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CS1616-FSZR Datasheet(PDF) 10 Page - Cirrus Logic |
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CS1616-FSZR Datasheet(HTML) 10 Page - Cirrus Logic |
10 / 16 page CS1615/16 10 DS961F1 Figure 14 illustrates the dual-mode buck-boost topology. The CS1615/16 regulates the output current by controlling the peak current to ensure that the target output charge is achieved every half line-cycle. Demagnetization time of the inductor is sensed by the FBAUX pin using an auxiliary winding and is used as an input to the control loop. 5.6.1 Primary-Side Current Control All input current shaping and output power transfer is attained using a peak current control algorithm. Demagnetization time of the primary inductor is sensed by the FBAUX pin using an auxiliary winding and is used as an input to the control algorithm. The values obtained from resistors RCTRL1 and RCTRL2 are the other inputs to the control algorithm that help shape the input current and control the LED current, respectively. 5.6.2 Output Current Regulation The CS1615/16 regulates output current by controlling the charge transferred over a half line-cycle. The full-scale output current target is set using resistor RCTRL2, which is connected on pin CTRL2. This pin is sampled periodically by an ADC. The value of this resistor can be determined using Equation 1. where, N = turns ratio IOUT = current through LED at maximum output RSense = resistor attached to pin FBSENSE When designing a buck-boost topology the turns ratio N is set to one. The CS1615/16 uses the value obtained from the resistor along with the phase-cut and line-cycle period information to determine the corresponding target full-scale output charge. The IC controls the inductor switching frequency and peak current to ensure that the target output charge is achieved every half line-cycle, thus regulating the output current. 5.6.3 Input Current Shaping The CS1615/16 shapes the input current by controlling the peak primary current and the flyback/buck-boost switching frequency. It shapes the currents differently when behind a dimmer compared to when not behind a dimmer. 5.6.3.1 Operation Behind a Dimmer Operating behind a dimmer, the CS1615/16 controls the switching frequency to ensure that the average input current is greater than the dimmer hold current requirement. The dimmer hold current level is sensed using resistor RCTRL1 on pin CTRL1, which is sampled periodically by an ADC. The value of this resistor can be determined using the formula shown in Equation 2. where, IIN(CC) = constant input current used when designing circuit RSense = resistor attached to pin FBSENSE 5.6.3.2 Operation in No-dimmer Mode Operating in No-dimmer Mode, the CS1615/16 controls the switching frequency to ensure that the average input current follows the line voltage to provide power factor correction. In No- dimmer Mode the controller is designed to operate in quasi- resonant mode to improve efficiency. 5.6.4 Max Primary-side Switching Current Maximum primary-side switching current IPK(max) is set using resistor RSense connected to pin FBSENSE of the CS1615/16. The maximum primary-side switching current can be calculated using Equation 3. 5.6.5 Auxiliary Winding Configuration The auxiliary winding is used for zero-current detection (ZCD), overvoltage protection (OVP), fast startup, and the steady-state power supply. The voltage on the auxiliary winding is sensed through pin FBAUX of the CS1615/16 for zero-current detection, overvoltage protection, and fast startup. The auxiliary winding is also used to provide the steady-state power supply to the CS1615/16. 5.6.6 Output Open Circuit Protection Output open circuit protection and output overvoltage protection (OVP) are 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 approximately one second. RCTRL2 CTRL2 FBAUX GND GD 9 12 CS1615/16 16 13 Q3 RSense Vrect FBSENSE 11 LED+ LED- L2 R7 R8 D4 D5 C8 VAUX C7 Figure 14. Buck-boost Model RCTRL2 1.4V N 4M 1.25 511 RSense IOUT ------------------------------------------------------------------------ = [Eq.1] RCTRL1 1.4V 4M 511 IIN CC RSense ------------------------------------------------------------ = [Eq.2] IPK max 1.4 RSense ------------------- = [Eq.3] |
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