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SI9730CBY-T1-E3 Datasheet(PDF) 11 Page - Vishay Siliconix |
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SI9730CBY-T1-E3 Datasheet(HTML) 11 Page - Vishay Siliconix |
11 / 14 page Si9730 Vishay Siliconix Document Number: 70658 S-40135—Rev. F, 16-Feb-04 www.vishay.com 11 Output Capacitor Depending on the MOSFET selected, the Si9730 can open the switch quite rapidly, in a matter of a few microseconds. However, the various monitoring operations take 10-100 times longer than this, and the basic period of the Si9730’s oscillator is 4 msec. In order to prevent false readings by the Si9730, it is necessary to attach a capacitor across the output of the battery charger/load (this is not in parallel with the battery, because of the switch). A 10-mF capacitor is recommended for this purpose; see Figure 8. Selecting a Current Sense Resistor The current sense resistor should be selected based on the maximum current the battery can source or charge at; above this current, the Si9730 will open the switch, disconnecting the battery from its load or charger. Rsense = VILIMIT/IILIMIT 28 mV/IILIMIT Of course, the resistor must be rated to take the power dissipated in it as well: PRSENSE = IILIMIT* VILIMIT 28 mV * IILIMIT For example, suppose that the maximum current the battery will see is 1.8 A. Then, ILIMIT might be chosen to be 2 A. We would then select a resistor of RSENSE = 28 mV/2 A = 14 mW. The power dissipation in this resistor is PRSENSE = 28 mV * 2 A = 56 mW and so a 100mW surface mount resistor would be suitable. Another possibility is to use a thin copper trace as the sense resistor. The copper has a temperature coefficient of 0.39%/_C, but this is partially compensated for by the temperature coefficient of the current limit comparator in the Si9730, which is 0.18%/_C. A simple formula for selecting a trace to act as a current sensor is: R + 0.5 mW length width 1 oz. Copper For example, to get a 14-mW. resistor, we need length/width = 28; with a trace width of 0.01”, the length of the trace should be 0.28”. MOSFET Selection Two MOSFETs in series, with their sources and gates connected together, are used as the switch. This prevents current from flowing in either direction when the gate is low; if only one MOSFET were used, the body diode could conduct current in the opposing direction. LITTLE FOOT MOSFETs are recommended for this application, because of their size, performance and cost benefits. SO-8 and TSSOP-8 MOSFETs allow for space efficient designs with performance equal to or better than their DPAK and TO-220 predecessors. Further, their availability from multiple sources permits a cost effective solution. There are two important parameters to consider in MOSFET selection: gate threshold voltage; and on-resistance, which determines power dissipation. Even when the DCO pin of the Si9730 is low, the specification allows its value to be as high as 0.4 V. If this voltage were close to the gate threshold voltage, leakage current through the MOSFETs could be hundreds of microamps, which would result in the battery quickly becoming discharged. To ensure that leakage is minimized, n-channel MOSFETs with a minimum gate threshold voltage of 0.8 V should be chosen. On resistance of the MOSFETs needs to be selected to limit power dissipation into the MOSFETs’ package. For example, a dual MOSFET SO-8 package is rated at 2 W, and a dual MOSFET TSSOP-8 package is rated at 1 W (both at 25_C; if the ambient temperature is higher, the allowable power dissipation in these packages is less). For example, if the maximum current is 2 A, and a dual MOSFET SO-8 package is being used, the maximum on-resistance of the two MOSFETs in series must not exceed 1 W = (2 A)2 * RON or RON = 0.25 W; each MOSFET can be allotted half of this, RON = 125 mW. Account must also be taken of the fact that MOSFETs’ on-resistance is a function of temperature; a conservative approach would give a discount of 1/3, RON = 125 mW * (2/3) = 80 mW per MOSFET. A list of recommended MOSFETs, which Vishay Silicoix supplies, follows. |
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