SLVSAW6H June   2011  – November 2024 LP2951-Q1

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics (Both Legacy and New Chip)
    6. 5.6 Timing Requirements (New Chip only)
    7. 5.7 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Output Enable
      2. 6.3.2 Dropout Voltage
      3. 6.3.3 Current Limit
      4. 6.3.4 Undervoltage Lockout (UVLO)
      5. 6.3.5 Thermal Shutdown
    4. 6.4 Device Functional Modes
      1. 6.4.1 Shutdown Mode
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Reverse Current
      2. 7.1.2 Input and Output Capacitor Requirements
      3. 7.1.3 Estimating Junction Temperature
      4. 7.1.4 Power Dissipation (PD)
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
        1. 7.2.1.1 Recommended Capacitor Types
          1. 7.2.1.1.1 Recommended Capacitors (Legacy Chip)
            1. 7.2.1.1.1.1 ESR Range (Legacy Chip)
          2. 7.2.1.1.2 Recommended Capacitors (New Chip)
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1 Feedback Resistor Selection
        2. 7.2.2.2 Feedforward Capacitor
      3. 7.2.3 Application Curves
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Development Support
      2. 8.1.2 Device Nomenclature
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

7.2.2.2 Feedforward Capacitor

Connect a feedforward capacitor (CFF) between the OUT pin and the FB pin. CFF improves transient, noise, and PSRR performance. A higher capacitance CFF is possible; however, the start-up time increases. For a detailed description of CFF tradeoffs, see the Pros and Cons of Using a Feedforward Capacitor with a Low-Dropout Regulator application note.

As shown in Figure 7-3, poor layout practices and using long traces at the FB pin results in the formation of a parasitic capacitor (CFB).

LP2951-Q1 Formation of Parasitic Capacitor at the FB Pin Figure 7-3 Formation of Parasitic Capacitor at the FB Pin

CFB, along with the feedback resistors R1 and R2 potentially result in the formation of an uncompensated pole in the transfer function of the loop gain. A CFB value as small as 6pF potentially causes the parasitic pole frequency, given by Equation 8, to fall within the bandwidth of the LDO and result in instability.

Equation 8. f P =   1 2 × π × C F B × R 1 ∥ R 2

Adding a feedforward capacitor (CFF), as shown in Figure 7-4, creates a zero in the loop gain transfer function that compensates for the parasitic pole created by CFB. Equation 9 and Equation 10 calculate the pole and zero frequencies.

LP2951-Q1 Feedforward Capacitor Compensates the Effects of the Parasitic Capacitor Figure 7-4 Feedforward Capacitor Compensates the Effects of the Parasitic Capacitor
Equation 9. f P =   1 2 × π × R 1 ∥ R 2 × C F F + C F B
Equation 10. f Z =   1 2 × π × C F F × R 1

The CFF value that makes fP equal to fZ, and results in a pole-zero cancellation, depends on the values of CFB and the feedback resistors used in the application. Alternatively, if the feedforward capacitor is selected so that CFF ≫ CFB, then the pole and zero frequencies given by Equation 9 and Equation 10 are related as:

Equation 11. f p f z   ≅   1 +   R 1 R 2   =   V O U T V F B

In most applications, particularly where a 3.3V or 5V VOUT is generated, this ratio is not very large. Thus, implying that the frequencies are located close to each other and therefore the parasitic pole is compensated. Even for large VOUT values, where this ratio is as large as 20, a CFF value in the range 100pF ≤ CFF ≤ 10nF typically helps prevent instability caused by the parasitic capacitance on the feedback node.