8.2.2.1 Output Filter Design (Inductor and Output Capacitor)

The device is optimized to operate with effective inductance values in the range of 0.7 μH to 4.3 μH and with effective output capacitance in the range of 2 μF to 15 μF. The internal compensation is optimized to operate with an output filter of L = 1 μH or 2.2 μH and C_OUT = 4.7 μF. Larger or smaller inductor and capacitor values can be used to optimize the performance of the device for specific operation conditions. For more details, see the Checking Loop Stability section.

8.2.2.2 Inductor Selection

The inductor value affects the peak-to-peak ripple current, the PWM-to-PFM transition point, the output voltage ripple, and the efficiency. The selected inductor must be rated for DC resistance and saturation current. The inductor ripple current (ΔI_L) decreases with higher inductance and increases with higher V_IN or V_OUT. Use Equation 5 to calculate the maximum inductor current under static load conditions. The saturation current of the inductor must be rated higher than the maximum inductor current as calculated with Equation 6. This rating is recommended because during heavy load transient the inductor current will rise above the calculated value.

Equation 5.

where

• L = Inductor value
• f = Switching frequency

Equation 6.

where

• I_Lmax = Maximum inductor current
• ΔI_L= Peak-to-peak inductor ripple current

In high-frequency converter applications, the efficiency is essentially affected by the inductor AC resistance (essentially the quality factor) and to a smaller extent by the inductor DCR value. To achieve high efficiency operation, use care when selecting inductors featuring a quality factor above 25 at the switching frequency. Increasing the inductor value produces lower RMS currents, but degrades transient response. For a given physical inductor size, increased inductance usually results in an inductor with lower saturation current.

The total losses of the coil consist of both the losses in the DC resistance, R_(DC), and the following frequency-dependent components:

• The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)
• Additional losses in the conductor from the skin effect (current displacement at high frequencies)
• Magnetic field losses of the neighboring windings (proximity effect)
• Radiation losses

Table 1 lists the inductor series from different suppliers that have been used with the TPS6223x-Q1 converters. These components must be verified and validated to determine whether the component is suitable for the end application.

8.2.2.3 Output Capacitor Selection

The unique hysteretic PWM control scheme of the TPS6223x-Q1 device allows the use of tiny ceramic capacitors. Ceramic capacitors with low ESR values have the lowest output voltage ripple and are recommended. The output capacitor requires either an X7R or X5R dielectric. Y5V and Z5U dielectric capacitors, aside from their wide variation in capacitance over temperature, become resistive at high frequencies.

At light load currents the converter operate in power save mode and the output voltage ripple is dependent on the output capacitor value and the PFM peak inductor current. Higher output capacitor values minimize the voltage ripple in PFM Mode and tighten DC output accuracy in PFM Mode.

8.2.2.4 Input Capacitor Selection

Because of the nature of the buck converter having a pulsating input current, a low-ESR input capacitor is required for best input voltage filtering and minimizing the interference with other circuits caused by high input voltage spikes. For most applications a 2.2-μF to 4.7-μF ceramic capacitor is recommended. The input capacitor can be increased without any limit for better input voltage filtering. Because a ceramic capacitor loses up to 80% of the initial capacitance at 5 V, TI recommends using 4.7-μF input capacitors for input voltages greater than 4.5 V.

Take care when using only small ceramic input capacitors. When a ceramic capacitor is used at the input and the power is being supplied through long wires, such as from a wall adapter, a load step at the output or V_IN step on the input can induce ringing at the VIN pin. This ringing can couple to the output and be mistaken as loop instability or could even damage the part by exceeding the maximum ratings.

Table 2 lists some tested input and output capacitors. These components must be verified and validated to determine whether the component is suitable for the end application.

8.2.2.5 Checking Loop Stability

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

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

These signals 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, L-C combination, or both.

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 turnon of the P-channel MOSFET, the output capacitor must supply all of the current required by the load. V_OUT immediately shifts by an amount equal to ΔI_(LOAD) × ESR, where ESR is the effective series resistance of C_OUT. ΔI_(LOAD) begins to charge or discharge C_O generating a feedback error signal used by the regulator to return V_OUT to the steady-state value. The results are most easily interpreted when the device operates in PWM mode.

During this recovery time, V_OUT can be monitored for settling time, overshoot, or ringing that helps judge the stability of the converter. 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 occur over the input voltage range, load current range, and temperature range.