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ACT337 Datasheet(PDF) 5 Page - Active-Semi, Inc |
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ACT337 Datasheet(HTML) 5 Page - Active-Semi, Inc |
5 / 12 page ACT337 Rev 2, 14-Nov-12 Innovative Power TM - 5 - www.active-semi.com Copyright © 2012 Active-Semi, Inc. As shown in the Functional Block Diagram, to regulate the output voltage in CV (constant voltage) mode, the ACT337 compares the feedback voltage at FB pin to the internal reference and generates an error signal to the pre-amplifier. The error signal, after filtering out the switching transients and compensated with the internal compensation network, modulates the external NPN transistor peak current at CS pin with current mode PFWM (Pulse Frequency and Width Modulation) control. To regulate the output current in CC (constant current) mode, the oscillator frequency is modulated by the output voltage. SW is a driver output that drives the emitter of an external high voltage NPN transistor. This base- emitter-drive method makes the drive circuit the most efficient. Fast Startup VDD is the power supply terminal for the ACT337. During startup, the ACT337 typically draws only 25μA supply current. The startup resistor from the rectified high voltage DC rail supplies current to the base of the NPN transistor. This results in an amplified emitter current to VDD through the SW pin via Active-Semi's proprietary fast-startup circuitry until it exceeds the VDDON threshold 19V. At this point, the ACT337 enters internal startup mode with the peak current limit ramping up in 10ms. After switching starts, the output voltage begins to rise. The VDD bypass capacitor must supply the ACT337 internal circuitry and the NPN base drive until the output voltage is high enough to sustain VDD through the auxiliary winding. The VDDOFF threshold is 5.5V; therefore, the voltage on the VDD capacitor must remain above 5.5V while the output is charging up. Constant Voltage (CV) Mode Operation In constant voltage operation, the ACT337 captures the auxiliary flyback signal at FB pin through a resistor divider network R5 and R6 in Figure 6. The signal at FB pin is pre-amplified against the internal reference voltage, and the secondary side output voltage is extracted based on Active-Semi's proprietary filter architecture. This error signal is then amplified by the internal error amplifier. When the secondary output voltage is above regulation, the error amplifier output voltage decreases to reduce the switch current. When the secondary output voltage is below regulation, the error amplifier output voltage increases to ramp up the switch current to bring the secondary output back to regulation. The output regulation voltage is determined by the following relationship: where RFB1 (R5) and RFB2 (R6) are top and bottom feedback resistor, NS and NA are numbers of transformer secondary and auxiliary turns, and VD is the rectifier diode forward drop voltage at approximately 0.1A bias. Standby (No Load) Mode In no load standby mode, the ACT337 oscillator frequency is further reduced to a minimum frequency while the current pulse is reduced to a minimum level to minimize standby power. The actual minimum switching frequency is programmable with an output preload resistor. Loop Compensation The ACT337 integrates loop compensation circuitry for simplified application design, optimized transient response, and minimal external components. Output Cable Resistance Compensation The ACT337 provides programmable output cable resistance compensation during constant voltage regulation, monotonically adding an output voltage correction up to predetermined percentage at full power. There are four levels to program the output cable compensation by connecting a resistor (R10 in Figure 6) from the SW pin to VDD pin. The percentage at full power is programmable to be 3%, 6%, 9% or 12%, and by using a resistor value of 300k, 150k, 75k or 33k respectively. If there is no resistor connection, there is no cord compensation. This feature allows for better output voltage accuracy by compensating for the output voltage droop due to the output cable resistance. Constant Current (CC) Mode Operation When the secondary output current reaches a level set by the internal current limiting circuit, the ACT337 enters current limit condition and causes the secondary output voltage to drop. As the output voltage decreases, so does the flyback voltage in a proportional manner. An internal current shaping circuitry adjusts the switching frequency based on the flyback voltage so that the transferred power remains proportional to the output voltage, resulting FUNCTIONAL DESCRIPTION (1) D A S 2 FB 1 FB OUTCV V N N R R 1 V 20 . 2 V − × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + × = |
Similar Part No. - ACT337_14 |
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Similar Description - ACT337_14 |
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