Datenblatt-Suchmaschine für elektronische Bauteile |
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TC3827-4.1VUA Datenblatt(PDF) 5 Page - Microchip Technology |
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TC3827-4.1VUA Datenblatt(HTML) 5 Page - Microchip Technology |
5 / 10 page 5 Lithium-Ion Battery Charger TC3827 TC3827-2 12/12/00 © 2001 Microchip Technology Inc. DS21558A control loop changes from current limiting to voltage regula- tion. If an external micro-controller determines battery con- ditions are unsafe for charge it can toggle the shutdown pin low and interrupt the charge cycle. Otherwise, once the pre- determined cell voltage is reached the TC3827 shifts into a constant-voltage mode (linear regulation) and a variable charge current is applied as required to maintain the battery cell voltage to within 1% accuracy of the cell voltage set- point. IMON – Charge Current Status The IMON pin provides an output voltage that is propor- tional to the battery charging current . It is an amplified version of the sense resistor voltage drop that the current loop uses to control the PMOS device. This voltage signal can be applied to the input of an A/D Converter and used by a controller to display information about the state of the battery or charge current profile. MODE – Charge Mode Status LED The MODE pin indicates the battery charging mode. An LED can be connected to the MODE for a visible indicator. Alternatively, a pull-up resistor (typically 100k Ω) from the interfacing logic supply to MODE provides a logic-level output. The MODE pin will toggle LOW and the LED will illuminate when the charger is in the current limited mode. The MODE pin toggles to a high impedance state and the LED will be off during constant-voltage mode charging or if the battery is not connected. The MODE pin toggles at a VOUT of VREG, typically. APPLICATION CIRCUIT DESIGN Due to the low efficiency of Linear Regulator Charging, the most important factors are thermal design and cost, which is a direct function of the input voltage, output current and thermal impedance between the PMOS and the ambi- ent cooling air. The worst-case situation is when the battery is shorted since the PMOS has to dissipate the maximum power. A tradeoff must be made between the charge cur- rent, cost and thermal requirements of the charger. Higher current requires a larger PMOS with more effective heat dissipation leading to a more expensive design. Lowering the charge current reduces cost by lowering the size of the PMOS, possibly allowing a smaller package such as 6-Pin SOT. The following designs consider both options. Higher Current Option The current sense resistor for the circuit shown in Figure 1 is calculated by: RSENSE = VCS /IMAX . Where VCS is the current limit threshold voltage of 40mV to 75mV, 50mV typical. If IMAX = 1A is desired, RSENSE = 50mΩ. Pre-regulated Input Voltage (5V ± 0%) For this application, the required θJA thermal imped- ance is calculated as follows: if: the PMOS data sheet allows a max junction temperature of TJMAX = 150°C, then: at 50 °C ambient with convection cooling, the maximum allowed junction temperature rise is: TJMAX – TAMAX = 150°C – 50°C = 100°C. θJA = ∆T/(IOx k x VIN) = 100/(1 x 0.46 x 5.5) = 39.5 °C/W This k factor is: k = ISC/IMAX ≈ 0.46. This thermal impedance can be realized using the transistor shown in Figure 1 when mouted to a heat sink. The θSA or thermal impedance of a suitable heatsink is calculated below: θSA ≤ (θJA – θJC – θCS) = 39.5 – 2.5 – 0.3 = 36.7°C/W Where the θJC, or junction-to-case thermal impedance is for the PMOS from the PMOS data sheet. A low cost heatsink is Thermalloy type PF430, with a θSA = +25.3°C/W. |
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