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A4401 Datasheet(PDF) 8 Page - Allegro MicroSystems |
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A4401 Datasheet(HTML) 8 Page - Allegro MicroSystems |
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8 / 17 page  Automotive Quasi-Resonant Flyback Control IC A4401 8 Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com Switching Turn-On Losses The turn-on losses are determined by the amount of energy the resonant capacitor, C11, has to discharge into the MOSFET. At low battery voltage, the resonant swing should force the volts across the capacitor to only a few volts, mak- ing this loss negligible. Total Losses The total MOSFET power loss can be estimated as follows: PTOTAL PSTATIC + PTURNOFF = . (11) The thermal resistance, RθJA, can be determined by two methods. One is by estimating a maximum junc- tion temperature, TJ(max). The other is to test for the operating junction temperature, using the given device package mounted on a printed circuit board with cop- per trace area connected to the device. RθJA is then calculated as follows: PTOTAL TJ – TA RθJA = . (12) The drain-to-source rating, VDS, is the sum of the maximum input voltage, VBAT (max), plus the reflected output voltage. Adequate margin should also be added, to allow for tolerancing effects and parasitic voltage ring. It can be calculated as follows: NOUT NP VDS VOUT ×+ VBAT (max) = . ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ (13) D2, D3, and D4 Output Diodes Selection For the low voltage outputs such as the filament sup- ply, it is recommended that a Schottky diode be used. For the higher voltage rails, ultrafast rectifier diodes are recommended. For each output, estimate the maximum reverse volt- age, VRRM, and the maximum average current that the diode is subjected to. The VRRM rating should exceed at least 20% of the maximum VDIODE voltage, calcu- lated as follows: NP NS1 VDIODE + VOUT VBAT (max) × = . (14) The maximum average current through the diode is simply the maximum load current. The diode should be rated to handle this current with some margin. In addition, the diode should be able to handle the power dissipation. The majority of the power loss is simply the static loss: PSTATIC Vf × ILOAD = . (15) The forward voltage drop, Vf, can be found from the diode characteristics at maximum load. C1 Input Capacitor Selection In the interests of cost and performance, it is recom- mended that ultralow impedance electrolytic capaci- tors be used. The ratio of the source impedance to the impedance of the input capacitor will determine how much of the input current is drawn from the input capacitor. For example, if the source impedance is relatively high, then the input capacitor would have to supply the triangular current that flows through the primary winding, the MOSFET Q1, and the current sense resistor. This rms current was worked out in the Current Sense Resistor Selection section. Electrolytic capacitors experience heating effects caused by the rms current flowing through the ESR of the device. The maximum rms current is normally quoted at 100 kHz and 105°C. Frequency correc- tion factors for the ripple current are provided when the operating frequencies are less than 100 kHz. A 50 VDC rating should be adequate for most applica- tions. C4, C6, and C8 Output Capacitors Selection The overall equivalent capacitance on the output should not be less 22 μF (see Control Loop section |
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