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LTC4059 Datasheet(PDF) 9 Page - Linear Technology |
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LTC4059 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page LTC4059/LTC4059A 9 4059fb Power Dissipation The conditions that cause the LTC4059/LTC4059A to reduce charge current through thermal feedback can be approximated by considering the power dissipated in the IC. For high charge currents, the LTC4059 power dissipa- tion is approximately: PD = (VCC – VBAT) • IBAT where PD is the power dissipated, VCC is the input supply voltage, VBAT is the battery voltage and IBAT is the charge current. It is not necessary to perform any worst-case power dissipation scenarios because the LTC4059/ LTC4059A will automatically reduce the charge current to maintain the die temperature at approximately 115 °C. However, the approximate ambient temperature at which the thermal feedback begins to protect the IC is: TA = 115°C – PDθJA TA = 115°C – (VCC – VBAT) • IBAT • θJA Example: Consider an LTC4059 operating from a 5V wall adapter providing 900mA to a 3.7V Li-Ion battery. The ambient temperature above which the LTC4059/LTC4059A begin to reduce the 900mA charge current is approximately: TA = 115°C – (5V – 3.7V) • (900mA) • 50°C/W TA = 115°C – 1.17W • 50°C/W = 115°C – 59°C TA = 56°C The LTC4059 can be used above 56 °C, but the charge current will be reduced from 900mA. The approximate current at a given ambient temperature can be calculated: I CT VV BAT A CC BAT JA = ° () 115 – –• θ Using the previous example with an ambient temperature of 65 °C,thechargecurrentwillbereducedtoapproximately: I CC VV C W C CA ImA BAT BAT = °° () ° = ° ° = 115 65 53 7 50 50 65 770 – –. • / / Furthermore, the voltage at the PROG pin will change proportionally with the charge current as discussed in the Programming Charge Current section. It is important to remember that LTC4059/LTC4059A applications do not need to be designed for worst-case thermal conditions since the IC will automatically reduce power dissipation when the junction temperature reaches approximately 115 °C. Board Layout Considerations In order to be able to deliver maximum charge current under all conditions, it is critical that the exposed metal pad on the backside of the LTC4059/LTC4059A package is soldered to the PC board ground. Correctly soldered to a 2500mm2 double sided 1oz copper board the LTC4059/ LTC4059A have a thermal resistance of approximately 60 °C/W. Failure to make thermal contact between the exposed pad on the backside of the package and the copper board will result in thermal resistances far greater than 60 °C/W.Asanexample,acorrectlysolderedLTC4059/ LTC4059A can deliver over 900mA to a battery from a 5V supply at room temperature. Without a backside thermal connection, this number could drop to less than 500mA. Stability Considerations The LTC4059 contains two control loops: constant voltage and constant current. The constant-voltage loop is stable without any compensation when a battery is connected with low impedance leads. Excessive lead length, how- ever, may add enough series inductance to require a bypass capacitor of at least 1 µF from BAT to GND. Further- more, a 4.7 µF capacitor with a 0.2Ω to 1Ω series resistor from BAT to GND is required to keep ripple voltage low when the battery is disconnected. High value capacitors with very low ESR (especially ce- ramic) reduce the constant-voltage loop phase margin. Ceramic capacitors up to 22 µF may be used in parallel with a battery, but larger ceramics should be decoupled with 0.2 Ω to 1Ω of series resistance. In constant-current mode, the PROG pin is in the feedback loop, not the battery. Because of the additional pole created by PROG pin capacitance, capacitance on this pin must be kept to a minimum. With no additional capaci- tance on the PROG pin, the charger is stable with program resistor values as high as 12k. However, additional ca- pacitance on this node reduces the maximum allowed APPLICATIO S I FOR ATIO |
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