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AAT3680ITP-8.2-T1 Datasheet(PDF) 10 Page - Advanced Analogic Technologies |
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AAT3680ITP-8.2-T1 Datasheet(HTML) 10 Page - Advanced Analogic Technologies |
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10 / 18 page  AAT3680 Lithium-Ion Linear Battery Charge Controller 10 3680.2003.4.0.91 Applications Information Choosing an External Pass Device (PNP or PMOS) The AAT3680 is designed to work with either a PNP transistor or P-Channel Power MOSFET. Selecting one or the other requires looking at the design tradeoffs including performance versus cost issues. Refer to the following design guide for selecting the proper device: PNP Transistor: In this design example, we will use the following conditions: VP=5V (with 10% supply tolerance), ICHARGE(REG) = 600mA, 4.2V single cell Lithium Ion pack. VP is the input voltage to the AAT3680, and ICHARGE(REG) is the desired fast-charge current. 1. The first step is to determine the maximum power dissipation (PD) in the pass transistor. Worst case is when the input voltage is the highest and the battery voltage is at the lowest during fast- charge (this is referred to as VMIN, nominally 3.1V when the AAT3680-4.2 transitions from trickle- charge to constant-current mode). In this equation VCS is the voltage across RSENSE. PD = (VP(MAX) - VCS - VMIN) · ICHARGE(REG) PD = (5.5V - 0.1V - 3.1V) · 600mA PD = 1.38W 2. The next step is to determine which size package is needed to keep the junction temperature below its rated value, TJ(MAX). Using this value, and the maxi- mum ambient temperature inside the system TA(MAX), calculate the thermal resistance RθJA required: RθJA = (TJ(MAX) - TA(MAX)) PD RθJA = (150 - 40) 1.38 RθJA = 80°C/W It is recommended to choose a package with a lower RθJA than the number calculated above. A SOT223 package would be an acceptable choice, as it has an RθϑΑ of 62.5°C/W when mounted to a PCB with ade- quately sized copper pad soldered to the heat tab. 3. Choose a collector-emitter (VCE) voltage rating greater than the input voltage. In this example, VP is 5.0V, so a 15V device is acceptable. 4. Choose a transistor with a collector current rating at least 50% greater than the programmed ICHARGE(REG) value. In this example we would select a device with at least 900mA rating. 5. Calculate the required current gain ( β or hFE): βMIN = IC(MAX) IB(MIN) βMIN = 0.60 0.02 βMIN = 30 where IC(MAX) is the collector current (which is the same as ICHARGE(REG)), and IB(MIN) is the minimum amount of base current drive shown in Electrical Characteristics as ISINK. Important Note: The cur- rent gain ( β or hFE) can vary a factor of 3 over tem- perature, and drops off significantly with increased collector current. It is critical to select a transistor with β, at full current and lowest temperature, greater than the βMIN calculated above. In summary, select a PNP transistor with ratings VCE ≥ 15V, RθJA ≤ 80°C/W, IC ≥ 900mA, βMIN ≥ 30 in a SOT223 (or better thermal) package. P-Channel Power MOSFET: In this design example, as shown in Figure 5, we will use the following conditions: VP = 5V (with 10% supply tolerance), ICHARGE(REG) = 750mA, 0.4V Schottky diode, 4.2V single cell Lithium Ion pack. VP is the input voltage to the AAT3680, and ICHARGE(REG) is the desired fast-charge current. 1. The first step is to determine the maximum power dissipation (PD) in the pass transistor. Worst case is when the input voltage is the highest and the battery voltage is at the lowest during fast- charge (this is referred to as VMIN, nominally 3.1V when the AAT3680-4.2 transitions from trickle- charge to constant-current mode). In this equation VCS is the voltage across RSENSE, and VD is the voltage across the reverse-current blocking diode. Refer to section below titled Schottky Diode for further details. Omit the value for VD in the equa- tion below if the diode is not used. |
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