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AN937 Datasheet(PDF) 4 Page - STMicroelectronics |
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AN937 Datasheet(HTML) 4 Page - STMicroelectronics |
4 / 29 page Power supply, UVLO and voltage reference AN937 4/29 Doc ID 5655 Rev 15 2 Power supply, UVLO and voltage reference The device is provided with an internal stabilized power supply (of about 12 V typ.) that powers the analog and digital control blocks and the bootstrap section. From this pre regulator, a 3.3V reference voltage ±2%, is internally available. 2.1 Oscillator and voltage feed forward Just one pin is necessary to implement the oscillator function, with inherent voltage feedforward. Figure 4. Oscillator internal circuit A resistor Rosc and a capacitor Cosc connected as shown in Figure 4, allow the setting of the desired switching frequency in agreement with the below formula: Where Fsw is in kHz, Rosc in kΩ and Cosc in nF. The oscillator capacitor, Cosc, is discharged by an internal MOS transistor of 100 Ω of Rdson (Q1) and during this period the internal threshold is set at 1 V by a second MOS, Q2. When the oscillator voltage capacitor reaches the 1V threshold the output comparator turn-off the MOS Q1 and turn-on the MOS Q2, restarting the Cosc charging. The oscillator block, shown in Figure 4, generates a sawtooth wave signal that sets the switching frequency of the system. This signal, compared with the output of the error amplifier, generates the PWM signal that will modulate the conduction time of the power output stage. The way the oscillator has been integrated, does not require additional external components to benefit of the voltage feed forward function. VCC Osc ROSC TO PWM COMPARATOR COSC Q1 1V R Q2 5R CLOCK D97IN655A - + F SW 1 R oasc C osc ⋅ ()In 6 5 --- ⎝⎠ ⎛⎞ 100 C osc ⋅ + ------------------------------------------------------------------------------------- = |
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