ZHCSFB9B June 2016 – March 2019 TPS63070
PRODUCTION DATA.
- 1 特性
- 2 應用
- 3 說明
- 4 修訂歷史記錄
- 5 Device Comparison Table
- 6 Pin Configuration and Functions
- 7 Specifications
- 8 Detailed Description
- 9 Application and Implementation
- 10Power Supply Recommendations
- 11Layout
- 12器件和文檔支持
- 13機械、封裝和可訂購信息
9.2.2.2 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, the peak current for the inductor in steady state operation is calculated using Equation 8. Only the equation which defines the switch current in boost mode is shown, because this provides 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 2.4MHz)
L = Selected inductor value
η = Estimated converter efficiency (use the number from the efficiency curves or 0.90 as an assumption)
Note: The calculation must be done for the minimum input voltage which is possible to have in boost mode
Calculating the maximum inductor current using the actual operating conditions gives the minimum saturation current of the inductor needed. It is recommended to choose an inductor with a saturation current 20% higher than the value calculated from Equation 8. The following inductors are recommended for use:
Table 5. Inductor Selection
| INDUCTOR VALUE | COMPONENT SUPPLIER(1) | SIZE (LxWxH /mm) | Isat/DCR |
|---|---|---|---|
| 1.2 µH | Coilcraft, XFL4015-122ME | 4 x 4 x 1.5 | 4.5 A / 18.8 mΩ |
| 1.5 µH | Coilcraft, XFL4020-152ME | 4 x 4 x 2.1 | 4.6 A / 14.4 mΩ |
| 1.0 µH | Coilcraft, XFL4020-102ME | 4 x 4 x 2.1 | 5.4 A / 10.8 mΩ |
| 1 µH | Murata, 1277AS-H-1R0M | 3.2 x 2.5 x 1.2 | 3.7 A / 45 mΩ |
The inductor value also affects the stability of the feedback loop. In particular the boost transfer function exhibits a right half-plane zero. The frequency of the right half plane zero is inverse proportional to the inductor value and the load current. This means the higher the value of the inductance and load current, the more the right half plane zero is moved to a lower frequency. This degrades the phase margin of the feedback loop. It is recommended to choose the inductor's value in order to have the frequency of the right half plane zero >400 kHz. The frequency of the RHPZ is calculated using Equation 9.
With,
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
If the operating conditions results in a frequency of the RHPZ of less than 400kHz, more output capacitance should be added to reduce the cross over frequency. The RHPZ moves to lowest frequency at lowest input voltage (highest boost factor) and largest output current. Device stability should therefore be observed mainly under these worst case operating conditions.