SLVSA44B November   2009  – February 2015 TPS22907

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1  Absolute Maximum Ratings
    2. 6.2  ESD Ratings
    3. 6.3  Recommended Operating Conditions
    4. 6.4  Thermal Information
    5. 6.5  Electrical Characteristics
    6. 6.6  Switching Characteristics: VIN = 3.6 V
    7. 6.7  Switching Characteristics: VIN = 1.8 V
    8. 6.8  Switching Characteristics: VIN = 1.1 V
    9. 6.9  Typical DC Characteristics
    10. 6.10 Typical AC Characteristics
  7. 7 Parameter Measurement Information
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 On and Off Control
    4. 8.4 Device Functional Modes
  9. 9 Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Input Capacitor
      2. 9.1.2 Output Capacitor
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Managing Inrush Current
        2. 9.2.2.2 Voltage Drop from VIN to VOUT
      3. 9.2.3 Application Curve
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Trademarks
    2. 12.2 Electrostatic Discharge Caution
    3. 12.3 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

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9 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.

9.1 Application Information

9.1.1 Input Capacitor

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 needs to be placed between VIN and GND. A 1-μF ceramic capacitor, CIN, place close to the pins is usually sufficient. Higher values of CIN can be use to further reduce the voltage drop during high-current application. When switching heavy loads, TI recommends having an input capacitor approximately ten times higher than the output capacitor to avoid excessive voltage drop.

9.1.2 Output Capacitor

Due to the integrated body diode in 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 turnon due to inrush currents.

9.2 Typical Application

TPS22907 TypicalApplication.gifFigure 23. Typical Application Schematic

9.2.1 Design Requirements

Table 3 lists the design requirements for the device.

Table 3. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE
VIN 3.6 V
CL 4.7 µF
Load current 1 A
Ambient Temperature 25 °C
Maximum inrush current 750 mA

9.2.2 Detailed Design Procedure

9.2.2.1 Managing Inrush Current

When the switch is enabled, the output capacitors must be charged up from 0 V to the set value (3.6 V in this example). This charge arrives in the form of inrush current. Inrush current can be calculated using the following equation:

Equation 1. TPS22907 Q3_Iinhush_slvsco0.gif

where

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

The TPS22907 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 because the amount of inrush current increases with output capacitance:

Equation 2. 750 mA = 4.7 µF × 3.6 V / dt

where

  • dt = 22.56 µs

To ensure an inrush current of less than 750 mA, a device with a rise time greater than 22.56 µs must be used. The TPS22907 has a typical rise time of 25 µs at 3.6 V which meets the above design requirements.

9.2.2.2 Voltage Drop from VIN to VOUT

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 = 3.6 V, the TPS22907 has an RON value of 44 mΩ. Using this value and the defined load current, the above equation can be evaluated:

Equation 4. ΔV = 1 A × 44 mΩ
ΔV = 44 mV

Therefore, the voltage drop across the device will be 44 mV.

9.2.3 Application Curve

Figure 24 shows the inrush current expected for different load capacitances at varying VIN voltages.

TPS22907 D001_TPS22907.gifFigure 24. Expected Inrush Current vs Load Capacitance