SD7530 is a high power factor flyback PWM controller special for LED lighting applications. It operates in
transition mode, it reduces the switch turn-on loss, improves the conversion efficiency and provides very
good power factor correction.
VCC UVLO and VCC OVP
Peripheral/internal circuit at pin Vcc of SD7530 is shown in figure 1. At the beginning of the power on,
capacitor C1 is charged by VIN through start resistor R1, and VCC increases slowly. As VCC is up to 17.5V,
IC starts to work and output driving pulse. Output voltage should be established in a period during which
VCC discharges and diode D2 is reverse biased, and IC is fully powered by start capacitor C1. When output
voltage increases to a certain value, diode D2 is forward conduction by auxiliary winding L3’s voltage, and
then VCC increases rapidly to the desired value. Fig. 2 shows the voltage waveform of VCC during start.
IC enters under voltage status as VCC is lower than 9.0V after start.
Moreover, the startup current of SD7530 is very low (less than 5uA), so large startup resistance R1 is
recommended for low power dissipation.
If Vcc exceeds the internal reference 34V, IC enters over voltage protection (OVP), and turns off the PWM
until next restart.
C1 is charged
C1 discharges till Vcc
Fig.1 Internal /peripheral circuit at pin VCC
Fig. 2 the voltage waveform of VCC during start
Error amplifier of SD7530 and peripheral circuit are shown in fig.3. Error amplifier compares the voltage of
pin INV which is fed back from output by optp-coupler with reference voltage (2.5V), amplifies the
difference and outputs Vcomp. Generally, the bandwidth of PFC loop is less than 20Hz for better power
factor correction, so compensation capacitor or resistor is connected between output (COMP) and inverting
input (INV) of error amplifier in applications.
Fig.3 Error amplifier and peripheral circuit
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
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