SLVSBI7C July   2012  – April 2015 TPS22908

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Options
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Switching Characteristics
    7. 7.7 Typical Characteristics
      1. 7.7.1 Typical DC Characteristics
      2. 7.7.2 Typical Switching Characteristics
  8. 8 Parameter Measurement Information
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 ON/OFF Control
      2. 9.3.2 Quick Output Discharge
    4. 9.4 Device Functional Modes
  10. 10Application and Implementation
    1. 10.1 Application Information
      1. 10.1.1 Input Capacitor (Optional)
      2. 10.1.2 Output Capacitor (Optional)
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1 Managing Inrush Current
        2. 10.2.2.2 VIN to VOUT Voltage Drop
      3. 10.2.3 Application Curve
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
    3. 12.3 Thermal Considerations
  13. 13Device and Documentation Support
    1. 13.1 Trademarks
    2. 13.2 Electrostatic Discharge Caution
    3. 13.3 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

10 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

10.1 Application Information

10.1.1 Input Capacitor (Optional)

To limit the voltage drop on the input supply caused by transient inrush currents when the switch turns on into a discharged load capacitor, a capacitor can be placed between VIN and GND. A 1 µF ceramic capacitor, CIN, placed close to the pins, is usually sufficient. Higher values of CIN can be used to further reduce the voltage drop during high-current application. When switching heavy loads, it is recommended to have an input capacitor about 10 times higher than the output capacitor to avoid excessive voltage drop.

10.1.2 Output Capacitor (Optional)

Due to the integrated body diode of the PMOS switch, a CIN greater than CL is highly recommended. A CL greater than CIN can cause VOUT to exceed VIN when the system supply is removed. This could result in current flow through the body diode from VOUT to VIN. A CIN to CL ratio of at least 10 to 1 is recommended for minimizing VIN dip caused by inrush currents during startup; however, a 10 to 1 ratio for capacitance is not required for proper functionality of the device. A ratio smaller than 10 to 1 (such as 1 to 1) could cause slightly more VIN dip at turn on due to inrush currents.

10.2 Typical Application

TPS22908 typ_app_lvsbi7.gifFigure 30. Typical Application Schematic

10.2.1 Design Requirements

The following input parameters will be used in these design examples.

Table 2. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE
VIN 1.8 V
CL 10 µF
Load current 500 mA
Ambient Temperature 25 °C
Maximum inrush current 200 mA

10.2.2 Detailed Design Procedure

10.2.2.1 Managing Inrush Current

When the switch is enabled, the output capacitors must be charged up from 0 V to the set value (1.8 V in this example). This charge arrives in the form of inrush current. Inrush current can be calculated using Equation 1:

Equation 1. TPS22908 Q3_Iinhush_slvsco0.gif

where

  • CL = Output capacitance
  • dVOUT = Output voltage
  • dt = Rise time

The TPS22908 offers a controlled rise time for minimizing inrush current. This device can be selected based upon the minimum acceptable rise time which can be calculated using the design requirements and the inrush current equation. An output capacitance of 4.7 µF will be used since the amount of inrush current increases with output capacitance:

Equation 2. 200 mA = 10 µF × 1.8V / dt

where

    To ensure an inrush current of less than 200 mA, a device with a rise time greater than 90 µs must be used. The TPS22908 has a typical rise time of 160 µs at 1.8 V which meets the above design requirements.

    10.2.2.2 VIN to VOUT Voltage Drop

    The voltage drop from VIN to VOUT is determined by the ON-resistance of the device and the load current. RON can be found in Electrical Characteristics and is dependent on temperature. When the value of RON is found, the following equation can be used to calculate the voltage drop across the device:

    Equation 3. ΔV = ILOAD × RON

    where

    • ΔV = Voltage drop across the device
    • ILOAD = Load current
    • RON = ON-resistance of the device

    At VIN = 1.8 V, the TPS22908 has an RON value of 42 mΩ. Using this value and the defined load current, the above equation can be evaluated:

    Equation 4. ΔV = 500 mA × 42 mΩ

    where

      Therefore, the voltage drop across the device will be 21 mV.

      10.2.3 Application Curve

      TPS22908 TPS22908_Inrush_Current.pngFigure 31. Inrush Current with a Load Capacitance of 10µF (VIN = 1.8 V, TA = 25°C)