8.1.1 Output Capacitance and Transient Response

The TPS73801 regulators are designed to be stable with a wide range of output capacitors. The ESR of the output capacitor affects stability, most notably with small capacitors. A minimum output capacitor of 10 μF with an ESR of 3 Ω or less is recommended 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 TPS73801, increase the effective output capacitor value.

Extra consideration must be given 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 dielectrics used are Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are good for providing high capacitances in a small package, but exhibit strong voltage and temperature coefficients. When used with a 5-V regulator, a 10-μF Y5V capacitor can exhibit an effective value as low as 1 μF to 2 μF over the operating temperature range. The X5R and X7R dielectrics result in more stable characteristics and are more suitable for use as the output capacitor. The X7R type has better stability across temperature, while the X5R is less expensive and is available in higher values.

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.

8.2.1 Design Requirements

Table 2 shows the design parameters for this application.

8.2.2 Detailed Design Procedure

The TPS73801 has an adjustable output voltage range of 1.21 to 20 V. The output voltage is set by the ratio of two external resistors R1 and R2 as shown in Figure 26. The device maintains the voltage at the FB pin at 1.21 V referenced to ground. The current in R1 is then equal to (1.21 V/R1), and the current in R2 is the current in R1 plus the FB pin bias current. The FB pin bias current, 3 µA at 25 °C, flows through R2 into the FB pin. The output voltage can be calculated using Equation 5.

Equation 5.

The value of R1 should be less than 4.17 kΩ to minimize errors in the output voltage caused by the FB pin bias current. Note that in shutdown the output is turned off, and the divider current is zero. For an output voltage of 2.50 V, R1 will be set to 4.0 kΩ. R2 is then found to be 4.22 kΩ using the equation above.

Equation 6.

The adjustable device is tested and specified with the FB pin tied to the OUT pin for an output voltage of 1.21 V. Specifications for output voltages greater than 1.21 V are proportional to the ratio of the desired output voltage to 1.21 V: V_OUT/1.21 V. For example, load regulation for an output current change of 1 mA to 1.5 A is –2 mV (typ) at V_OUT = 1.21 V. At V_OUT = 2.50 V, the typical load regulation is:

Figure 27 shows the actual change in output is about 3 mV for a 1-A load step. The maximum load regulation at 25°C is –8 mV. At V_OUT = 2.50 V, the maximum load regulation is:

Since 16.53 mV is only 0.7% of the 2.5 V output voltage, the load regulation will meet the design requirements.

8.2.3 Application Curve

Figure 27. 1-A Load Transient Response