10.2.1.2.1 Inductor Selection

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 TPS6132x device integrates a current limit protection circuitry. The valley current of the PMOS rectifier is sensed to limit the maximum current flowing through the synchronous rectifier and the inductor. The valley peak current limit (250 mA, 1150 mA, 1600 mA) is user selectable through the I²C interface.

To optimize solution size the TPS6132x device is designed to operate with inductance values from 1.3 μH to 2.9 μH. TI recommends 2.2-µH inductance be used in typical high current white LED applications.

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 2 and Equation 3:

Equation 2.

Equation 3.

where

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

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

10.2.1.2.2 Input Capacitor

TI recommends low ESR ceramic capacitors for good input voltage filtering. TI recommends a 10-μF input capacitor to improve transient behavior of the regulator and EMI behavior of the total power supply circuit. The input capacitor must be placed as close as possible to the input pin of the converter.

10.2.1.2.3 Output Capacitor

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 required for the defined ripple, supposing that the ESR is zero, by using Equation 4:

Equation 4.

where

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

With a chosen ripple voltage of 10 mV, a minimum capacitance of 10 μF is required. The total ripple is larger due to the ESR of the output capacitor. This additional component of the ripple can be calculated using Equation 5:

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 standard current white LED application (HC_SEL = 0, TPS6132x), 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 or X7R ceramic capacitors.

Depending on the material, size, and therefore 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. TI recommends checking that the selected capacitors are showing enough effective capacitance under real operating conditions.

To support high-current camera flash application (HC_SEL = 1), the converter is designed to work with a low voltage super-capacitor on the output to take advantage of the benefits they offer. A low-voltage super-capacitor in the 0.1-F to 1.5-F range, and with ESR larger than 40 mΩ, is suitable in the TPS6132x application circuit. For this device the output capacitor must be connected between the VOUT pin and a good ground connection.

10.2.1.2.4 NTC Selection

The TPS6132x requires a negative thermistor (NTC) for sensing the LED temperature. Once the temperature monitoring feature is activated, a regulated bias current, approximately 24 μA, is driven out of the TS port and produce a voltage across the thermistor.

If the temperature of the NTC-thermistor rises due to the heat dissipated by the LED, the voltage on the TS input pin decreases. When this voltage goes below the warning threshold, the LEDWARN bit in REGISTER6 is set. This flag is cleared by reading the register.

If the voltage on the TS input decreases further and falls below hot threshold, the LEDHOT bit in REGISTER6 is set and the device automatically goes into shutdown mode to avoid damaging the LED. This status is latched until the LEDHOT flag gets cleared by software.

The selection of the NTC-thermistor value strongly depends on the power dissipated by the LED and all components surrounding the temperature sensor and on the cooling capabilities of each specific application. With a 220-kΩ (at 25°C) thermistor, the valid temperature window is set from 60°C to 90°C. The temperature window can be enlarged by adding external resistors to the TS pin application circuit. To ensure proper triggering of the LEDWARN and LEDHOT flags in noisy environments, the TS signal may require additional filtering capacitance.

Figure 63. Temperature Monitoring Characteristic

10.2.1.2.5 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_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 improper board layout or inductor and capacitor combinations.

The load transient response must be tested as a next step in the evaluation of the regulation loop. V_OUT can be monitored for settling time, overshoot or ringing that helps judge the converter's stability. Without any ringing, the loop usually has 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.