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FL7734MX Datasheet(PDF) 11 Page - ON Semiconductor |
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FL7734MX Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 14 page © 2014 Fairchild Semiconductor Corporation www.fairchildsemi.com FL7734 • Rev. 1.0 10 As a function of dimming reference modulation shown in Figure 18, output current is constantly regulated with constant VREF when VDIM is higher than 3 V and VREF is set lower than VEAI (TRUECURRENT ® calculation result) when VDIM is lower than 2.25 V. Once VDIM is less than 2.25 V, the error amplifier always pulls down current in the output and FB voltage is clamped by MOD voltage so that open loop control starts for stable LED current control at low phase angle range. IDIM VVIN-DIM (3 V) Input Voltage VIN RVIN1 RVIN2 VDIM-CLAMP (3.25 V) CDIM RDIM1 DIM RDIM2 MOD FB CFB Dimming Reference Modulation VEAI Error Amp. VREF Figure 18. Dimming Control High dimmer compatibility is implemented by bleeding current generated through BLD to RBLD and having the switching mode management by TCIC. PFC and THD In a conventional boost converter, Boundary Conduction Mode (BCM) is generally used to keep input current in phase with input voltage for Power Factor (PF) and Total Harmonic Distortion (THD). However, BCM switching distorts input current in the single stage flyback / buck boost converter because power inductor current is not the same as input current. Moreover, it becomes more difficult to meet PF and THD once passive bleeder (resistor and capacitor in the driver input) is added for successful firing in the single stage TRIAC dimming system. IIN IIN_AVG GATE Constant Frequency Figure 19. PFC Control in DCM In order to optimize PF and THD in the single stage flyback topology, constant turn-on time and constant frequency in Discontinuous Conduction Mode (DCM) are the best method to make the input current proportional to the input voltage as shown in Figure 19. FL7734 basically adopts the DCM operation with constant turn-on and frequency in a half line cycle. Once dimmer absence is detected during startup sequence, FL7734 selects voltage mode in which constant turn-on time is maintained by an internal error amplifier and a large external capacitor (typically >1 µF) at the FB pin. Constant frequency and DCM operation are managed by linear frequency control. Linear Frequency Control DCM should be guaranteed for high power factor in flyback topology. To maintain DCM in the wide range of output voltage, frequency is linearly adjusted by output voltage in linear frequency control. Output voltage is detected by auxiliary winding and resistive divider connected to the VS pin, as shown in Figure 20. When output voltage decreases, secondary diode conduction time is increased and the linear frequency control lengthens switching period, which retains DCM operation in the wide output voltage range. The frequency control lowers primary rms current for better power efficiency in full-load condition. OSC Linear Frequency Controller VVS freq. VOUT VS tDIS Detector DCM BCM Figure 20. Linear Frequency Control BCM Control The end of secondary diode conduction time (tDIS) can be over a switching period set by linear frequency control. In this case, FL7734 doesn ’t allow CCM and operation mode changes from DCM to BCM. Therefore, magnetizing inductance can be largely designed to add BCM for better efficiency if PF and THD meet specification with enough margin. |
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