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CS1613 Folha de dados(PDF) 11 Page - Cirrus Logic |
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CS1613 Folha de dados(HTML) 11 Page - Cirrus Logic |
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11 / 17 page  CS1610/11/12/13 DS929F5 11 A quasi-resonant buck stage is illustrated in Figure 13. The buck stage is controlled by measuring current in the buck inductor and voltage on the auxiliary winding. Figure 13. Buck Model The digital buck algorithm ensures monotonic dimming from 2% to 100% of the dimming range with a linear relationship between the dimming signal and the LED current. Quasi-resonant operation is achieved by detecting second stage inductor demagnetization via an auxiliary winding. The digital control algorithm rejects line-frequency ripple created on the second stage input by the front-end boost stage, resulting in the highest possible LED efficiency and long LED life. 5.7.1 Auxiliary Winding Configuration The auxiliary winding is also used for zero-current detection (ZCD) and overvoltage protection (OVP). The auxiliary winding is sensed through the FBAUX pin of the IC. 5.7.2 Control Parameters The second stage control parameters assure: • Line Regulation — The LED current remains constant despite a ±10% AC line voltage variation. • Effect of Variation in Transformer Magnetizing Inductance — The LED current remains constant over a ±20% variation in magnetizing inductance. The second stage requires three inputs and generates one key output. The FBSENSE input is used to sense the current in the second stage inductor. When the current reaches a certain threshold, the gate drive turns off (output on pin GD). The sensed current and the FBGAIN input are used to determine the switching period Ttotal. The zero-current detect input on pin FBAUX is used to determine the demagnetization period T2. The controller then uses the time Ttotal to determine gate turn-on time. The FBGAIN input is set using an external resistor, RFBGAIN. Resistor RFBGAIN must be selected to ensure that the switching period Ttotal is greater than the resonant switching period Tcritical at maximum output power. See Equation 5: where, Tcritical = resonant switching period at max power T1 = gate turn-on time T2 = demagnetization time The switching period Ttotal is computed for flyback topology using Equation 6: where, = dimming factor, proportional to the duty cycle of the dimmer, between 0 and 1 IPK(FB) = transformer primary winding current FBgain =Ttotal/T2; a constant, computed at full load 13 11 R FB GA IN FBGAIN FBAUX GND GD FBSENSE 15 9 12 CS1612/13 R12 R11 R13 Q4 LED + LED - VBST C8 D8 C9 L3 Ttotal Tcritical T1 T2 + = [Eq.5] Ttotal IPK FB T2 FBgain ------------------ [Eq.6] |
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