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Today, many portable applications come equipped with Li-ion and Li-polymer batteries, since they provide a very high-energy density relative to their volume. An attractively priced, space-saving charge IC must then be provided for these cells, as the circumstances allow. In the QFN package, the bq2401x from Texas Instruments is just such a charge-management IC with what is known as a power pad — a metal surface under the IC for heat dissipation. The heat from the power pad is conducted through thermal vias designed into the printed-circuit board (PCB) to a copper layer on the opposite side. From there, the heat is dissipated to the ambient air. Each board layout is different, and consequently the maximum possible power dissipation also differs, so each board layout must be tested. This article illustrates one way to determine the maximum power dissipation for a given layout during the development phase using the circuit shown in the figure and the components listed in the table.
The bq2401x is an IC designed for a power dissipation of 1.5 W during continuous operation. As a rule of thumb, the service life of a semiconductor is halved by each 10°C increase in semiconductor temperature. This means that an IC designed for a lifespan of 20 years with a continuous semiconductor temperature of 130°C would have its service life reduced to 10 years at an operating temperature of 140°C. To predict the potential lifespan of the IC, it is helpful to document a complete charging process by logging the charge current.
When charging a fully discharged cell, the initial charge current will be C/10 until the cell voltage has reached a value of 3 V. (For a 700-mAh battery, 1 C = 700 mA, so C/10 = 70 mA.)
After the cell voltage rises to 3 V, the charge regulator switches to a “fast-charge” mode and the charge current becomes 700 mA. As a result of the impedance, the cell voltage then climbs abruptly to around 3.25 V and in the following 2 to 3 minutes will continue to rise to 3.5 V. Typically, a circuit board also takes a few minutes to warm up completely when the vias are placed under the power pad (because the mass of the board material and copper pad are combined into a single thermal mass). Once the voltage has risen to 3.5 V, the calculated power dissipation for the main part of the charge period is [(5 V - 3.5 V) × 0.7 A] = 1.05 W. Even if the IC gets somewhat hotter for a few minutes, this will not have a noticeable effect on the lifespan of the IC.
To test the power dissipation range discussed above, approximately 10 minutes after the IC has charged in “fast-charge” mode at 700 mA, the previous 5-V starting voltage of the laboratory power supply unit can be slowly increased (maximum 16.5 V) until the IC starts the transition to the thermal shutdown range, during which the IC will start to suspend and resume charging (thermal cycling). The power dissipation that occurs under these conditions is the maximum power dissipation for the PCB layout.
On Texas Instruments' test board (EVM) a power dissipation of around [(7 V - 3.5 V) × 0.7 A] = 2.45 W at 25°C ambient temperature can be achieved before the module commences overtemperature shutdown and then cyclically suspends and resumes charging. For hysteresis, the overtemperature shutdown point is at around 155°C and the resumption point is 135°C.
Therefore, the maximum possible power dissipation that the IC can achieve is directly related to the thermal layout and the PCB. With a high ambient temperature and no heat dissipation on a board, the maximum possible power dissipation will be less than 1.5 W. The IC would rather switch to overtemperature shutdown mode, and suspend and resume charging cyclically. This shutdown and cyclical suspension and resumption of charging does not represent a desirable behavior in charge mode and should be avoided if possible, but does not present a safety risk.
Editor's Note
This article was originally published in German in the March 2005 issue of Design & Elektronik.