The MAX38911/MAX38912 are designed to have stable operation using low equivalent series resistance (ESR) ceramic capacitors at the input and output pins. Multilayer ceramic capacitors (MLCC) with X7R dielectric are commonly used for these types of applications and are recommended due to their relatively stable capacitance across temperature. Nevertheless, the amount of effective capacitance depends on the operating DC bias, capacitance tolerance with temperature, choice of dielectric. Therefore, the capacitor data sheet must be properly examined.
The MAX38911/MAX38912 are designed and characterized for operation with X7R ceramic capacitors of 4.7μF (2.0μF of effective capacitance) at both the input and output. These capacitors shall be placed as close as possible to the respective input and output pins to minimize trace parasitics. There is no maximum output capacitance limitation due to stability. However, for 5V output voltage applications, it is recommended to keep output capacitance to a maximum of 4.7µF effective capacitance in order to minimize short-circuit current buildup in an inductive shorting path.
In order to optimize MAX38911/MAX38912 performance, special consideration is given to the device power dissipation and PCB thermal design. Power dissipation in the regulator depends on the input-to-output voltage difference and load conditions. This can be calculated by the following equation:
Loss (W) = (VIN – VOUT) x ILOAD
The optimal power dissipation can be achieved by carefully choosing the input voltage for a given output target voltage.
The main thermal conduction path for the device is through the exposed pad of the package. As a result, the thermal pad must be soldered to a copper pad area under the device. Thermal plated vias must be placed inside the thermal PCB pad to transfer heat to different GND layers in the system. The vias should be capped to minimize solder voids. The maximum power dissipation is determined by using thermal resistance from the device junction to ambient keeping the maximum junction temperature below +125°C. Thermal properties of the package are given in the Package Information section.
The first-order power dissipation estimate for the 3.3V VIN and 2.5V VOUT with a load current of 300mA is:
Loss (W) = (VIN – VOUT) x ILOAD = (3.3V – 2.5V) x 0.3A = 0.24W
Assuming the MAX38911ATA+ is used, this power dissipation will raise the junction temperature to:
TJ = (PD x θJA) + 25°C = (0.24W x 85.3°C/W) + 25°C = 45.47°C