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MC33260DG Datasheet(PDF) 10 Page - ON Semiconductor |
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MC33260DG Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 22 page MC33260 http://onsemi.com 10 Pin Numbers are Relevant to the PDIP−8 Version Ro + Vo regH * Vpin1 I regH Consequently: In practice, Vpin1 is small compared to (Vo)regH and this equation can be simplified as follows (IregH being also replaced by its typical value 200 mA): Ro [ 5 Vo regH (kW) The regulation block output is connected to the Pin 2 through a 300 k W resistor. The Pin 2 voltage (Vcontrol) is compared to the oscillator sawtooth for PWM control. An external capacitor must be connected between Pin 2 and ground, for external loop compensation. The bandwidth is typically set below 20 Hz so that the regulation block output should be relatively constant over a given ac line cycle. This integration that results in a constant on−time over the ac line period, prevents the mains frequency output ripple from distorting the ac line current. OSCILLATOR SECTION The oscillator consists of three phases: • Charge Phase: The oscillator capacitor voltage grows up linearly from its bottom value (ground) until it exceeds Vcontrol (regulation block output voltage). At that moment, the PWM latch output gets low and the oscillator discharge sequence is set. • Discharge Phase: The oscillator capacitor is abruptly discharged down to its valley value (0 V). • Waiting Phase: At the end of the discharge sequence, the oscillator voltage is maintained in a low state until the PWM latch is set again. Figure 26. Oscillator Icharge = 2 Io Io / Iref 0 1 CT 3 15 pF 0 1 Output_Ctrl The oscillator charge current is dependent on the feedback current (Io). In effect I charge + 2 I2 o I ref where: Icharge is the oscillator charge current, Io is the feedback current (drawn by Pin 1), Iref is the internal reference current (200 mA). So, the oscillator charge current is linked to the output voltage level as follows: I charge + 2 Vo * Vpin1 2 R2 o I ref where: Vo is the output voltage, Ro is the feedback resistor, Vpin1 is the Pin 1 clamp voltage. In practice, Vpin1 that is in the range of 2.5 V, is very small compared to Vo. The equation can then be simplified by neglecting Vpin1: I charge [ 2 V2 o R2 o I ref It must be noticed that the oscillator terminal (Pin 3) has an internal capacitance (Cint) that varies versus the Pin 3 voltage. Over the oscillator swing, its average value typically equals 15 pF (min 10 pF, max 20 pF). The total oscillator capacitor is then the sum of the internal and external capacitors. C pin3 + C T ) C int PWM LATCH SECTION The MC33260 operates in voltage mode: the regulation block output (Vcontrol − Pin 2 voltage) is compared to the oscillator sawtooth so that the gate drive signal (Pin 7) is high until the oscillator ramp exceeds Vcontrol. The on−time is then given by the following equation: ton + C pin3 V control I ch where: ton is the on−time, Cpin3 is the total oscillator capacitor (sum of the internal and external capacitor), Icharge is the oscillator charge current (Pin 3 current), Vcontrol is the Pin 2 voltage (regulation block output). Consequently, replacing Icharge by the expression given in the Oscillator Section: ton + R2 o I ref C pin3 V control 2 V2 o One can notice that the on−time depends on Vo (preconverter output voltage) and that the on−time is maximum when Vcontrol is maximum (1.5 V typically). At a given Vo, the maximum on−time is then expressed by the following equation: ton max + C pin3 R2 o I ref V control max 2 V2 o This equation can be simplified replacing 2 [(Vcontrol) max *Iref] by Kosc Refer to Electrical Characteristics, Oscillator Section. Then: ton max + C pin3 R2 o Kosc V2 o |
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