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ADP3820ART-42 Arkusz danych(PDF) 7 Page - Analog Devices |
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ADP3820ART-42 Arkusz danych(HTML) 7 Page - Analog Devices |
7 / 8 page ADP3820 –7– REV. A The current sense resistor for this application: RS = VS/IO = 0.05/0.5 = 100 m Ω FET Selection The type and size of the pass transistor are determined by the threshold voltage, input-output voltage differential and load current. The selected PMOS must satisfy the physical and ther- mal design requirements. To ensure that the maximum VGS provided by the controller will turn on the FET at worst case conditions, (i.e., temperature and manufacturing tolerances) the maximum available VGS must be determined. Maximum VGS is calculated as follows: VGS = VIN – VBE – IOUTMAX × R S where IOUTMAX = Maximum Output Current RS = Current Sense Resistor VBE ~ 0.7 V (Room Temperature) ~ 0.5 V (Hot) ~ 0.9 V (Cold) For example: VIN = 5 V, and IOUTMAX = 1 A, VGS = 5 V – 0.7 V – 1 A × 50 m Ω = 4.25 V If VGS < 5 V, logic level FET should be considered. If VGS > 5 V, either logic level or standard MOSFET can be used. The difference between VIN and VO (VDS) must exceed the voltage drop due to the sense resistor plus the ON-resistance of the FET at the maximum charge current. The selected MOSFET must satisfy these criteria; otherwise, a different pass device should be used. VDS = VIN – VO = 5 V – 4.2 V = 0.8 V The maximum RDS(ON) required at the available gate drive (VDR) and Drain-to-Source voltage (VDS) is: RDS(ON) = VDS/IOUTMAX From the Drain-to Source current vs. Drain-to-Source voltage vs. gate drive graph off the MOSFET data sheet, it can be de- termined if the above calculated RDS(ON) is higher than the graph indicates. However, the value read from the MOSFET data sheet graph must be adjusted based on the junction temperature of the MOSFET. This adjustment factor can be obtained from the normalized RDS(ON) vs. junction temperature graph in the MOSFET data sheet. External Capacitors The ADP3820 is stable with or without a battery load, and virtually any good quality output filter capacitors can be used (anyCAP™), independent of the capacitor’s minimum ESR (Effective Series Resistance) value. The actual value of the capacitor and its associated ESR depends on the gm and capaci- tance of the external PMOS device. A 10 µF tantalum or alumi- num electrolytic capacitor at the output is sufficient to ensure stability for up to a 10 A output current. Shutdown Mode Applying a TTL high signal to the SD pin or tying it to the input pin will enable the output. Pulling this pin low or tying it to ground will disable the output. In shutdown mode, the controller’s quiescent current is reduced to less than 1 µA. Gate-to-Source Clamp A 6 V gate-to-source voltage clamp is provided by the ADP3820 to protect most MOSFET gates in the event the VIN > VGS allowed and the output is suddenly shorted to ground. This allows use of the new, low RDS(ON) MOSFETs. Short Circuit Protection The power FET is protected during short circuit conditions with a foldback type of current limiting that significantly re- duces the current. See Figure 13 for foldback current limit information. Current Sense Resistor Current limit is achieved by setting an appropriate current sense resistor (RS) across the current limit threshold voltage. Current limit sense resistor, RS, is calculated as shown above. Proper derating is advised to select the power dissipation rating of the resistor. The simplest and cheapest sense resistor for high current appli- cations, (i.e., Figure 1) is a PCB trace. However, the tempera- ture dependence of the copper trace and the thickness tolerances of the trace must be considered in the design. The resistivity of copper has a positive temperature coefficient of +0.39%/ °C. Copper’s Tempco, in conjunction with the proportional-to- absolute temperature ( ±0.3%) current limit voltage, can provide an accurate current limit. Table I provides the typical resistance values for PCB copper traces. Alternately, an appropriate sense resistor, such as surface mount sense resistors, available from KRL, can be used. Table I. Printed Circuit Copper Resistance Conductor Conductor Resistance Thickness Width/Inch m /In 1/2oz/ft2 (18 µm) 0.025 39.3 0.050 19.7 0.100 9.83 0.200 4.91 0.500 1.97 1oz/ft2 (35 µm) 0.025 19.7 0.050 9.83 0.100 4.91 0.200 2.46 0.500 0.98 2oz/ft2 (70 µm) 0.025 9.83 0.050 4.91 0.100 2.46 0.200 1.23 0.500 0.49 3oz/ft2 (106 µm) 0.025 6.5 0.050 3.25 0.100 1.63 0.200 0.81 0.500 0.325 anyCAP is a trademark of Analog Devices, Inc. |
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Podobny opis - ADP3820ART-42 |
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