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FAN7529 Datasheet(PDF) 12 Page - Fairchild Semiconductor |
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FAN7529 Datasheet(HTML) 12 Page - Fairchild Semiconductor |
12 / 20 page ![]() © 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN7529 Rev. 1.0.2 12 Applications Information 1. Error Amplifier Block The error amplifier block consists of a transconductance amplifier, output OVP comparator, and disable compara- tor. For the output voltage control, a transconductance amplifier is used instead of the conventional voltage amplifier. The transconductance amplifier (voltage con- trolled current source) aids the implementation of OVP and disable function. The output current of the amplifier changes according to the voltage difference of the invert- ing and non-inverting input of the amplifier. The output voltage of the amplifier is compared with the internal ramp signal to generate the switch turn-off signal. The OVP comparator shuts down the output drive block when the voltage of the INV pin is higher than 2.675V and there is 0.175V hysteresis. The disable comparator dis- ables the operation of the FAN7529 when the voltage of the inverting input is lower than 0.45V and there is 100mV hysteresis. An external small signal MOSFET can be used to disable the IC, as shown in Figure 33. The IC operating current decreases below 65µA to reduce power consumption if the IC is disabled. Figure 33. Error Amplifier Block 2. Zero Current Detection Block The zero current detector (ZCD) generates the turn-on signal of the MOSFET when the boost inductor current reaches zero using an auxiliary winding coupled with the inductor. If the voltage of the ZCD pin goes higher than 1.5V, the ZCD comparator waits until the voltage goes below 1.4V. If the voltage goes below 1.4V, the zero cur- rent detector turns on the MOSFET. The ZCD pin is pro- tected internally by two clamps, 6.7V-high clamp and 0.65V-low clamp. The 150µs timer generates a MOSFET turn-on signal if the drive output has been low for more than 150µs from the falling edge of the drive output. Figure 34. Zero Current Detector Block 3. Sawtooth Generator Block The output of the error amplifier and the output of the sawtooth generator are compared to determine the MOSFET turn-off instance. The slope of the sawtooth is determined by an external resistor connected to the MOT pin. The voltage of the MOT pin is 2.9V and the slope is proportional to the current flowing out of the MOT pin. The internal ramp signal has a 1V offset; there- fore, the drive output is shut down if the voltage of the COMP pin is lower than 1V. The MOSFET on-time is maximum when the COMP pin voltage is 5V. According to the slope of the internal ramp, the maximum on-time can be programmed. The necessary maximum on-time depends on the boost inductor, lowest AC line voltage, and maximum output power. The resistor value should be designed properly. Figure 35. Sawtooth Generator Block INV Error Amp OVP COMP 2.675V 2.5V 2 1 Gm 0.45V 0.35V Disable V out V ref1 (2.5V) Disable Signal FAN7529 Rev. 00 Timer ZCD 6.7V Zero Current Detector R S Q 5 1.5V 1.4V V in R ZCD 150 μs Turn-on Signal FAN7529 Rev. 00 1V Offset MOT 3 Error Amp Output Off Signal 2.9V Saw Tooth Generator FAN7529 Rev. 00 |
Similar Part No. - FAN7529_07 |
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Similar Description - FAN7529_07 |
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