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ACT4072SH-T Datasheet(PDF) 4 Page - Active-Semi, Inc |
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ACT4072SH-T Datasheet(HTML) 4 Page - Active-Semi, Inc |
4 / 10 page Innovative Products. Active Solutions. - 4 - www.active-semi.com Copyright © 2006 Active-Semi, Inc. ACT4072 FUNCTIONAL BLOCK DIAGRAM FUNCTIONAL DESCRIPTION As seen in the Functional Block Diagram, the ACT4072 is a current mode pulse width modulation (PWM) converter. The converter operates as fol- lows: A switching cycle starts when the rising edge of the Oscillator clock output causes the High-Side Power Switch to turn on and the Low-Side Power Switch to turn off. With the SW side of the inductor now con- nected to IN, the inductor current ramps up to store energy in its magnetic field. The inductor current level is measured by the Current Sense Amplifier and added to the Oscillator ramp signal. If the result- ing summation is higher than the COMP voltage, the output of the PWM Comparator goes high. When this happens or when Oscillator clock output goes low, the High-Side Power Switch turns off and the Low-Side Power Switch turns on. At this point, the SW side of the inductor swings to a diode voltage below ground, causing the inductor current to de- crease and magnetic energy to be transferred to the output. This state continues until the cycle starts again. The High-Side Power Switch is driven by logic using the BS bootstrap pin as the positive rail. This pin is charged to VSW + 6V when the Low-Side Power Switch turns on. The COMP voltage is the integration of the error between the FB input and the internal 1.222V refer- ence. If FB is lower than the reference voltage, COMP tends to go higher to increase current to the output. Current limit happens when COMP reaches its maximum clamp value of 2.55V. The Oscillator normally switches at 400kHz. How- ever, if the FB voltage is less than 0.7V, then the switching frequency decreases until it reaches a minimum of 60kHz at VFB = 0.5V. Shutdown Control The ACT4072 has an enable input EN for turning the IC on or off. When EN is less than 0.7V, the IC is in 6 μA low current shutdown mode and the out- put is discharged through the Low-Side Power Switch. When EN is higher than 1.3V, the IC is in normal operation mode. EN is internally pulled up with a 2 μA current source and can be left uncon- nected for always-on operation. Thermal Shutdown The ACT4072 automatically turns off when its junc- tion temperature exceeds 160°C. |
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