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CS1611 データシート(PDF) 11 Page - Cirrus Logic |
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CS1611 データシート(HTML) 11 Page - Cirrus Logic |
11 / 16 page CS1610/11/12/13 DS929F3 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 using the formula shown in 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 An appropriate value for RFBGAIN needs to be selected to provide the correct FBgain. Resistor RFBGAIN is calculated using the formula shown in Equation 7: The value of FBgain also has a bearing on the linearity of the dimming factor versus the LED current curve and must be chosen using Application Note AN364: CS1610/11 Design Guide. 5.7.3 Output Open Circuit Protection Output open circuit protection and output overvoltage protection (OVP) is implemented by monitoring the output voltage through the transformer auxiliary winding. If the voltage on the FBAUX pin exceeds a threshold (VOVP(th)) of 1.25V, a fault condition occurs. The IC output is disabled and the controller attempts to restart after one second. 5.7.4 Overcurrent Protection (OCP) Overcurrent protection is implemented by monitoring the voltage across the second stage sense resistor. If this voltage exceeds a threshold (VOCP(th)) of 1.69V, a fault condition occurs. The IC output is disabled and the controller attempts to restart after one second. 5.7.5 Open Loop Protection (OLP) Both open loop protection and protection against a short of the second stage sense resistor are implemented by monitoring the voltage across the resistor. If the voltage on pin FBSENSE does not reach the protection threshold (VOLP(th)) of 200mV, the IC output is disabled and the controller attempts to restart after one second. 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] RFBGAIN 4000000 FBgain 128 64 – ---------------------------------------------------- = [Eq.7] |
同様の部品番号 - CS1611 |
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同様の説明 - CS1611 |
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