Datasheet FL7701 (ON Semiconductor) - 5

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FL7701 FUNCTIONAL DESCRIPTION
The FL7701 is a basic PWM controller for buck converter instead uses fluctuation in the signal on the VCC pin. topology in Continuous Conduction Mode (CCM) with Basically, the VCC pin, which is supplies power for the IC, intelligent PFC function using a digital control algorithm. has voltage ripple as well as the rectification voltage after The FL7701 has an internal self−biasing circuit that uses the bridge, changing voltage level according to the VCC high−voltage switching device. The IC does not need an capacitor value. Using this kind of voltage fluctuation on the auxiliary powering path to the VCC pin typical in flyback VCC pin, the IC can detect the time reference and create the control ICs or PSR product family. internal ZCD signal. When the input voltage applied to the HV pin is over 25 V For precise and reliable internal reference for input to 500 V, the FL7701 maintains a 15.5 V DC voltage at the voltage signal, the FL7701 uses a digital technique VCC pin for stable operation. The FL7701 has UVLO block (sigma/delta modulation) and creates a new internal signal functions such that when the VCC voltage rises higher than (DAC_OUT) that has the same phase as the input voltage, VCCST+, the internal UVLO block releases and starts as shown in Figure 5. This signal enters the final comparator operation. Otherwise the VCC goes down to the VCCST− and and is compared with current information from the sensing IC operation stops. Normally, the hysteresis function resistor. provides stable operation even if the input voltage is operating under the very noisy or unstable circumstances. Bridge Diode The FL7701 has a smart internal digital block for Vbridge Output determining input condition: AC or DC. When an AC source Voltage Input JFET with 50 Hz or 60 Hz is applied to the IC, the IC automatically Voltage Output Voltage changes its internal reference signal, which is similar to Peak input signal, for creating high power factor. Otherwise, once VCC V Charging dd t the DC source connects to the IC, the internal reference Voltage JFET Output immediately changes to DC. Voltage t ZCD
Soft−Start Function
t The FL7701 has an internal soft−start function to reduce DAC_OUT inrush current at startup. When the IC starts operation following an internal sequence, the internal reference slowly t increases for a pre−determined fixed time. After this
Figure 5. Internal PFC Function
transient period, the internal reference goes to a steady−state level. In this time, the IC continually tries to find phase
Self−Biasing Function
information from the VCC pin. If the IC succeeds in getting The self−biasing function, using an HV device, can supply phase information, it automatically follows a similar shape enough operating current to the IC and guarantee similar reference made during the transient times, 7 periods. If not, startup time across the whole input voltage range the IC has a DC reference level. (80 V~308 VAC). However, self−biasing has a weakness in high−voltage condition. Normally, the HV device acts as Vbridge constant current source, so the internal HV device has power loss when high input voltage connects to the HV pin. This power loss is proportional to input voltage. To reduce this power loss, one of the possible solutions is an additional ILED resistor between the input voltage source and the HV pin, as shown in Figure 6. T / 2 = 1 / (Input Frequency * 2) Bridge Diode EMI filter 7 * (T / 2) Normal Operation
Figure 4. Soft Starting Function in AC Input Mode Internal PFC Function: How to Achieve High Power
U1
Factor
ADIM HV RT OUT The FL7701 has a simple, but smart, internal PFC VCC function that does not require additional detection pins for CS GND detecting input phase information or an electrolytic capacitor for supply voltage stabilization. For achieving high PF, the FL7701 does not use the rectification capacitor
Figure 6. High−Voltage Application
after the bridge diode. This is important because the IC
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