ZHCSA37B August 2012 – August 2019 TPS63036
PRODUCTION DATA.
8.2.2.1 Inductor Selection
For high efficiencies, the inductor should have a low DC resistance to minimize conduction losses. Especially at high-switching frequencies the core material has a higher impact on efficiency. When using small chip inductors, the efficiency is reduced mainly due to higher inductor core losses. This needs to be considered when selecting the appropriate inductor. The inductor value determines the inductor ripple current. The larger the inductor value, the smaller the inductor ripple current and the lower the conduction losses of the converter. Conversely, larger inductor values cause a slower load transient response. To avoid saturation of the inductor, with the chosen inductance value, the peak current for the inductor in steady-state operation can be calculated. Only the equation which defines the switch current in boost mode is reported because this is providing the highest value of current and represents the critical current value for selecting the right inductor.
where
- D = Duty cycle in boost mode
- f = Converter switching frequency (typical 2 MHz)
- L = Selected inductor value
- η = Estimated converter efficiency (use the number from the efficiency curves or 0.80 as an assumption)
- ISW_MAX = Maximum average input current (Figure 6)
NOTE
The calculation must be done for the minimum input voltage which is possible to have in boost mode.
Consider the load transients and error conditions that can cause higher inductor currents. Consider when selecting an appropriate inductor. Please refer to Table 3 for typical inductors.
The size of the inductor can also affect the stability of the feedback loop. In particular the boost transfer function exhibits a right half-plane zero, whose frequency is inverse proportional to the inductor value and the load current. This means as higher the value of inductance and load current is the more possibilities has the right plane zero to be moved at lower frequency. This could degrade the phase margin of the feedback loop. TI recommends to choose the value of the inductor in order to have the frequency of the right half plane zero >400 kHz. The frequency of the RHPZ can be calculated using equation Equation 2.
where
- D =Duty cycle in boost mode
NOTE
The calculation must be done for the minimum input voltage which is possible to have in boost mode.
Table 3. Inductor Selection
| INDUCTOR VALUE | COMPONENT SUPPLIER | SIZE (LxWxH mm) | Isat/DCR |
|---|---|---|---|
| 1 µH | TOKO 1286AS-H-1R0M | 2x1.6x1.2 | 2.3A/78mΩ |
| 1 µH | Coilcraft XFL4020-102 | 4 x 4 x 2.1 | 5.1A/10.8 mΩ |
| 1 µH | Coilcraft XFL3012-102 | 3 x 3 x 1.2 | 2.2 A/35 mΩ |
| 1.5µH | TOKO, 1286AS-H-1R5M | 2 x 1.6 x 1.2 | 4.4A/ 14.40mΩ |
| 1.5µH | Coilcraft, LPS3015-152MLC | 3 x 3 x 1.5 | 2.1A/100mΩ |
| 1.5µH | TOKO, 1269AS-H-1R5M | 2.5 x 2 x 1 | 2.1A/108mΩ |
| 2.2µH | TOKO D1286AS-H-2R2M | 2 x 1.6 x 1.2 | 1.6A/192mΩ |