Table 2. Design Parameters

9.2.2.1 Output Capacitance and Transient Response

The TPS7A4501-SP regulator is designed to be stable with a wide range of output capacitors. The ESR of the output capacitor affects stability, most notably with small capacitors. TI recommends a minimum output capacitor of 10 μF with an ESR of 3 Ω or less to prevent oscillations. Larger values of output capacitance can decrease the peak deviations and provide improved transient response for larger load current changes. Bypass capacitors, used to decouple individual components powered by the TPS7A4501-SP, increase the effective output capacitor value.

Give extra consideration to the use of ceramic capacitors. Ceramic capacitors are manufactured with a variety of dielectrics, each with different behavior over temperature and applied voltage. The most common dielectric used for a harsh environment is X7R. Ceramic capacitors lose capacitance when DC bias is applied across the capacitor. This capacitance loss is due to the polarization of the ceramic material. The capacitance loss is not permanent: after a large DC bias is applied, reducing the DC bias reduces the degree of polarization and capacitance increases. DC bias effects vary dramatically with voltage rating, case size, capacitor value, and capacitor manufacturer. Because a capacitor could lose more than 50% of its capacitance with DC bias voltages near the voltage rating of the capacitor, it is important to consider DC bias when selecting a ceramic capacitor for an application.

Ceramic capacitors' dielectric also changes over the temperature range. For example X7R, the first letter X denotes lower temperature range –55°C whereas 7 denotes a higher temperature range 125°C and R denotes capacitance variation over the temperature range (±15%). For harsh environment applications, minimum dielectric thickness must be 1 mil for 100-V DC-rated capacitor and 0.8 mil for 50-V DC-rated capacitors.

Voltage and temperature coefficients are not the only sources of problems. Some ceramic capacitors have a piezoelectric response. A piezoelectric device generates voltage across its terminals due to mechanical stress, similar to the way a piezoelectric accelerometer or microphone works. For a ceramic capacitor, the stress can be induced by vibrations in the system or thermal transients.

Tantalum capacitors can provide higher capacitance per unit volume. Tantalum capacitors can be either manganese dioxide (MNO2)-based capacitors where the cathode is MN02 or polymer. MN02-based tantalum capacitors exhibit high ESR as compared to polymer-based tantalum capacitors. MN02-based tantalum capacitors require in excess of 60% voltage derating. Thus, a 10-V rated capacitor can only be used for 3.3-V application. Whereas polymer-based capacitors only require 10% voltage derating. Paralleling ceramic and tantalum capacitors provide optimum balance between capacitance and ESR.

Table 3 highlights some of the capacitors used in the device.

9.2.2.2 Compensation

TPS7A4501-SP is internally compensated. However, the user can implement a lead network using C₃ to boost the phase margin as well as reduce output noise.

Figure 24. Compensation Schematic

R₁, the bottom resistor, and R₂, the top resistor, form the output voltage divider network. C₃ across R₂ adds a lead network.

For R₁ = 3.2 kΩ and R₂ = 10 kΩ, V_OUT is set at 5 V and C₃ = 470 pF.

Zero and pole can be calculated as shown in the following equations.

Equation 7.

Equation 9.

Equation 11.