the Current Reading Method selected. When reading across LS mosfet, the maximum switching frequency per phase must be limited to 500kHz to avoid current reading errors causing, as a consequence, current sharing errors. When reading across the inductor, higher switching frequency can be approached (device power dissipation must be checked prior to design high switching frequency systems). Figure 19. ROSC vs. switching frequency R OSC vs. GND: FSW 150kHz 1.237 R OSC kΩ () ---------------------------- 6 kHz µA ----------- ⋅ + 150kHz 7.422 10 6 ⋅ R OSC kΩ () ----------------------------- + == R OSC vs. 12V: FSW 150kHz 12-1.237 R OSC kΩ () ---------------------------- 6 kHz µA ----------- ⋅ + 150kHz 6.457 10 7 ⋅ R OSC kΩ () ----------------------------- + == 0 2000 4000 6000 8000 10000 12000 14000 25 50 75 100 125 150 0 200 400 600 800 1000 150 250 350 450 550 650 750 850 ROSC [kΩ] to SGND vs. Selected FSW [kHz] ROSC [kΩ] to 12V vs. Selected FSW [kHz] |
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