Table 1. Design Parameters

8.2.2.1 External Capacitors

The LP5922 is designed to be stable using low equivalent series resistance (ESR) ceramic capacitors at the input, output, and the noise-reduction pin (SS/NR). Multilayer ceramic capacitors have become the industry standard for these types of applications and are recommended, but must be used with good judgment. Ceramic capacitors that employ X7R-, X5R-, and COG-rated dielectric materials provide relatively good capacitive stability across temperature, whereas the use of Y5V-rated capacitors is discouraged because of large variations in capacitance. Additionally, the case size has a direct impact on the capacitance versus applied voltage derating.

Regardless of the ceramic capacitor type selected, the actual capacitance varies with the applied operating voltage and temperature. As a rule of thumb, derate ceramic capacitors by at least 50%. The input and output capacitors recommended herein account for a effective capacitance derating of approximately 50%, but at high applied voltage conditions the capacitance derating can be greater than 50% and must be taken into consideration. The minimum capacitance values declared in Input and Output Capacitors must be met across the entire expected operating voltage range and temperature range.

8.2.2.2 Input Capacitor, C_IN

An input capacitor is required for stability. A capacitor with a value of at least 22 μF must be connected between the LP5922 IN pin and ground for stable operation over full load current range. It is acceptable to have more output capacitance than input, as long as the input is at least 22 μF.

The input capacitor must be located as close as possible to, but at a distance not more than 1 cm from, the IN pin and returned to the device GND pin with a clean analog ground. This will minimize the trace inductance between the capacitor and the device. Any good quality ceramic or tantalum capacitor may be used at the input.

8.2.2.3 Output Capacitor, C_OUT

The LP5922 is designed to work specifically with a low ESR ceramic (MLCC) output capacitor, typically 22 μF. A ceramic capacitor (dielectric types X5R or X7R) in the 22-μF to 100-μF range, with an ESR not exceeding 500 mΩ, is suitable in the LP5922 application circuit having an output voltage greater than 0.8 V. For output voltages of 0.8 V or less, the output capacitance must be increased to typically 47 μF. The output capacitor must be connected between the device OUT and GND pins. The output capacitor must meet the requirement for the minimum value of capacitance and have an ESR value that does not exceed 500 mΩ to ensure stability.

It is possible to use tantalum capacitors at the device output, but these are not as attractive for reasons of size, cost, and performance.

A combination of multiple output capacitors in parallel boosts the high-frequency PSRR. The combination of one 0805-sized, 47-µF ceramic capacitor in parallel with two 0805-sized, 10-µF ceramic capacitors with a sufficient voltage rating optimizes PSRR response in the frequency range of 400 kHz to 700 kHz (which is a typical range for dc-dc supply switching frequency). This 47-µF || 10-µF || 10-µF combination also ensures that at high input voltage and high output voltage configurations, the minimum effective capacitance is met. Many 0805-sized, 47-µF ceramic capacitors have a voltage derating of approximately 60% to 75% at 5 V, so the addition of the two 10-µF capacitors ensures that the capacitance is at or above 22 µF.

8.2.2.4 Soft-Start and Noise-Reduction Capacitor, C_SS/NR

Recommended value for C_SS/NR is 100 nF or larger. The soft-start period can be calculated by Equation 2. The C_SS/NR capacitor is also the filter capacitor for internal reference for noise reduction purpose.

8.2.2.5 Feed-Forward Capacitor, C_FF

Although a feed-forward capacitor (C_FF) from the FB pin to the OUT pin is not required to achieve stability, a 10-nF external C_FF optimizes the transient, noise, and PSRR performance. A higher capacitance C_FF value can be used; however, the start-up time may be longer and the Power-Good signal may incorrectly indicate that the output voltage is settled. The maximum recommended value is 100 nF

To ensure proper PGx functionality, the time constant defined by CNR/SSx must be greater than or equal to the time constant from CFFx. For a detailed description, see the application report Pros and Cons of Using a Feed-Forward Capacitor with a Low Dropout Regulator (SBVA042).

8.2.2.6 No-Load Stability

The LP5922 remains stable, and in regulation, with no external load.

8.2.2.7 Power Dissipation

Knowing the device power dissipation and proper sizing of the thermal plane connected to the exposed thermal pad is critical to ensuring reliable operation. Device power dissipation depends on input voltage, output voltage, and load conditions and can be calculated with Equation 3.

Power dissipation can be minimized, and greater efficiency can be achieved, by using the lowest available voltage drop option that is greater than the dropout voltage (V_DO). However, keep in mind that higher voltage drops result in better dynamic (that is, PSRR and transient) performance.

On the WSON (DSC) package, the primary conduction path for heat is through the exposed thermal pad into the PCB. To ensure the device does not overheat, connect the exposed thermal pad, through multiple thermal vias, to an internal ground plane with an appropriate amount of PCB copper area.

Power dissipation and junction temperature are most often related by the junction-to-ambient thermal resistance (R_θJA) of the combined PCB and device package and the temperature of the ambient air (T_A), according to Equation 4 or Equation 5:

If the V_IN-V_OUT voltage is known, the maximum allowable output current can be calculated with Equation 6

Unfortunately, the R_θJA value is highly dependent on the heat-spreading capability of the particular PCB design, and therefore varies according to the PCB size, total copper area, copper weight, any thermal vias, and location of the planes. The R_θJA recorded in Thermal Information is determined by the specific EIA/JEDEC JESD51-7 standard for PCB and copper spreading area, and is to be used only as a relative measure of package thermal performance. For a well designed thermal layout, R_θJA is actually the sum of the package junction-to-case (bottom) thermal resistance (R_θJC(bot)) plus the thermal resistance contribution by the PCB copper area acting as a heat sink.

8.2.2.8 Estimating Junction Temperature

The JEDEC standard now recommends the use of psi (Ψ) thermal metrics to estimate the junction temperatures of the LDO when in-circuit on a typical PCB board application. These metrics are not strictly speaking thermal resistances, but rather offer practical and relative means of estimating junction temperatures. These psi metrics are determined to be significantly independent of the copper-spreading area. The key thermal metrics (Ψ_JT and Ψ_JB) are given in the Thermal Information table and are used in accordance with Equation 7 and Equation 8.

For more information about the thermal characteristics Ψ_JT and Ψ_JB, see Semiconductor and IC Package Thermal Metrics ; for more information about measuring T_TOP and T_BOARD, see Using New Thermal Metrics ; and for more information about the EIA/JEDEC JESD51 PCB used for validating R_θJA, see the TI Application Report Thermal Characteristics of Linear and Logic Packages Using JEDEC PCB Designs. These application notes are available at www.ti.com.

8.2.2.9 Recommended Continuous Operating Area

The continuous operational area of an LDO is limited by the input voltage (V_IN), the output voltage (V_OUT), the dropout voltage (V_DO), the output current (I_OUT), and the junction temperature (T_J). The recommended area for continuous operation for a linear regulator can be separated into the following steps, and is shown in Figure 16.

• Limited by dropout: Dropout voltage limits the minimum differential voltage between the input and the output (V_IN – V_OUT) at a given output current level.
• Limited by the rated output current: The rated output current limits the maximum recommended output current level. Exceeding this rating causes the device to fall out of specification.
• Limited by thermals: This portion of the boundary is defined by Equation 6. The slope is nonlinear because the junction temperature of the LDO is controlled by the power dissipation (P_D) across the LDO; therefore, when V_IN – V_OUT increases, the output current must decrease in order to ensure that the rated maximum operating junction temperature of the device is not exceeded. Exceeding the maximum operating junction temperature rating can cause the device to fall out of specifications, reduces long-term reliability, and may activate the thermal shutdown protection circuitry.
• Limited by V_IN range: The rated operating input voltage range governs both the minimum and maximum of V_IN – V_OUT.