Table 3. Design Parameters

DESIGN PARAMETERS	EXAMPLE VALUES
Input voltage range	3.3 V to 4.2 V
Output voltage	5 V
Flash current	500 mA

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:

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

8.2.1.2.3 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.