SLVS931B November   2009  – December 2016 TPS2556 , TPS2557

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  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
  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 Overcurrent Conditions
      2. 9.3.2 FAULT Response
      3. 9.3.3 Undervoltage Lockout (UVLO)
      4. 9.3.4 Enable (EN OR EN)
      5. 9.3.5 Thermal Sense
    4. 9.4 Device Functional Modes
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Applications
      1. 10.2.1 Current-Limiting Power-Distribution Switch
        1. 10.2.1.1 Design Requirements
        2. 10.2.1.2 Detailed Design Procedure
          1. 10.2.1.2.1 Input and Output Capacitance
          2. 10.2.1.2.2 Programming the Current-Limit Threshold
            1. 10.2.1.2.2.1 Designing Above a Minimum Current Limit
            2. 10.2.1.2.2.2 Designing Below a Maximum Current Limit
            3. 10.2.1.2.2.3 Accounting for Resistor Tolerance
          3. 10.2.1.2.3 Auto-Retry Functionality
          4. 10.2.1.2.4 Two-Level Current-Limit Circuit
        3. 10.2.1.3 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 Related Links
    2. 13.2 Receiving Notification of Documentation Updates
    3. 13.3 Community Resources
    4. 13.4 Trademarks
    5. 13.5 Electrostatic Discharge Caution
    6. 13.6 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

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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

The TPS2556 and TPS2557 are precision power-distribution switches for applications where heavy capacitive loads and short circuits are expected to be encountered. The following design procedures can be used to choose the input and output capacitors as well as to calculate the current limit programming resistor value for a typical design. Additional application examples are provided including an auto-retry circuit and a two-level current limit circuit.

10.2 Typical Applications

10.2.1 Current-Limiting Power-Distribution Switch

TPS2556 TPS2557 usb_swt_lvs931.gif Figure 18. Typical Current-Limiting Application

10.2.1.1 Design Requirements

For this example, use the parameters listed in Table 1 as the input parameters.

Table 1. Design Parameters

PARAMETER VALUE
Input voltage 5 V
Output voltage 5 V
Above a minimum current limit 3000 mA
Below a maximum current limit 5000 mA

10.2.1.2 Detailed Design Procedure

10.2.1.2.1 Input and Output Capacitance

Input and output capacitance improves the performance of the device; the actual capacitance must be optimized for the particular application. TI recommends a 0.1-µF or greater ceramic bypass capacitor between IN and GND as close to the device as possible for local noise decoupling for all applications. This precaution reduces ringing on the input due to power-supply transients. Additional input capacitance may be needed on the input to reduce voltage overshoot from exceeding the absolute-maximum voltage of the device during heavy transient conditions. This is especially important during bench testing when long, inductive cables are used to connect the evaluation board to the bench power supply.

Output capacitance is not required, but TI recommends placing a high-value electrolytic capacitor on the output pin when large transient currents are expected on the output.

10.2.1.2.2 Programming the Current-Limit Threshold

The overcurrent threshold is user programmable through an external resistor. The TPS255x uses an internal regulation loop to provide a regulated voltage on the ILIM pin. The current-limit threshold is proportional to the current sourced out of ILIM. The recommended 1% resistor for RILIM is 20 kΩ ≤ RILIM ≤ 187 kΩ to ensure stability of the internal regulation loop. Many applications require that the minimum current limit is above a certain current level or that the maximum current limit is below a certain current level, so it is important to consider the tolerance of the overcurrent threshold when selecting a value for RILIM. Equation 1 approximates the resulting overcurrent threshold for a given external resistor value (RILIM). See Electrical Characteristics for specific current limit settings. The traces routing the RILIM resistor to the TPS255x must be as short as possible to reduce parasitic effects on the current-limit accuracy.

Equation 1. TPS2556 TPS2557 eq_pg_tcur_lvs931.gif
TPS2556 TPS2557 cur_lim_thres_lvs931.gif Figure 19. Current-Limit Threshold vs RILIM

10.2.1.2.2.1 Designing Above a Minimum Current Limit

Some applications require that current limiting cannot occur below a certain threshold. For this example, assume that 3 A must be delivered to the load so that the minimum desired current-limit threshold is 3000 mA. Use the IOS equations and Figure 19 to select RILIM.

Equation 2. TPS2556 TPS2557 eq_app1_1st_lvs841.gif

Select the closest 1% resistor less than the calculated value: RILIM = 33.2 kΩ. This sets the minimum current-limit threshold at 3000 mA . Use the IOS equations, Figure 19, and the previously calculated value for RILIM to calculate the maximum resulting current-limit threshold.

Equation 3. TPS2556 TPS2557 eq_app1_2nd_lvs841.gif

The resulting maximum current-limit threshold is 3592 mA with a 33.2-kΩ resistor.

10.2.1.2.2.2 Designing Below a Maximum Current Limit

Some applications require that current limiting must occur below a certain threshold. For this example, assume that the desired upper current-limit threshold must be below 5000 mA to protect an upstream power supply. Use the IOS equations and Figure 19 to select RILIM.

Equation 4. TPS2556 TPS2557 eq_app2_1st_lvs841.gif

Select the closest 1% resistor greater than the calculated value: RILIM = 23.7 kΩ. This sets the maximum current-limit threshold at 5000 mA . Use the IOS equations, Figure 19, and the previously calculated value for RILIM to calculate the minimum resulting current-limit threshold.

Equation 5. TPS2556 TPS2557 eq_app2_2nd_lvs841.gif

The resulting minimum current-limit threshold is 4316 mA with a 23.7-kΩ resistor.

10.2.1.2.2.3 Accounting for Resistor Tolerance

The analysis of resistor selection focused only on the TPS255x performance and assumed an exact resistor value. However, resistors sold in quantity are not exact and are bounded by an upper and lower tolerance centered around a nominal resistance. The additional RILIM resistance tolerance directly affects the current-limit threshold accuracy at a system level. Table 2 shows a process that accounts for worst-case resistor tolerance assuming 1% resistor values. Using the selection process outlined, determine the upper and lower resistance bounds of the selected resistor. Then calculate the upper and lower resistor bounds to determine the threshold limits. It is important to use tighter tolerance resistors (0.5% or 0.1%) when precision current limiting is desired.

Table 2. Common RILIM Resistor Selections

DESIRED NOMINAL CURRENT LIMIT (mA) IDEAL RESISTOR (kΩ) CLOSEST 1% RESISTOR (kΩ) RESISTOR BOUNDS (kΩ) IOS ACTUAL LIMITS (mA)
1% LOW 1% HIGH MIN NOM MAX
750 146.9 147 145.5 148.5 605 749 886
1000 110.2 110 108.9 111.1 825 1002 1166
1250 88.2 88.7 87.8 89.6 1039 1244 1430
1500 73.6 73.2 72.5 73.9 1276 1508 1715
1750 63.1 63.4 62.8 64 1489 1742 1965
2000 55.2 54.9 54.4 55.4 1737 2012 2252
2250 49.1 48.7 48.2 49.2 1975 2269 2523
2500 44.2 44.2 43.8 44.6 2191 2501 2765
2750 40.2 40.2 39.8 40.6 2425 2750 3025
3000 36.9 36.5 36.1 36.9 2689 3030 3315
3250 34 34 33.7 34.3 2901 3253 3545
3500 31.6 31.6 31.3 31.9 3138 3501 3800
3750 29.5 29.4 29.1 29.7 3390 3764 4068
4000 27.7 27.4 27.1 27.7 3656 4039 4349
4250 26 26.1 25.8 26.4 3851 4241 4554
4500 24.6 24.9 24.7 25.1 4050 4446 4761
4750 23.3 23.2 23 23.4 4369 4773 5091
5000 22.1 22.1 21.9 22.3 4602 5011 5331
5250 21.1 21 20.8 21.2 4861 5274 5595
5500 20.1 20 19.8 20.2 5121 5539 5859

10.2.1.2.3 Auto-Retry Functionality

Some applications require that an overcurrent condition disables the part momentarily during a fault condition and re-enables after a pre-set time. This auto-retry functionality can be implemented with an external resistor and capacitor. During a fault condition, FAULTpulls EN low. The part is disabled when EN is pulled below the turn-off theshold, and FAULT goes high impedance allowing CRETRY to begin charging. The part re-enables when the voltage on EN reaches the turn-on threshold. The auto-retry time is determined by the resistor and capacitor time constant. The part continues to cycle in this manner until the fault condition is removed. The time between retries is given in Equation 6.

Equation 6. TBR = –RFAULT × CRETRY × LN (1 – VEN / (VIN – VOL)) + TFAULT

where

  • VEN is the EN pin typical threshold voltage
  • VIN is the input voltage
  • VOL is the FAULT pin typical saturation voltage
  • TFAULT is the internal FAULT typical deglitch time

The retry duty cycle is calculated with Equation 7, and the average current is D × IOS.

Equation 7. D = TFAULT / (TFAULT + TBR)
TPS2556 TPS2557 auto_func_lvs931.gif Figure 20. Auto-Retry Functionality

Some applications require auto-retry functionality and the ability to enable and disable with an external logic signal. The figure below shows how an external logic signal can drive EN through RFAULT and maintain auto-retry functionality. The resistor and capacitor time constant determines the auto-retry time-out period.

TPS2556 TPS2557 ext_en_sig_lvs931.gif Figure 21. Auto-Retry Functionality With External EN Signal

10.2.1.2.4 Two-Level Current-Limit Circuit

TPS2556 TPS2557 two_cur_limt_lvs931.gif Figure 22. Two-Level Current-Limit Circuit

Some applications require different current-limit thresholds depending on external system conditions. Figure 22 shows an implementation for an externally-controlled, two-level current-limit circuit. The current-limit threshold is set by the total resistance from ILIM to GND (see Programming the Current-Limit Threshold). A logic-level input enables and disables MOSFET Q1 and changes the current-limit threshold by modifying the total resistance from ILIM to GND. Additional MOSFET and resistor combinations can be used in parallel to Q1 and R2 to increase the number of additional current-limit levels.

NOTE

ILIM must never be driven directly with an external signal.

10.2.1.3 Application Curve

In Figure 23, the load current setpoint is 5.05 A, as programmed by the 22.1-kΩ resistor. Load current is stepped mildly from approximately 4.9 A to 5.2 A. The internal FAULT timer runs and after 9 ms, FAULT goes low and current continues to be regulated at approximately 5 A. Due to the high power dissipation within the device, thermal cycling occurs.

In Figure 24, the load current setpoint is 597 mA, as programmed by the 187-kΩ resistor. Load current is stepped mildly from approximately 560 mA to 620 mA. The internal FAULT timer runs and after 9 ms, FAULT goes low and current continues to be regulated at approximately 580 mA.

TPS2556 TPS2557 appgraph2_f23.gif Figure 23. 5-A Current Limit With Thermal Cycling
TPS2556 TPS2557 appgraph1_f23.gif Figure 24. 600-mA Current Limit Without Thermal Cycling