Table 1. TPS61240 5V Output Design Requirements

Table 2. List of Components

10.2.2.1 Programming the Output Voltage

The output voltage is set by a resistor divider internally. The FB pin is used to sense the output voltage. So to configure the output properly, the FB pin needs to be connected directly to the output.

10.2.2.2 Inductor Selection

To make sure that the TPS6124x devices can operate, an inductor must be connected between pin V_IN and pin L. A boost converter normally requires two main passive components for storing energy during the conversion. A boost inductor and a storage capacitor at the output are required. 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. The highest peak current through the inductor and the switch depends on the output load, the input (V_IN), and the output voltage (V_OUT). Estimation of the maximum average inductor current can be done using Equation 2.

Equation 2.

For example, for an output current of 200 mA at 5.0 V V_OUT, at least 540 mA of average current flows through the inductor at a minimum input voltage of 2.3 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 size and cost. With these parameters, it is possible to calculate the value of the minimum inductance by using Equation 3.

Equation 3.

Parameter f is the switching frequency and ΔI_L is the ripple current in the inductor, i.e., 20% x I_L. In this example, the desired inductor has the value of 1.7 μH. With this calculated value and the calculated currents, it is possible to choose a suitable inductor. In typical applications a 1.0-μH inductance is recommended. The device has been optimized to operate with inductance values between 1.0 μH and 2.2 μH. It is recommended that inductance values of at least 1.0 μH is used, even if Equation 3 yields something lower. 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.

With the chosen inductance value, the peak current for the inductor in steady state operation can be calculated. Equation 4 shows how to calculate the peak current I.

Equation 4.

This would be the critical value for the current rating for selecting the inductor. It also needs to be taken into account that load transients and error conditions may cause higher inductor currents.

10.2.2.3 Input Capacitor

At least 2.2-μF input capacitor is recommended to improve transient behavior of the regulator and EMI behavior of the total power supply circuit. It is recommended to place a ceramic capacitor as close as possible to the VIN and GND pins.

10.2.2.4 Output Capacitor

For the output capacitor, it is recommended to use small ceramic capacitors placed as close as possible to the V_OUT and GND pins of the IC. If, for any reason, the application requires the use of large capacitors which can not be placed close to the IC, using a smaller ceramic capacitor in parallel to the large one is recommended. This small capacitor should be placed as close as possible to the V_OUT and GND pins of the IC. To get an estimate of the recommended minimum output capacitance, Equation 5 can be used.

Equation 5.

where

• f is the switching frequency
• ΔV is the maximum allowed ripple

With a chosen ripple voltage of 10 mV, a minimum effective capacitance of 2.7 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 ΔV_ESR = I_OUT x R_ESR.

A capacitor with a value in the range of the calculated minimum should be used. This is required to maintain control loop stability. There are no additional requirements regarding minimum ESR. There is no upper limit for the output capacitance value. Larger capacitors cause lower output voltage ripple as well as lower output voltage drop during load transients.

Note that ceramic capacitors have a DC Bias effect, which will have a strong influence on the final effective capacitance needed. Therefore the right capacitor value has to be chosen carefully. Package size and voltage rating in combination with material are responsible for differences between the rated capacitor value and the effective capacitance.

10.2.2.5 Checking Loop Stability

The first step of circuit and stability evaluation is to look from a steady-state perspective at the following signals:

• Switching node, SW
• Inductor current, I_L
• Output ripple voltage, V_O(AC)

These are the basic signals that need to be measured when evaluating a switching converter. When the switching waveform shows large duty cycle jitter or the output voltage or inductor current shows oscillations, the regulation loop may be unstable. This is often a result of board layout and/or L-C combination.

As a next step in the evaluation of the regulation loop, the load transient response is tested. The time between the application of the load transient and the turn on of the P-channel MOSFET, the output capacitor must supply all of the current required by the load. V_O immediately shifts by an amount equal to ΔI_(LOAD) × ESR, where ESR is the effective series resistance of C_O. ΔI_(LOAD) begins to charge or discharge C_O generating a feedback error signal used by the regulator to return V_O to its steady-state value. The results are most easily interpreted when the device operates in PWM mode. During this recovery time, V_O can be monitored for settling time, overshoot or ringing that helps judge the converter’s stability. Without any ringing, the loop has usually more than 45° of phase margin. Because the damping factor of the circuitry is directly related to several resistive parameters (for example, MOSFET r_DS(on)) that are temperature dependant, the loop stability analysis has to be done over the input voltage range, load current range, and temperature range.