JAJSBZ4B June   2014  – January 2018

PRODUCTION DATA.

1. 特長
2. アプリケーション
3. 概要
1.     Device Images
4. 改訂履歴
5. Pin Configuration and Functions
6. Specifications
7. Detailed Description
1. 7.1 Overview
2. 7.2 Functional Block Diagram
3. 7.3 Feature Description
4. 7.4 Device Functional Modes
8. Applications and Implementation
1. 8.1 Application Information
2. 8.2 Typical Applications
1. 8.2.1 Design Requirements
2. 8.2.2 Detailed Design Procedure
3. 8.2.3 Application Curves
9. Power Supply Recommendations
10. 10Layout
1. 10.1 Layout Guidelines
2. 10.2 Layout Example
11. 11デバイスおよびドキュメントのサポート
1. 11.1 デバイス・サポート
1. 11.1.1 開発サポート
2. 11.2 ドキュメントの更新通知を受け取る方法
3. 11.3 コミュニティ・リソース
4. 11.4 商標
5. 11.5 静電気放電に関する注意事項
6. 11.6 Glossary
12. 12メカニカル、パッケージ、および注文情報

• PWP|16
• PWP|16

#### 7.3.9 Internal Compensation and CFF

The LM46001 is internally compensated with RC = 400 kΩ and CC = 50 pF as shown in Functional Block Diagram. The internal compensation is designed such that the loop response is stable over the entire operating frequency and output voltage range. Depending on the output voltage, the compensation loop phase margin can be low with all ceramic capacitors at the output. TI recommends placing an external feed-forward capacitor CFF in parallel with the top resistor divider RFBT for optimum transient performance.

The feed-forward capacitor CFF in parallel with RFBT places an additional zero before the crossover frequency of the control loop to boost phase margin. The zero frequency can be found by:

Equation 8. fZ-CFF = 1 / ( 2π × RFBT × CFF )

An additional pole is also introduced with CFF at the frequency of

Equation 9. fP-CFF = 1 / ( 2π × CFF × ( RFBT // RFBB ))

Select the CFF so that the bandwidth of the control loop without the CFF is centered between fZ-CFF and fP-CFF. The zero fZ-CFF adds phase boost at the crossover frequency and improves transient response. The pole fP-CFF helps maintaining proper gain margin at frequency higher than the crossover frequency.

Designs with different combinations of output capacitors need different CFF. Different types of capacitors have different Equivalent Series Resistance (ESR). Ceramic capacitors have the smallest ESR and need the most CFF. Electrolytic capacitors have much larger ESR and the ESR zero frequency

Equation 10. fZ-ESR = 1 / ( 2π × ESR × COUT)

would be low enough to boost the phase up around the crossover frequency. Designs using mostly electrolytic capacitors at the output may not need any CFF.

The CFF creates a time constant with RFBT that couples in the attenuated output voltage ripple to the FB node. If the CFF value is too large, it can couple too much ripple to the FB and affect VOUT regulation. It could also couple too much transient voltage deviation and falsely trip PGOOD thresholds. Therefore, calculate CFF based on output capacitors used in the system. At cold temperatures, the value of CFF might change based on the tolerance of the chosen component. This may reduce its impedance and ease noise coupling on the FB node. To avoid this, more capacitance can be added to the output or the value of CFF can be reduced. Please refer to the Detailed Design Procedure for the calculation of CFF.