Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The TPS6224x device is a high-efficiency synchronous step-down DC-DC converter featuring power save mode or 2.25-MHz fixed-frequency operation.
The device operates over an input voltage range from 2 V to 6 V. The output voltage is adjustable using an external feedback divider.
|COMPONENT REFERENCE||PART NUMBER||MANUFACTURER||VALUE|
|CIN||GRM188R60J475K||Murata||4.7 μF, 6.3 V. X5R Ceramic|
|COUT||GRM188R60J106M||Murata||10 μF, 6.3 V. X5R Ceramic|
|C1||Murata||22 pF, COG Ceramic|
|L1||LPS3015||Coilcraft||2.2 μH, 110 mΩ|
|R1, R2||Values depending on the programmed output voltage|
The output voltage can be calculated to:
To minimize the current through the feedback divider network, R2 should be 180 kΩ or 360 kΩ. The sum of R1 and R2 should not exceed approximately 1 MΩ, to keep the network robust against noise.
An external feedforward capacitor C1 is required for optimum load transient response. The value of C1 should be in the range from 22 pF to 33 pF.
Route the FB line away from noise sources, such as the inductor or the SW line.
The TPS6224x is designed to operate with inductors in the range of 1.5 μH to 4.7 μH and with output capacitors in the range of 4.7 μF to 22 μF. The device is optimized for operation with a 2.2-μH inductor and 10-μF output capacitor.
Larger or smaller inductor values can be used to optimize the performance of the device for specific operation conditions. For stable operation, the L and C values of the output filter may not fall below 1-μH effective inductance and 3.5-μF effective capacitance. Selecting larger capacitors is less critical because the corner frequency of the L-C filter moves to lower frequencies with fewer stability problems.
The inductor value has a direct effect on the ripple current. The selected inductor must be rated for its DC resistance and saturation current. The inductor ripple current (ΔIL) decreases with higher inductance and increases with higher VIN or VOUT.
The inductor selection also has an impact on the output voltage ripple in the PFM mode. Higher inductor values will lead to lower output voltage ripple and higher PFM frequency, and lower inductor values will lead to a higher output voltage ripple but lower PFM frequency.
Equation 3 calculates the maximum inductor current in PWM mode under static load conditions. The saturation current of the inductor should be rated higher than the maximum inductor current as calculated with Equation 4. This is recommended because during heavy load transients the inductor current will rise above the calculated value.
A more conservative approach is to select the inductor current rating just for the maximum switch current limit ILIMF of the converter.
Accepting larger values of ripple current allows the use of low inductance values, but results in higher output voltage ripple, greater core losses, and lower output current capability.
The total losses of the coil have a strong impact on the efficiency of the DC-DC conversion and consist of both the losses in the DC resistance (R(DC)) and the following frequency-dependent components:
|DIMENSIONS (mm3)||INDUCTANCE (μH)||INDUCTOR TYPE||SUPPLIER|
|2.5 × 2 × 1||2||MIPS2520D2R2||FDK|
|2.5 × 2 × 1.2||2||MIPSA2520D2R2||FDK|
|2.5 × 2 × 1||2.2||KSLI-252010AG2R2||Hitachi Metals|
|2.5 × 2 × 1.2||2.2||LQM2HPN2R2MJ0L||Murata|
|3 × 3 × 1.4||2.2||LPS3015||Coilcraft|
The advanced fast-response voltage mode control scheme of the TPS6224x 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 overtemperature, become resistive at high frequencies.
At nominal load current, the device operates in PWM mode and the RMS ripple current is calculated as:
At nominal load current, the device operates in PWM mode and the overall output voltage ripple is the sum of the voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and discharging the output capacitor:
At light load currents, the converter operates in power save mode and the output voltage ripple depends on the output capacitor and inductor value. Larger output capacitor and inductor values minimize the voltage ripple in PFM mode and tighten DC output accuracy in PFM mode.
The buck converter has a natural pulsating input current; therefore, 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 4.7-μF to 10-μF ceramic capacitor is recommended. Because ceramic capacitors lose up to 80% of their initial capacitance at 5 V, it is recommended that a 10-μF input capacitor be used for input voltages greater than 4.5 V. The input capacitor can be increased without any limit for better input voltage filtering.
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 VIN step on the input, can induce ringing at the VIN pin. The ringing can couple to the output and be mistaken as loop instability, or could even damage the part by exceeding the maximum ratings
|4.7 μF||GRM188R60J475K||0603: 1.6 × 0.8 × 0.8 mm3||Murata|
|10 μF||GRM188R60J106M69D||0603: 1.6 × 0.8 × 0.8 mm3||Murata|