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TC3827 Folha de dados(PDF) 6 Page - Microchip Technology |
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TC3827 Folha de dados(HTML) 6 Page - Microchip Technology |
6 / 10 page 6 Lithium-Ion Battery Charger TC3827 TC3827-2 12/12/00 © 2001 Microchip Technology Inc. DS21558A Lower Current Option Preregulated Input Voltage (5V +/- 10%) If lower charging current is allowed, the ΘJA value can be increased, and the system cost decreased. This is accom- plished by using a FDC638P PMOS, for example, in a 6-Pin SOT package mounted on a small 1in x 1in area of 2oz Cu on FR-4 board. This provides a convection cooled thermal impedance of ΘJA = +78 °C/W. Allowing a maximum FET junction temperature of +150 °C, at +50°C ambient, with convection cooling the maximum allowed heat rise is: 150 °C–50°C = 100°C. The maximum short circuit current, ISC, is found as: ISC = ∆T/(ΘJA x VIN) = 100/(78 x 5.5) = 0.23A Thus the maximum charging current, IMAX, is: IMAX = ISC/k = 0.51A The current sense resistor for this application is then: RSENSE = VCS/IMAX = 0.05/0.51 = 98mΩ ≈ 100mΩ FET Selection The type and size of the pass transistor is determined by the threshold voltage, input-output voltage differential and load current. The selected PMOS must satisfy the physical and thermal design requirements. To ensure that the maxi- mum 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 deter- mined. Maximum VGS is calculated as follows: VGSMAX = VIN – (IMAX x RSENSE) – VDRVMAX For example: VIN = 5V, and IMAX = 1A, VGSMAX = 5V - (1A x 50mΩ) – 1V= 3.95V The difference between VIN and VO (VDS) must exceed the voltage drop due to the sense resistor plus the ON- resistance of the PMOS at the maximum charge current. VDS ≤ VIN – VO – VCS = 5V – 4.2V – 0.075V = 0.725V The maximum RDS(ON) required at the available gate drive (VDR) and Drain-to-Source voltage (VDS) is: RDS(ON) = VDS/IMAX ≤ 0.725V/1A = 725mΩ The selected PMOS must satisfy these criteria. External Capacitors The TC3827 is stable with or without a battery load, and virtually any good quality output filter capacitors can be used, independent of the capacitor’s minimum ESR (Effec- tive Series Resistance) value. The actual value of the capacitor and its associated ESR depends on the gm and capacitance of the external PMOS device. A 22 µF tantalum or aluminum electrolytic capacitor at the output is sufficient to ensure stability for up to a 10A output current. Shutdown Mode Applying a logic high signal to the SHDN 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 typically 1 µA. Short Circuit Protection The PMOS is protected during short circuit conditions with a foldback type of current limiting that reduces the power dissipation. Current Sense Resistor The current limit sense resistor, RSENSE, is calculated previously. Proper de-rating is advised to select the power dissipation rating of the resistor. The simplest and cheapest sense resistor for high current applications, is a PCB trace. However, the temperature dependence of the copper trace and the thickness tolerances of the trace must be consid- ered in the design. Copper’s Tempco, in conjunction with the proportional- to-absolute temperature ( ±0.3%) current limit voltage, can provide an accurate current limit. Alternately, an appropriate sense resistor, such as surface mount sense resistors, available from KRL, can be used. PCB Layout Issues For optimum voltage regulation, place the load as close as possible to the device’s VOUT and GND pins. It is recommended to use dedicated PCB traces to connect the PMOS drain to the positive terminal and VSS to the negative terminal of the load to avoid voltage drops along the high current carrying PCB traces. If the PCB layout is used as a heatsink, adding many vias around the power FET helps conduct more heat from the FET to the back-plane of the PCB, thus reducing the maximum FET junction temperature. |
Nº de peça semelhante - TC3827 |
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Descrição semelhante - TC3827 |
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