An LDO Primer - Microchip

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Figure 2 – Integrated Li-Ion Battery-Charger Circuit and Charging ProfileDue to the battery-life and size constraints of PMPs, a fan and/or heat sink cannot beused to transfer heat generated by the PMP’s components. Common design mistakesinclude inadequate thermal optimization of the PCB layout, as well as the improperplacement of components on the PCB and overall enclosure design. All of thesemistakes create heat-transfer problems.For example, on a multi-layer PCB, if the power ground does not have enough surfacearea or metal for heat dissipation, there is not enough surface or thermal conductivity forthe heat to dissipate. As a result, heat may become trapped, which can cause thermalrelatedhazards. To combat this, engineers should practice careful board layout,including enlargement of the power ground with more metal, to avoid thermal traps onthe PCB.Quite often, design engineers neglect to monitor temperatures at potential “hot spots” inportable designs. For example, the backlight white LEDs for LCD displays may becomevery hot. Without a strategically placed temperature sensor in the LCD display module,this overheat is undetectable and, therefore, cannot be eliminated by reducing thecurrent to the white LEDs. This may result in damage to the LCD screen, or even otherparts of the PMP.The placement of temperature sensors at thermally critical points throughout theportable device prevents such overheat. In conjunction with the sensors, a low pin-count,embedded thermal-management microcontroller responds quickly to data from thetemperature sensors, signaling the microcontroller to shut down or reduce power to thehot points in the system.Figure 3 illustrates some thermally critical points in a PMP that must be monitored on areal-time basis in order to avoid thermal runaway.
------------- Power------------- SignalFigure 3 – Thermally Critical Points in a PMPIII. Battery ManagementBattery-management related design problems usually involve battery overheat duringthe charging phase. The charging current cannot be adjusted according to the battery’stemperature if the battery’s temperature is not monitored during charging. Eventually,the battery overheats and is damaged. In extreme cases, the battery may even explodeand inflict physical injury. A temperature sensor or equivalent device, such as athermistor, can be used to monitor battery temperature during charging so that thecharging current can be reduced when overheat occurs.In addition, fuel-gauge ICs that provide real-time monitoring of battery voltage, current,and temperature can be used to monitor the battery. Fuel gauge ICs report this data, aswell as battery capacity data, to the embedded thermal-management microcontroller,which then instructs the system to adjust itself accordingly.ConclusionThere are several steps that designers can take to develop thermally optimized, powerefficientportable device designs. For example, designers can use components withpower conservation options (such as shutdown), low power consumption and highpower-conversion efficiency features.
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An LDO Primer - Microchip

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