SLVSD76C February   2016  – July 2017 TPS22918

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
    7. 6.7 Typical DC Characteristics
    8. 6.8 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
      2. 8.3.2 Quick Output Discharge (QOD)
        1. 8.3.2.1 QOD when System Power is Removed
        2. 8.3.2.2 Internal QOD Considerations
      3. 8.3.3 Adjustable Rise Time (CT)
    4. 8.4 Device Functional Modes
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Input Capacitor (CIN)
        2. 9.2.2.2 Output Capacitor (CL) (Optional)
        3. 9.2.2.3 Shutdown Sequencing During Unexpected System Power Loss
        4. 9.2.2.4 VIN to VOUT Voltage Drop
        5. 9.2.2.5 Inrush Current
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
    3. 11.3 Thermal Considerations
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Developmental Support
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

8 Detailed Description

8.1 Overview

The TPS22918 is a 5.5-V, 2-A load switch in a 6-pin SOT-23 package. To reduce voltage drop for low voltage and high current rails, the device implements a low resistance N-channel MOSFET which reduces the drop out voltage across the device.

The device has a configurable slew rate which helps reduce or eliminate power supply droop because of large inrush currents. Furthermore, the device features a QOD pin, which allows the configuration of the discharge rate of VOUT once the switch is disabled. During shutdown, the device has very low leakage currents, thereby reducing unnecessary leakages for downstream modules during standby. Integrated control logic, driver, charge pump, and output discharge FET eliminates the need for any external components which reduces solution size and bill of materials (BOM) count.

8.2 Functional Block Diagram

TPS22918 FBD_SLVSD76.gif

8.3 Feature Description

8.3.1 On and Off Control

The ON pin controls the state of the switch. ON is active high and has a low threshold, making it capable of interfacing with low-voltage signals. The ON pin is compatible with standard GPIO logic threshold. It can be used with any microcontroller with 1 V or higher GPIO voltage. This pin cannot be left floating and must be driven either high or low for proper functionality.

8.3.2 Quick Output Discharge (QOD)

The TPS22918 includes a QOD feature. The QOD pin can be configured in one of three ways:

  • QOD pin shorted to VOUT pin. Using this method, the discharge rate after the switch becomes disabled is controlled with the value of the internal resistance RPD. The value of this resistance is listed in the Electrical Characteristics table.
  • QOD pin connected to VOUT pin using an external resistor REXT. After the switch becomes disabled, the discharge rate is controlled by the value of the total resistance of the QOD. To adjust the total QOD resistance, Equation 1 can be used:
    • Equation 1. RQOD = RPD + REXT

    where

    • RQOD = Total output discharge resistance
    • RPD = Internal pulldown resistance
    • REXT = External resistance placed between the VOUT and QOD pin.
  • QOD pin is unused and left floating. Using this method, there will be no quick output discharge functionality, and the output will remain floating after the switch is disabled.

The fall times of the device depend on many factors including the total resistance of the QOD, VIN, and the output capacitance. When QOD is shorted to VOUT, the fall time will change over VIN as the internal RPD varies over VIN. To calculate the approximate fall time of VOUT for a given RQOD, use Equation 2 and Table 1.

Equation 2. VCAP = VIN × e-t/τ

where

  • VCAP = Voltage across the capacitor (V)
  • t = Time since power supply removal (s)
  • τ = Time constant equal to RQOD × CL

The fall times' dependency on VIN becomes minimal as the QOD value increases with additional external resistance. See Table 1 for QOD fall times.

Table 1. QOD Fall Times

VIN (V) (1)FALL TIME (μs) 90% - 10%, CIN = 1 μF, IOUT = 0 A , VON = 0 V
TA = 25°C TA = 85°C
CL = 1 μF CL = 10 μF CL = 100 μF CL = 1 μF CL = 10 μF CL = 100 μF
5.5 42 190 1880 40 210 2150
5 43 200 1905 45 220 2200
3.3 47 230 2150 50 260 2515
2.5 58 300 2790 60 345 3290
1.8 75 430 4165 80 490 4950
1.2 135 955 9910 135 1035 10980
1 230 1830 19625 210 1800 19270
(1) TYPICAL VALUES WITH QOD SHORTED TO VOUT

8.3.2.1 QOD when System Power is Removed

The adjustable QOD can be used to control the power down sequencing of a system even when the system power supply is removed. When the power is removed, the input capacitor discharges at VIN. Past a certain VIN level, the strength of the RPD will be reduced. If there is still remaining charge on the output capacitor, this will result in longer fall times. For further information regarding this condition, see the Shutdown Sequencing During Unexpected System Power Loss section.

8.3.2.2 Internal QOD Considerations

Special considerations must be taken when using the internal RPD by shorting the QOD pin to the VOUT pin. The internal RPD is a pulldown resistance designed to quickly discharge a load after the switch has been disabled. Care must be used to ensure that excessive current does not flow through RPD during discharge so that the maximum TJ of 125°C is not exceeded. When using only the internal RPD to discharge a load, the total capacitive load must not exceed 200 µF. Otherwise, an external resistor, REXT, must be used to ensure the amount of current flowing through RPD is properly limited and the maximum TJ is not exceeded. To ensure the device is not damaged, the remaining charge from CL must decay naturally through the internal QOD resistance and should not be driven.

8.3.3 Adjustable Rise Time (CT)

A capacitor to GND on the CT pin sets the slew rate of VOUT. The CT capacitor will charge up until shortly after the switch is turned on and VOUT becomes stable. Once VOUT become stable, the capactior will discharge to ground. An approximate formula for the relationship between CT and the slew rate is shown in Equation 3:

Equation 3. SR = 0.55 × CT + 30

where

  • SR = slew rate (in µs/V)
  • CT = the capacitance value on the CT pin (in pF)
  • The units for the constant 30 are µs/V. The units for the constant 0.55 are µs/(V × pF).

This equation accounts for 10% to 90% measurement on VOUT and does not apply for CT = 0 pF. Use Table 2 to determine rise times for when CT = 0 pF.

Rise time can be calculated by multiplying the input voltage by the slew rate. Table 2 contains rise time values measured on a typical device.

Table 2. Rise Time Table

CT× (pF) RISE TIME (µs) 10% - 90%, CL = 0.1 µF, CIN = 1 µF, RL = 10 Ω
(1)
VIN = 5 V VN = 3.3 V VIN = 2.5 V VIN = 1.8 V VIN = 1.5 V VIN = 1.2 V VIN = 1 V
0 135 95 75 60 50 45 40
220 650 455 350 260 220 185 160
470 1260 850 655 480 415 340 300
1000 2540 1680 1300 960 810 660 560
2200 5435 3580 2760 2020 1715 1390 1220
4700 12050 7980 6135 4485 3790 3120 2735
10000 26550 17505 13460 9790 8320 6815 5950
(1) Typical values at 25°C with a 25 V X7R 10% ceramic capacitor on CT.

As the voltage across the capacitor approaches the capacitor rated voltage, the effective capacitance reduces. Depending on the dielectric material used, the voltage coefficient changes. See Table 3 for the recommended minimum voltage rating for the CT capacitor. If using VIN = 1.2 V or 4 V, it is recommended to use the higher of the two CT Voltage ratings specified.

Table 3. Recommended CT Capacitor Voltage Rating

VIN (V) RECOMMENDED CT CAPACITOR VOLTAGE RATING (V)
1 V to 1.2 V 10
1.2 V to 4 V 16
4 V to 5.5 V 20

8.4 Device Functional Modes

Table 4 describes the connection of the VOUT pin depending on the state of the ON pin.

Table 4. VOUT Connection

ON QOD CONFIGURATION TPS22918 VOUT
L QOD pin connected to VOUT with REXT GND (via REXT+RPD)
L QOD pin tied to VOUT directly GND (via RPD)
L QOD pin left open Open
H Any valid QOD configuration VIN