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A4401KL-T Datasheet(PDF) 5 Page - Allegro MicroSystems |
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A4401KL-T Datasheet(HTML) 5 Page - Allegro MicroSystems |
5 / 17 page Automotive Quasi-Resonant Flyback Control IC A4401 5 Allegro MicroSystems, LLC 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com Functional Description Basic Operation A peak current-mode control scheme is used to regu- late one of the converter outputs, which will typically be the highest output voltage. The regulated output voltage is potentially divided down and fed into a Gm stage, where the resulting error signal acts as the control reference. This reference signal is compared against the signal that is produced by the inductor magnetization current flowing through the sense resis- tor. As shown in figure 1, at the beginning of a switching cycle, the external MOSFET, Q1, is turned on. After the sense resistor signal reaches the control reference amplitude, the PWM comparator resets the synchronous rectification (SR) latch and turns off the MOSFET. When the MOSFET is turned off, the voltage on the LX node rises until the voltage clamps at the bat- tery voltage, VBAT, plus the reflected output voltage, VOUT(RFL). The secondary rectification diodes are forward biased and the energy stored in the coupled inductor is released to the output circuits. During this period, the current through the inductor decreases lin- early. As the current falls to 0 A, a resonance is set up between the primary magnetizing inductance and any capacitance appearing between the drain and ground. A damped voltage ringing occurs, which resonates around the battery voltage, VBAT. As the resonant ring swings negative, the adaptive turn-on circuit moni- tors to detect the point at which the voltage reaches a minimum. At this point the MOSFET is commanded on, thereby minimizing the turn-on losses. Also, the relatively slow resonant dV/dt helps to reduce EMI. In most applications, the converter will be operated with a battery input voltage of 13.5 V. To optimize the performance of the regulator at this voltage, the magnetics can be designed to force 0 V across the MOSFET at turn-on. This minimizes switching losses and perhaps more importantly reduces EMI caused by voltage ringing due to the drain to ground capacitor resonating with the primary inductance. The voltage resonance at the MOSFET turn-off can be reduced by a simple low-loss R-C snubber, as described in the Electromagnetic Interference section. If a small enough load is applied to the outputs, and the output of the Gm stage falls below a certain level, the converter will enter a burst mode of operation. Burst mode reduces switching losses while maintain- ing regulation of the outputs. During startup, assuming the battery voltage is above the turn-on threshold and the EN input is enabled, the controller turns on. A soft start circuit controls the ref- erence voltage, limiting the amount of current drawn on the input and the amount of charge transferred to the output, preventing voltage overshoot. During the initial phase of the soft start, very little or no voltage is present on the output. This means that there will be no resonant phase and the converter will operate in con- tinuous-conduction mode. The converter effectively operates in constant-current mode until regulation is achieved. Coupled inductor goes discontinuous; resonant ring occurs VOUT(RFL) Current released from coupled inductor into output circuit MOSFET turns on VOUT(RFL) VBAT +V +I 0 MOSFET turns off ½ resonant period Current builds up in primary winding of coupled inductor Figure 1. External MOSFET voltage and current |
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