Table 3. Design Parameters

8.2.1.2.1 Inductor Selecton

A boost converter requires two main passive components for storing energy during the conversion. A boost inductor and a storage capacitor at the output are required. The TPS6105x device integrates a current limit protection circuitry. The peak current of the NMOS switch is sensed to limit the maximum current flowing through the switch and the inductor. The typical peak current limit (1000 mA / 1500 mA / 2000 mA) is user selectable through the I²C interface.

To optimize solution size the TPS6105x device has been designed to operate with inductance values from a minimum of 1.3 μH to a maximum of 2.9 μH. In typical high-current white LED applications, TI recommends an inductance of 2.2 μH.

To select the boost inductor, TI recommends keeping 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 power switch depends on the output load, the input and output voltages. Estimation of the maximum average inductor current and the maximum inductor peak current can be done using Equation 1 and Equation 2:

Equation 1.

Equation 2.

with:

f = switching frequency (2 MHz)
L = inductance value (2.2 μH)
η = estimated efficiency (85%)

For example, for an output current of 500 mA at 5 V, the TPS6105x device must be set for a 1000 mA current limit operation together with an inductor supporting this peak current.

The losses in the inductor caused by magnetic hysteresis losses and copper losses are a major parameter for total circuit efficiency.

8.2.1.2.2 Capacitor Selection

8.2.1.2.2.2 Output Capacitor

The primary 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 3:

Equation 3.

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

With a chosen ripple voltage of 10mV, 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 4:

Equation 4. ΔV_ESR = I_OUT × R_ESR

The total ripple is the sum of the ripple caused by the capacitance and the ripple caused by the ESR of the capacitor. Additional ripple is caused by load transients. This means that the output capacitor must completely supply the load during the charging phase of the inductor. A reasonable value of the output capacitance depends on the speed of the load transients and the load current during the load change.

For the high current white LED application, a minimum of 3-μF effective output capacitance is usually required when operating with 2.2-μH (typical) inductors. For solution size reasons, this is usually one or more X5R/X7R ceramic capacitors. For stable operation of the internally compensated control loop, a maximum of 50 μF effective output capacitance is tolerable.

Depending on the material, size and margin to the rated voltage of the used output capacitor, degradation on the effective capacitance can be observed. This loss of capacitance is related to the DC bias voltage applied. It is therefore always recommended to check that the selected capacitors are showing enough effective capacitance under real operating conditions.

8.2.1.2.3 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_OUT(AC)

These are the basic signals that must 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.

The next step in regulation loop evaluation is to perform a load transient test. Output voltage settling time after the load transient event is a good estimate of the control loop bandwidth. The amount of overshoot and subsequent oscillations (ringing) indicates the stability of the control loop. 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 must be done over the input voltage range, output current range, and temperature range.