Table 2. TPS61010 3.3 V Output Design Parameters

10.2.1.2.1 Programming the TPS61010 Adjustable Output Voltage Device

The output voltage of the TPS61010 can be adjusted with an external resistor divider. The typical value of the voltage on the FB pin is 500 mV in fixed frequency operation and 485 mV in the power-save operation mode. The maximum allowed value for the output voltage is 3.3 V. The current through the resistive 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 R4 is typically 500 mV. Based on those two values, the recommended value for R4 is in the range of 500 kΩ in order to set the divider current at 1 µA. From that, the value of resistor R3, depending on the needed output voltage (V_O), can be calculated using Equation 1.

Equation 1.

If, as an example, an output voltage of 2.5 V is needed, a 2-MΩ resistor should be chosen for R3.

Figure 20. Typical Application Circuit for Adjustable Output Voltage Option

The output voltage of the adjustable output voltage version changes with the output current. Due to device-internal ground shift, which is caused by the high switch current, the internal reference voltage and the voltage on the FB pin increases with increasing output current. Since the output voltage follows the voltage on the FB pin, the output voltage rises as well with a rate of 1 mV per 1-mA output current increase. Additionally, when the converter goes into pulse-skip mode at output currents around 5 mA and lower, the output voltage drops due to the hysteresis of the controller. This hysteresis is about 15 mV, measured on the FB pin.

10.2.1.2.2 Programming the Low Battery Comparator Threshold Voltage

The current through the resistive divider should be about 100 times greater than the current into the LBI pin. The typical current into the LBI pin is 0.01 µA, the voltage across R2 is equal to the reference voltage that is generated on-chip, which has a value of 500 mV ±15 mV. The recommended value for R2 is therefore in the range of 500 kΩ. From that, the value of resistor R1, depending on the desired minimum battery voltage V_BAT, can be calculated using Equation 2.

Equation 2.

For example, if the low-battery detection circuit should flag an error condition on the LBO output pin at a battery voltage of 1 V, a resistor in the range of 500 kΩ should be chosen for R1. The output of the low battery comparator is a simple open-drain output that goes active low if the battery voltage drops below the programmed threshold voltage on LBI. The output requires a pullup resistor with a recommended value of 1 MΩ, and should only be pulled up to the V_O. If not used, the LBO pin can be left floating or tied to GND.

10.2.1.2.3 Inductor Selection

A boost converter normally requires two main passive components for storing energy during the conversion. A boost inductor is required and a storage capacitor at the output. To select the boost inductor, it is recommended to keep the possible peak inductor current below the current limit threshold of the power switch in the chosen configuration. For example, the current limit threshold of the TPS61010's switch is 1100 mA at an output voltage of 3.3 V. The highest peak current through the inductor and the switch depends on the output load, the input (V_BAT), and the output voltage (V_O). Estimation of the maximum average inductor current can be done using Equation 3.

Equation 3.

For example, for an output current of 100 mA at 3.3 V, at least 515-mA of current flows through the inductor at a minimum input voltage of 0.8 V.

The second parameter for choosing the inductor is the desired current ripple in the inductor. Normally, it is advisable to work with a ripple of less than 20% of the average inductor current. A smaller ripple reduces the magnetic hysteresis losses in the inductor, as well as output voltage ripple and EMI. But in the same way, regulation time at load changes rises. In addition, a larger inductor increases the total system costs.

With those parameters, it is possible to calculate the value for the inductor by using Equation 4.

Equation 4.

Parameter 7 is the switching frequency and Δ I_L is the ripple current in the inductor, that is, 20% × I_L.

In this example, the desired inductor has the value of 12 µH. With this calculated value and the calculated currents, it is possible to choose a suitable inductor. Care must be taken that load transients and losses in the circuit can lead to higher currents as estimated in Equation 3. Also, the losses in the inductor caused by magnetic hysteresis losses and copper losses are a major parameter for total circuit efficiency.

The following inductor series from different suppliers were tested. All work with the TPS6101x converter within their specified parameters:

10.2.1.2.4 Capacitor Selection

The major parameter necessary to define the output capacitor is the maximum allowed output voltage ripple of the converter. This ripple is determined by two parameters of the capacitor, the capacitance and the ESR. It is possible to calculate the minimum capacitance needed for the defined ripple, supposing that the ESR is zero, by using Equation 5.

Equation 5.

Parameter f is the switching frequency and ΔV is the maximum allowed ripple.

With a chosen ripple voltage of 15 mV, a minimum capacitance of 10 µF is needed. The total ripple is larger due to the ESR of the output capacitor. This additional component of the ripple can be calculated using Equation 6.

Equation 6.

An additional ripple of 30 mV is the result of using a tantalum capacitor with a low ESR of 300 mΩ. The total ripple is the sum of the ripple caused by the capacitance and the ripple caused by the ESR of the capacitor. In this example, the total ripple is 45 mV. It is possible to improve the design by enlarging the capacitor or using smaller capacitors in parallel to reduce the ESR or by using better capacitors with lower ESR, like ceramics. For example, a 10 µF ceramic capacitor with an ESR of 50 mΩ is used on the evaluation module (EVM). Tradeoffs must be made between performance and costs of the converter circuit.

A 10-µF input capacitor is recommended to improve transient behavior of the regulator. A ceramic capacitor or a tantalum capacitor with a 100 nF ceramic capacitor in parallel placed close to the IC is recommended.

10.2.1.2.5 Compensation of the Control Loop

An R/C/C network must be connected to the COMP pin in order to stabilize the control loop of the converter. Both the pole generated by the inductor L1 and the zero caused by the ESR and capacitance of the output capacitor must be compensated. The network shown in Figure 21 satisfies these requirements.

Figure 21. Compensation of Control Loop

Resistor R_C and capacitor C_C2 depend on the chosen inductance. For a 10 µH inductor, the capacitance of C_C2 should be chosen to 10 nF, or in other words, if the inductor is XX µH, the chosen compensation capacitor should be XX nF, the same number value. The value of the compensation resistor is then chosen based on the requirement to have a time constant of 1 ms, for the R/C network R_C and C_C2, hence for a 33 nF capacitor, a 33 kΩ resistor should be chosen for R_C.

Capacitor C_C1 depends on the ESR and capacitance value of the output capacitor, and on the value chosen for R_C. Its value is calculated using Equation 7.

Equation 7.

For a selected output capacitor of 22 µF with an ESR of 0.2Ω , an R_C of 33 kΩ, the value of C_C1 is in the range of 100 pF.