feature

Green Current Mode PWM Control Fixed 65kHz Frequency Modulation Operation Internal High Voltage Start Switch Burst Mode Operation Voltage Feedforward Limit Maximum Power Line Under Voltage Protection Latch Protection and Internal Soft Start (10ms) Function Overload Protection Over Voltage Protection Low Operating Current: Typical 1 mA 8-pin dip/SOP

application

Adapter LCD Display Power Supply Auxiliary Power Supply Related Application Description: AN6014-Green Current Mode PWM Controller Fan 7602

illustrate

FAN7602 is a green current mode PWM controller. It is specially designed for off-line adapter application, DVDP, VCR, LCD display application and auxiliary power supply. Internal high voltage start switch and burstmode operation reduce power loss in standby mode. Because of the internal start switch and pulse mode, it can supply 0.5W load, limited when the input line voltage is 265VAC . Under no-load conditions, the input power is 0.3 watts. The maximum power can be limited, regardless of line voltage changes, using the power limit. Function. The switching frequency is internally fixed at 65kHz FM technology reduces electromagnetic interference. The FAN7602 includes various system reliability protections, and an internal soft-start prevents output voltage overshoot during startup.

Absolute Maximum Ratios "Absolute Maximum Ratios" are those values that do not guarantee the safety of the device. Tea appliances should not operate at these limits. Parameter values in the electrical characteristics table are defined as absolute maximum ratios are not guaranteed.

application information

1. Startup Circuit and Soft Start Block The fan 7602 contains a start switch to reduce power loss in the starter circuit external to conventional PWM converters. Internal start-up circuit if AC line is connected. The start switch turns off the IC 15 ms after startup, as shown in Figure 19. When the VCC voltage reaches the start-up threshold voltage of 12V, when the internal soft-start voltage reaches 1V. The internal start-up circuit restarts charging the VCC capacitor if the VCC voltage drops to the minimum operating voltage of 8V. The UVLO module turns off the output driver circuit and reduces the IC operating current and the internal soft-start voltage to zero. If the VCC voltage reaches the start-up threshold voltage, the IC starts to switch again, and the soft-start block can also work. During soft-start, a pulse-width modulation (PWM) comparator compares the CS/FB pin voltage to the soft-start voltage. The soft-start voltage starts from 0.5V and ends when it reaches 1V. The soft-start end time is 10ms. The start switch reaches 1.5V at the soft start voltage.

2. Oscillator block The oscillator frequency is set internally and the modulation (FM) function reduces EMI. The average frequency is 65kHz, and the modulation frequency is ±2kHz. Frequency from 63kHz to 67kHz, 16 steps. The frequency step is 250Hz, and the FM frequency is 125Hz, as shown.

3. The current sensing and feedback block FAN7602 passes through only one pin, pin 3. Use one for two function pins, an internal LEB (leading edge blanking) circuit to filter current induced noise, not included because an external RC filter is required to increase output voltage feedback information and current sensing information. Figure shows the current sense and feedback circuit. RS is a current sense resistor and is used to sense switching current. The current sensing information is filtered by RC according to the output voltage feedback information of the filter composed of RF and CF, IFB charges or stops charging CF to adjust the bias voltage. If IFB is zero, CF is discharged through RF and RS to reduce the offset voltage.

Figure shows typical voltage waveforms for CS/FB pins. The current sense waveform is added to the offset voltage as shown. The CS/FB pin voltage is as shown compared to PWM+ with a 1V-Plimit offset as shown. If the CS/FB voltage meets PWM+, the output driver is turned off. As shown in Figure 22, if the feedback offset voltage is low, the on-time is increased. If the feedback offset voltage is high, the on-time is shortened. In this way, the duty cycle is controlled according to the output load condition. In general, the maximum output power increases with the input. The voltage increase is due to the increased power-on time. The limited output power converter continues, including the power limiting function in the fan 7602. The sense converter input voltage goes through the latch/Plimit pin, and the Plimit offset voltage is subtracted from 1V. As shown, subtracting the Plimit offset voltage from 1V and the switch on-time decreases as the Plimit offset voltage increases. If the converter input voltage increases, the switch on-time decreases, controlling the output power constant. This bias voltage is proportional to the latch/Plimit pin voltage with a gain of 0.16. If the latch/lock voltage is 1V, the offset voltage is 0.16V.

4. Burst mode block FAN 7602 contains a fan 7601 to reduce the power loss at light and no load of the burst mode block. The hysteretic comparator senses the burst+ voltage in offset burst mode, as shown in Figure 23. Burst+ is the CS/FB voltage and Plimit offset voltage. Fan 7602 enters burst mode when the offset voltage of burst+ is 0.95V above the offset voltage and exits burst mode below 0.88V. at closing time.

5. The protection block FAN7602 has a variety of protection functions to improve system reliability. 5.1 Overload Protection Fan 7602 has overload protection function. If the output load is higher than the rated output current, the output voltage drop and the feedback error amplifier are saturated. The offset of the CS/FB voltage representing the feedback information is almost zero. As shown in the figure, the CS/FB voltage is compared to 50mV. When the internal clock signal is high and the voltage is lower than 50mV, the OLP timer starts counting. If the OLP condition persists for 22 milliseconds, the timer generates the OLP signal. This protection is reset by UVLO. The OLP block is done at soft start.

5.2 Line under-voltage protection If the input voltage of the converter is lower than the minimum operating voltage, the input current of the converter increases too much, resulting in component failure. Therefore, if the input voltage is low, the converter should be protected. In the FAN7602, the LUVP circuit senses the input voltage using the LUVP pin, and if the voltage is lower than 2V, a LUVP signal is generated. The comparator has a hysteresis of 0.5V. If a LUVP signal is generated, the output driver block turns off and the output voltage feedback loop saturates if the LUVP condition persists for more than 22ms.

5.3 Latch Protection Latch protection is provided to protect the system from the use of a latch/spring pin under abnormal conditions. The Latch/Plimit pin can be used for output overvoltage protection and/or other protections. If the latch/Plimit pin voltage passes through the external circuit, the IC shuts down. Latch protection resets when VCC voltage is lower than 5V. 5.4 Over Voltage Protection (OVP) If the VCC voltage reaches 19V, the IC shuts down when the VCC voltage is lower than 5V.

6. The output driver block FAN7602 contains a totem pole output stage to drive the power MOSFET. The driver output is capable of source currents up to 450mA and sink currents of 600mA with typical rise and fall times of 45ns and 35ns, respectively, with a load of 1nF.

feature

Low standby power (<0.3W@265VAC) „Constant output power control key design instructions All IC-related components should be close to the IC, especially C107 and C110. If the value of R106 is too low, there may be sub-harmonic oscillation. The design of R109 Care should be taken, when the input voltage is 265VAC, the VCC voltage should be loaded above 8V. R110 should be designed with care to keep the VCC voltage below the OVP level when the input voltage is 85VAC fully loaded. When the output is shorted, R103 should be designed to Keep the MOSFET VDS voltage below the maximum rating.