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MC44608 bảng dữ liệu(PDF) 5 Page - ON Semiconductor |
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MC44608 bảng dữ liệu(HTML) 5 Page - ON Semiconductor |
5 / 16 page MC44608 http://onsemi.com 5 PIN FUNCTION DESCRIPTION Pin Name Description 1 Demag The Demag pin offers 3 different functions: Zero voltage crossing detection (50mV), 24 µA current detection and 120 µA current detection. The 24µA level is used to detect the secondary reconfiguration status and the 120 µA level to detect an Over Voltage status called Quick OVP. 2 Isense The Current Sense pin senses the voltage developed on the series resistor inserted in the source of the power MOSFET. When Isense reaches 1V, the Driver output (pin 5) is disabled. This is known as the Over Current Protection function. A 200 µA current source is flowing out of the pin 3 during the start–up phase and during the switching phase in case of the Pulsed Mode of operation. A resistor can be inserted between the sense resistor and the pin 3, thus a programmable peak current detection can be performed during the SMPS stand–by mode. 3 Control Input A feedback current from the secondary side of the SMPS via the opto–coupler is injected into this pin. A resistor can be connected between this pin and GND to allow the programming of the Burst duty cycle during the Stand–by mode. 4 Ground This pin is the ground of the primary side of the SMPS. 5 Driver The current and slew rate capability of this pin are suited to drive Power MOSFETs. 6 VCC This pin is the positive supply of the IC. The driver output gets disabled when the voltage becomes higher than 15V and the operating range is between 6.6V and 13V. An intermediate voltage level of 10V creates a disabling condition called Latched Off phase. 7 This pin is to provide isolation between the Vi pin 8 and the VCC pin 6. 8 Vi This pin can be directly connected to a 500V voltage source for start–up function of the IC. During the Start–up phase a 9 mA current source is internally delivered to the VCC pin 6 allowing a rapid charge of the VCC capacitor. As soon as the IC starts–up, this current source is disabled. OPERATING DESCRIPTION Regulation Figure 1. Regulator S3 10 & 3 Regulation 20 W S2 10 Control Input VCC V LP–stby 5 V 4 kHz Filter Output Vdd PWM Comparator Switching Phase Stand–by Latched off Phase 1.6 V The pin 3 senses the feedback current provided by the opto coupler. During the switching phase the switch S2 is closed and the shunt regulator is accessible by the pin 3. The shunt regulator voltage is typically 5V. The dynamic resistance of the shunt regulator represented by the zener diode is 20 W. The gain of the Control input is given on Figure 10 which shows the duty cycle as a function of the current injected into the pin 3. A 4kHz filter network is inserted between the shunt regulator and the PWM comparator to cancel the high frequency residual noise. The switch S3 is closed in Stand–by mode during the Latched Off Phase while the switch S2 remains open. (See section PULSED MODE DUTY CYCLE CONTROL). The resistor Rdpulsed (Rduty cycle burst) has no effect on the regulation process. This resistor is used to determine the burst duty cycle described in the chapter “Pulsed Duty Cycle Control” on page 8. PWM Latch The MC44608 works in voltage mode. The on–time is controlled by the PWM comparator that compares the oscillator sawtooth with the regulation block output (refer to the block diagram on page 2). The PWM latch is initialized by the oscillator and is reset by the PWM comparator or by the current sense comparator in case of an over current. This configuration ensures that only a single pulse appears at the circuit output during an oscillator cycle. Current Sense The inductor current is converted to a positive voltage by inserting a ground reference sense resistor RSense in series with the power switch. The maximum current sense threshold is fixed at 1V. The peak current is given by the following equation: Ipkmax + 1 Rsense(W) (A) |
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