Table 2. TPS6100x Output Design Parameters

10.2.2.1 Programming the TPS61000 and TPS61007 Adjustable Output Voltage Devices

The output voltage of the TPS61000 and TPS61007 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_OUT, can be calculated using the following equation:

Equation 1.

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

The TPS61007 is an improved version of the TPS61000 adjustable output voltage device. The FBGND pin is internally connected to GND.

10.2.2.2 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 the following equation:

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 1MΩ, and should only be pulled up to the V_OUT. If not used, the LBO pin can be left floating.

10.2.2.3 Inductor Selection

The output filter of inductive switching regulators is a low pass filter of second order. It consists of an inductor and a capacitor, often referred to as storage inductor and output capacitor.

To select an inductor, keep the possible peak inductor current below the current limit threshold of the power switch in your chosen configuration. For example, the current limit threshold of the TPS61006’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_OUT). Estimation of the maximum average inductor current can be done using the following equation:

Equation 3.

For example, for an output current of 100 mA at 3.3 V, at least 515-mA 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, the regulation time at load change rises. In addition, a larger inductor increases the total system cost.

With those parameters it is possible to calculate the value for the inductor:

Equation 4.

where

• f is the switching frequency
• ΔI_L is the ripple current in the inductor, that is 20% x 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 chose a suitable inductor. Care has to 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 inductors from different suppliers were tested. All work with the TPS6100x converter within their specified parameters:

10.2.2.4 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.

Equation 5.

where

• f is the switching frequency
• 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 will be larger due to the ESR of the output capacitor. This additional component of the ripple can be calculated using the following equation:

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 have to 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.

10.2.2.5 Rectifier Selection

The rectifier diode has a major impact on the overall converter efficiency. Standard diodes are not suitable for low-voltage switched mode power supplies. A Schottky diode with low forward voltage and fast reverse recovery should be used as a rectifier to minimize overall losses of the dc-dc converter. The maximum current rating of the diode must be high enough for the application. The maximum diode current is equal to the maximum current in the inductor that was calculated in equation 3. The maximum reverse voltage is the output voltage. The chosen diode should therefore have a reverse voltage rating higher than the output voltage.

The typical forward voltage of those diodes is in the range of 0.35 to 0.45 V assuming a peak diode current of 600 mA.

10.2.2.6 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 18 satisfies these requirements.

Figure 18. Compensation of the Control Loop

Resistor R_C and capacitor C_C2 depend on the chosen inductance. For a 33-µH inductor, the capacitance of C_C2 should be chosen to 33 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 0.3 ms for the R/C network of R_C and C_C2; hence for a 33-nF capacitor, a 10-kΩ resistor should be chosen for R_C.

Capacitor C_C1 is depending on the ESR and capacitance value of the output capacitor, and on the value chosen for R_C. Its value is calculated using following equation:

Equation 7.

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