9.2.2 Detailed Design Procedure
9.2.2.1 Inductor Selection
The TPS6262x device family of step-down converters have been optimized to operate with an effective inductance value in the range of 0.3 μH to 1.3 μH and with output capacitors in the range of 4.7 μF to 10 μF. The internal compensation is optimized to operate with an output filter of L = 0.47 μH and COUT = 4.7 μF. Larger or smaller inductor values can be used to optimize the performance of the device for specific operation conditions. For more details, see section Checking Loop Stability.
The inductor value affects its peak-to-peak ripple current, the PWM-to-PFM transition point, the output voltage ripple and the efficiency. The selected inductor has to 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.
Equation 1.
where
- fSW = Switching frequency (6 MHz typical)
- L = Inductor value
- ΔIL = Peak-to-peak inductor ripple current
Equation 2.
where
- ΔIL = Peak-to-peak inductor ripple current
- IL(MAX) = Maximum inductor current
In high-frequency converter applications, the efficiency is essentially affected by the inductor AC resistance (i.e. quality factor) and to a smaller extent by the inductor DCR value. To achieve high efficiency operation, care should be taken in 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
The following inductor series from different suppliers have been used with the TPS6262x converters.
Table 2. List Of Inductors(1)
| MANUFACTURER |
SERIES |
DIMENSIONS |
| MURATA |
LQM21PN1R0NGR |
2.0 x 1.2 x 1.0 max. height |
| LQM21PNR54MG0 |
2.0 x 1.2 x 1.0 max. height |
| LQM21PNR47MC0 |
2.0 x 1.2 x 0.55 max. height |
| LQM21PN1R0MC0 |
2.0 x 1.2 x 0.55 max. height |
| LQM21PN1R5MC0 |
2.0 x 1.2 x 0.55 max. height |
| HITACHI METALS |
HSLI-201210AG-R47 |
2.0 x 1.2 x 1.0 max. height |
| HSLI-201210SW-R85 |
2.0 x 1.2 x 1.0 max. height |
| JSLI-201610AG-R70 |
2.0 x 1.6 x 1.0 max. height |
| TOKO |
MDT2012-CX1R0A |
2.0 x 1.2 x 1.0 max. height |
| FDK |
MIPS2012D1R0-X2 |
2.0 x 1.2 x 1.0 max. height |
| TAIYO YUDEN |
NM2012NR82 |
2.0 x 1.2 x 1.0 max. height |
| NM20121NR0 |
2.0 x 1.2 x 1.0 max. height |
9.2.2.2 Output Capacitor Selection
The advanced fast-response voltage mode control scheme of the TPS6262x converters allows the use of tiny ceramic capacitors. Ceramic capacitors with low ESR values have the lowest output voltage ripple and are recommended. For best performance, the device should be operated with a minimum effective output capacitance of 1.6 μF. 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 nominal load current, the device operates in PWM mode and the overall output voltage ripple is the sum of the voltage step caused by the output capacitor ESL and the ripple current flowing through the output capacitor impedance.
At light loads, the output capacitor limits the output ripple voltage and provides holdup during large load transitions. A 4.7 μF capacitor typically provides sufficient bulk capacitance to stabilize the output during large load transitions. The typical output voltage ripple is 1% of the nominal output voltage VOUT.
The output voltage ripple during PFM mode operation can be kept very small. The PFM pulse is time controlled, which allows to modify the charge transferred to the output capacitor by the value of the inductor. The resulting PFM output voltage ripple and PFM frequency depend in first order on the size of the output capacitor and the inductor value. The PFM frequency decreases with smaller inductor values and increases with larger ones. Increasing the output capacitor value and the effective inductance will minimize the output ripple voltage.
9.2.2.3 Input Capacitor Selection
Because of the nature of the buck converter having a pulsating input current, a low ESR input capacitor is required to prevent large voltage transients that can cause misbehavior of the device or interferences with other circuits in the system. For most applications, a 2.2-μF capacitor is sufficient.
Take care when using only 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 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. Additional "bulk" capacitance (electrolytic or tantalum) should in this circumstance be placed between CIN and the power source lead to reduce ringing than can occur between the inductance of the power source leads and CIN.
9.2.2.4 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, IL
- Output ripple voltage, VOUT(AC)
These are the basic signals that need to 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.
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 turn on of the P-channel MOSFET, the output capacitor must supply all of the current required by the load. VOUT immediately shifts by an amount equal to ΔI(LOAD) x ESR, where ESR is the effective series resistance of COUT. ΔI(LOAD) begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value. The results are most easily interpreted when the device operates in PWM mode.
During this recovery time, VO can be monitored for settling time, overshoot or ringing that helps judge the converter’s stability. 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 (e.g., MOSFET RDS(on)) that are temperature dependant, the loop stability analysis has to be done over the input voltage range, load current range, and temperature range.
Figure 5. TPS6262x Typical Application Schematic
