10.2.1.2.1 Device Choice

The TPS61222 DC/DC converter is intended for systems powered by anything from a single cell through up to three Alkaline, NiCd or NiMH cells with a total typical pin voltage between 0.7 V and 5.5 V. They can also be used in systems powered by one-cell Li-Ion or Li-Polymer batteries with a typical voltage between 2.5 V and 4.2 V. Additionally, any other voltage source with a typical output voltage between 0.7 V and 5.5 V can be used with the TPS61222.

10.2.1.2.2 Programming The Output Voltage

In the fixed-voltage version used for this example, the output voltage is set by an internal resistor divider. The FB pin is used to sense the output voltage. To configure the device properly, connect the FB pin directly to VOUT as shown in Figure 26.

10.2.1.2.3 Inductor Selection

To make sure that the device can operate, a suitable inductor must be connected between pin VIN and pin L. Inductor values of 4.7 μH show good performance over the whole input and output voltage range.

Choosing other inductance values affects the switching frequency f proportional to 1/L as shown in Equation 1.

Equation 1.

Choosing inductor values higher than 4.7 μH can improve efficiency due to reduced switching frequency and therefore with reduced switching losses. Using inductor values below 2.2 μH is not recommended.

Having selected an inductance value, the peak current for the inductor in steady-state operation can be calculated. Equation 2 gives the peak-current estimate.

Equation 2.

Equation 2 provides a suitable inductor current rating. However, remember that load transients and error conditions may cause higher inductor currents.

Equation 3 provides an easy way to estimate whether the device will work in continuous or discontinuous operation depending on the operating points. As long as the Equation 3 is true, continuous operation is typically established. If Equation 3 becomes false, discontinous operation is typically established.

Equation 3.

The following inductor series from different suppliers have been used with TPS6122x converters:

10.2.1.2.4 Capacitor Selection

10.2.1.2.4.2 Output Capacitor

For the output capacitor C₂, small ceramic capacitors are recommended, placed as close as possible to the VOUT and GND pins of the IC. If, for any reason, the application requires the use of large capacitors which cannot be placed close to the IC, the use of a small ceramic capacitor with a capacitance value of around 2.2μF in parallel to the large one is recommended. This small capacitor should be placed as close as possible to the VOUT and GND pins of the IC.

A minimum capacitance value of 4.7 μF should be used, 10 μF is recommended. If the inductor value exceeds 4.7 μH, the value of the output capacitance value needs to be half the inductance value or higher for stability reasons, see Equation 4.

Equation 4.

The TPS6122x is not sensitive to the ESR in terms of stability. However, low ESR capacitors, such as ceramic capacitors, are recommended anyway to minimize output voltage ripple. If heavy load changes are expected, increase the output capacitor value to avoid output voltage drops during fast load transients.

10.2.2.2.1 Device Selection

This application example uses the TPS61220 so that the output voltage can be set at 4.2 V.

10.2.2.2.2 Programming The Output Voltage

In the adjustable output versions, an external resistor divider is used to adjust the output voltage. The resistor divider must be connected between VOUT, FB and GND as shown in Figure 28. When the output voltage is regulated properly, the typical voltage value at the FB pin is 500 mV for the adjustable devices. The maximum recommended value for the output voltage is 6.0 V. The current through the resistor divider should be about 100 times greater than the current into the FB pin. The typical current into the FB pin is 0.01 μA, and the voltage across the resistor between FB and GND, R₂, is typically 500 mV. Based on those two values, the recommended value for R₂ should be lower than 500 kΩ, in order to set the divider current to 1 μA or higher. The value of the resistor connected between VOUT and FB, R₁, depending on the needed output voltage (V_OUT), can be calculated using Equation 5:

Equation 5.

For this example, if an output voltage of 4.2 V is needed, a 1.2-MΩ resistor is calculated for R₁ when 160 kΩ is selected for R₂. This would yield an output voltage of 4.25 V, neglecting resistor tolerances.

10.2.2.2.3 Inductor Selection

See Inductor Selection for a discussion on inductor choice.

10.2.2.2.4 Capacitor Selection

The procedure for selecting capacitors is the same as for the fixed output voltage circuit. See Capacitor Selection.