SNVSB02C March   2020  – July 2021 TPS54JA20

PRODUCTION DATA  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  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 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Internal VCC LDO And Using External Bias On VCC Pin
      2. 7.3.2  Enable
      3. 7.3.3  Output Voltage Setting
        1. 7.3.3.1 Remote Sense
      4. 7.3.4  Internal Fixed Soft Start and External Adjustable Soft Start
      5. 7.3.5  External REFIN For Output Voltage Tracking
      6. 7.3.6  Frequency and Operation Mode Selection
      7. 7.3.7  D-CAP3 Control
      8. 7.3.8  Low-side FET Zero-Crossing
      9. 7.3.9  Current Sense and Positive Overcurrent Protection
      10. 7.3.10 Low-side FET Negative Current Limit
      11. 7.3.11 Power Good
      12. 7.3.12 Overvoltage and Undervoltage Protection
      13. 7.3.13 Out-Of-Bounds (OOB) Operation
      14. 7.3.14 Output Voltage Discharge
      15. 7.3.15 UVLO Protection
      16. 7.3.16 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Auto-Skip Eco-mode Light Load Operation
      2. 7.4.2 Forced Continuous-Conduction Mode
      3. 7.4.3 Powering The Device From A 12-V Bus
      4. 7.4.4 Powering The Device From A 3.3-V Bus
      5. 7.4.5 Powering The Device From A Split-rail Configuration
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1  Output Voltage Setting Point
        2. 8.2.2.2  Choose the Switching Frequency and the Operation Mode
        3. 8.2.2.3  Choose the Inductor
        4. 8.2.2.4  Set the Current Limit (TRIP)
        5. 8.2.2.5  Choose the Output Capacitor
        6. 8.2.2.6  Choose the Input Capacitors (CIN)
        7. 8.2.2.7  Soft Start Capacitor (SS/REFIN Pin)
        8. 8.2.2.8  EN Pin Resistor Divider
        9. 8.2.2.9  VCC Bypass Capacitor
        10. 8.2.2.10 BOOT Capacitor
        11. 8.2.2.11 PGOOD Pullup Resistor
      3. 8.2.3 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
      1. 10.2.1 Thermal Performance On TI EVM
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Support Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

D-CAP3 Control

The TPS54JA20 uses D-CAP3 mode control to achieve the fast load transient while maintaining the ease-of-use feature. The D-CAP3 control architecture includes an internal ripple generation network, enabling the use of very low-ESR output capacitors such as multi-layered ceramic capacitors (MLCC) and low-ESR polymer capacitors. No external current sensing network or voltage compensators are required with D-CAP3 control architecture. The role of the internal ripple generation network is to emulate the ripple component of the inductor current information and then combine it with the voltage feedback signal to regulate the loop operation. The amplitude of the ramp is determined by VIN, VOUT, operating frequency, and the R-C time-constant of the internal ramp circuit. At different switching frequency settings (see Table 7-1), the R-C time-constant varies to maintain relatively constant ramp amplitude. Also, the device utilizes internal circuitry to cancel the dc offset caused by injected ramp and significantly reduce the dc offset caused by the output ripple voltage, especially under light load condition.

For any control topologies that do not support external compensation design, there is a minimum range of the output filter, maximum range of the output filter, or both, it can support. The output filter used with the TPS54JA20 is a low-pass L-C circuit. This L-C filter has double pole that is described in Equation 3.

Equation 3. GUID-00A874E3-1D18-4DC1-B7F7-82F5398D7318-low.gif

At low frequencies, the overall loop gain is set by the output set-point resistor divider network and the internal gain of the TPS54JA20. The low frequency L-C double pole has a 180-degree drop in-phase. At the output filter frequency, the gain rolls off at a –40-dB per decade rate and the phase drops rapidly. The internal ripple generation network introduces a high-frequency zero that reduces the gain roll off from –40-dB to –20-dB per decade and increases the phase by 90 degrees per decade above the zero frequency.

After identifying the application requirements, the output inductance should be designed so that the inductor peak-to-peak ripple current is approximately between 15% and 40% of the maximum output current.

The inductor and capacitor selected for the output filter must be such that the double pole of Equation 3 is located no higher than 1/30 of operating frequency. Choose very small output capacitance leads to relative high frequency L-C double pole which allows that overall loop gain stays high until the L-C double frequency. Given the zero from the internal ripple generation network is relatively high frequency as well, the loop with very small output capacitance can have too high of crossover frequency which is not desired. Use Table 7-2 to help locate the internal zero based on the selected switching frequency.

Table 7-2 Locating the Zero
SWITCHING
FREQUENCIES
(fSW) (kHz)
ZERO (fZ) LOCATION (kHz)
60084.5
80084.5
1000106

In general, where reasonable (or smaller) output capacitance is desired, the output ripple requirement and load transient requirement can be used to determine the necessary output capacitance for stable operation.

For the maximum output capacitance recommendation, select the inductor and capacitor values so that the L-C double pole frequency is no less than 1/100 of operating frequency. With this starting point, verify the small signal response on the board using the following one criteria:

  • Phase margin at the loop crossover is greater than 50 degrees

The actual maximum output capacitance can go higher as long as phase margin is greater than 50 degrees. However, small signal measurement (bode plot) should be done to confirm the design.

If MLCC is used, consider the derating characteristics to determine the final output capacitance for the design. For example, when using an MLCC with specifications of 10 µF, X5R, and 6.3 V, the derating by DC bias and AC bias are 80% and 50%, respectively. The effective derating is the product of these two factors, which in this case, is 40% and 4 µF. Consult with capacitor manufacturers for specific characteristics of the capacitors to be used in the system/applications.

For higher output voltage at or above 2 V, additional phase boost can be required to secure sufficient phase margin due to phase delay/loss for higher output voltage (large on-time (tON)) setting in a fixed-on-time topology based operation. A feedforward capacitor placing in parallel with RFB_HS is found to be very effective to boost the phase margin at loop crossover. Refer to the Optimizing Transient Response of Internally Compensated dc-dc Converters With Feedforward Capacitor application report for details.

Besides boost the phase, a feedforward capacitor feeds more VOUT node information into the FB node by AC coupling. This feedforward during load transient event enables the control loop to a faster response to VOUT deviation. However, this feedforward during steady state operation also feeds more VOUT ripple and noise into FB. High ripple and noise on FB usually leads to more jitter, or even double pulse behavior. To determine the final feedforward capacitor value, impacts to phase margin, load transient performance and ripple, and noise on FB should be all considered. Using Frequency Analysis equipment to measure the crossover frequency and the phase margin is recommended.