SLUSAH4D MARCH   2011  – February 2016 TPS56121

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 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Voltage Reference
      2. 7.3.2  Enable Functionality, Startup Sequence and Timing
        1. 7.3.2.1 COMP Pin Impedance Sensing
        2. 7.3.2.2 Overcurrent Protection (OCP) Setting
      3. 7.3.3  Soft-Start Time
      4. 7.3.4  Oscillator
      5. 7.3.5  Overcurrent Protection (OCP)
      6. 7.3.6  Switching Node (SW)
      7. 7.3.7  Input Undervoltage Lockout (UVLO)
      8. 7.3.8  Pre-Bias Startup
      9. 7.3.9  Power Good
      10. 7.3.10 Thermal Shutdown
    4. 7.4 Device Functional Modes
  8. 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  Switching Frequency Selection
        2. 8.2.2.2  Inductor Selection (L1)
        3. 8.2.2.3  Output Capacitor Selection
        4. 8.2.2.4  Inductor Peak Current Rating
        5. 8.2.2.5  Input Capacitor Selection
        6. 8.2.2.6  Bootstrap Capacitor (C14)
        7. 8.2.2.7  Bootstrap Resistor (R2)
        8. 8.2.2.8  RC Snubber (R9 and C18)
        9. 8.2.2.9  VDD Bypass Capacitor (C11)
        10. 8.2.2.10 BP5 Bypass Capacitor (C12)
        11. 8.2.2.11 Soft-Start Capacitor (C13)
        12. 8.2.2.12 Current Limit (R1)
        13. 8.2.2.13 Feedback Divider (R4, R7)
        14. 8.2.2.14 Compensation (C15, C16, C17, R3, R6)
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Trademarks
    3. 11.3 Electrostatic Discharge Caution
    4. 11.4 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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8 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.

8.1 Application Information

The TPS56121 is a highly-integrated synchronous step-down DC-DC converter. It is used to convert a higher DC input voltage (4.5 V to 14 V recommended) to a lower DC output voltage (as low as 0.6 V), with a maximum output current of 15 A, for a variety of applications.

8.2 Typical Application

This design example describes a 15-A, 12-V to 1.0-V design using the TPS56121 high-current integrated buck converter. The system specifications are listed in Table 1. Use the following design procedure to select key component values for this device.

TPS56121 deq_schematic_lusah4.gif Figure 20. Design Example Schematic

8.2.1 Design Requirements

Table 1. TPS56121 Design Example Parameters

PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
VIN Input voltage 8 12 14 V
VIN(ripple) Input ripple IOUT = 15 A 0.15 V
VOUT Output voltage 0 A ≤ IOUT ≤ 15 A 0.98 1.00 1.02 V
Line regulation 8 V ≤ VIN ≤ 14 V 0.1%
Load regulation 0 A ≤ IOUT ≤ 15 A 0.5%
VRIPPLE Output ripple IOUT= 15 A 20 mV
VOVER Output overshoot ITRAN = 5 A 50 mV
VUNDER Output undershoot ITRAN = 5 A 50 mV
IOUT Output current 8 V ≤ VIN ≤ 14 V 0 15 A
tSS Soft-start time VIN = 12 V 2.0 ms
IOUT(max) Short- circuit current trip point 20 A
η Efficiency VIN = 12 V, IOUT = 15 A 90%
fSW Switching frequency 500 kHz

8.2.2 Detailed Design Procedure

Table 2. List of Materials for TPS56121 Design Example

REFERENCE
DESiGNATOR		 QTY VALUE DESCRIPTION SIZE PART NUMBER MANUFACTURER
C1, C2, C3, C4 4 22 µF Capacitor, Ceramic, 25 V, X5R, 20% 1210 Std Std
C5, C11 2 1.0 µF Capacitor, Ceramic, 25V, X7R, 20% 0805 Std Std
C6 0 100 µF Capacitor, Aluminum, 16 VDC, ±20% Code D8 EEEFP1C101AP Panasonic
C7, C8, C9, C10, C19 5 100 µF Capacitor, Ceramic, 6.3V, X5R, 20% 1210 Std Std
C12 1 4.7 µF Capacitor, Ceramic, 10 V, X5R, 20% 0805 Std Std
C13 1 33 nF Capacitor, Ceramic, 16 V, X7R, 20% 0603 Std Std
C14 1 100 nF Capacitor, Ceramic, 50 V, X7R, 20% 0603 Std Std
C15 1 2200 pF Capacitor, Ceramic, 50 V, X7R, 10% 0603 Std Std
C16 1 100 pF Capacitor, Ceramic, 50 V, C0G, 5% 0603 Std Std
C17 1 680 pF Capacitor, Ceramic, 50 V, C0G, 5% 0603 Std Std
C18 1 1000 pF Capacitor, Ceramic, 50 V, X7R, 20% 0603 Std Std
C20, C21 0 100 µF Capacitor, Ceramic, 6.3 V, X5R, 20% 1210 Std Std
J1, J2 2 Terminal Block, 4-pin, 15 A, 5.1 mm 0.80 x 0.35 inch ED120/4DS OST
J3 1 Header, Male 2-pin, 100mil spacing 0.100 inch x 2 PEC02SAAN Sullins
L1 1 440 nH Inductor, 440 nH, 30A, 0.32 mΩ 0.530 x 0.510 inch PA0513.441NLT Pulse
R1 1 1.78 kΩ Resistor, Chip, 1/16W, 1% 0603 Std Std
R2 1 5.10 Ω Resistor, Chip, 1/16W, 1% 0603 Std Std
R3 1 7.87 kΩ Resistor, Chip, 1/16W, 1% 0603 Std Std
R4 1 20.5 kΩ Resistor, Chip, 1/16W, 1% 0603 Std Std
R5 1 49.9 Ω Resistor, Chip, 1/16W, 1% 0603 Std Std
R6 1 1.00 kΩ Resistor, Chip, 1/16W, 1% 0603 Std Std
R7 1 30.1 kΩ Resistor, Chip, 1/16W, 1% 0603 Std Std
R8 1 0 Ω Resistor, Chip, 1/16 W, 1% 0603 Std Std
R9 1 1.00 Ω Resistor, Chip, 1/8 W, 1% 0805 Std Std
R10 1 100 kΩ Resistor, Chip, 1/16W, 1% 0603 Std Std
TP1, TP3, TP11 3 Test Point, Red, Thru Hole 0.125 x 0.125 inch 5010 Keystone
TP2, TP4, TP8, TP9, TP12 5 Test Point, Black, Thru Hole 0.125 x 0.125 inch 5011 Keystone
TP5, TP6 2 Test Point, Yellow, Thru Hole 0.125 x 0.125 inch 5014 Keystone
TP7, TP10 2 Test Point, White, Thru Hole 0.125 x 0.125 inch 5012 Keystone
U1 1 4.5-V to 14-V input, 15-A, synchronous buck converter QFN-22
6 × 5 mm		 TPS56121DQP TI

8.2.2.1 Switching Frequency Selection

To achieve a balance between small size and high efficiency for this design, use switching frequency of 500 kHz.

8.2.2.2 Inductor Selection (L1)

Synchronous buck power inductors are typically sized for between approximately 20% and 40% peak-to-peak ripple current (IP-P).

Using this target ripple current, the required inductor size can be calculated as shown in Equation 3.

Equation 3. TPS56121 deq_l_lusah4.gif

Selecting a standard 440-nH inductor value, IP-P = 4.2 A.

The RMS current through the inductor is approximated in Equation 4.

Equation 4. TPS56121 deq_irms_lusah4.gif

8.2.2.3 Output Capacitor Selection

The output transient response typically drives the selection of the output capacitor. For applications where VIN(min) > 2 × VOUT, use overshoot to calculate the minimum output capacitance, as shown in Equation 5.

Equation 5. TPS56121 deq_cout1_lusah4.gif

For applications where VIN(min) < 2 × VOUT, use overshoot to calculate the minimum output capacitance. The equation is shown in Equation 6

Equation 6. TPS56121 deq_cout2_lusah4.gif

In order to meet the low ESR and high capacitance requirements, this design uses five 100-µF, 1210 ceramic capacitors. With a minimum capacitance, maximum ripple voltage determines the maximum allowable ESR. The ESR is approximated in Equation 7.

Equation 7. TPS56121 deq_esrmax_lusah4.gif

8.2.2.4 Inductor Peak Current Rating

With output capacitance, it is possible to calculate the charge current during start-up and determine the minimum saturation current rating for the inductor. Equation 8 approximates the start-up charging current (ICHARGE).

Equation 8. TPS56121 deq_icharge_lusah4.gif

Equation 9 approximates the peak current in the inductor, IL(peak).

Equation 9. TPS56121 deq_ilpeak1_lusah4.gif

With the short circuit current trip point IOUT(max) set at 20 A, the maximum allowable peak current IL_PEAK(max) is shown in Equation 10.

Equation 10. TPS56121 deq_ilpeak2_lusah4.gif

The selection of output capacitor meets the maximum allowable peak current requirement.

Table 3. Inductor Requirements Summary

PARAMETER VALUE UNIT
L Inductance 440 nH
IL(rms) RMS current (thermal rating) 15.1 A
IL_PEAK(max) Peak current (saturation rating) 22.1 A

Thie design uses a PA0513.441NLT, 440-nH, 0.32-mΩ, 30-A inductor.

8.2.2.5 Input Capacitor Selection

The input voltage ripple is divided between capacitance and ESR. For this design VIN_RIPPLE(CAP) = 100 mV and VIN_RIPPLE(ESR) = 50 mV. Use Equation 11 to estimate the minimum capacitance. Use Equation 12 to estimate the maximum ESR.

Equation 11. TPS56121 deq_cinminx_lusah4.gif
Equation 12. TPS56121 deq_esrmax3_lusah4.gif

Equation 13 estimates the RMS current in the input capacitors.

Equation 13. TPS56121 deq_irmscinx_lusah4.gif

Four 1210, 22-µF, 25-V, X5R, ceramic capacitors with approximately 2.5-mΩ ESR and a 2.5-A RMS current rating are selected. Higher voltage capacitors are selected to minimize capacitance loss at the DC bias voltage to ensure the capacitors will have sufficient capacitance at the working voltage while a 1.0-µF capacitor in smaller case size is used to reduce high frequency noise from the MOSFET switching.

8.2.2.6 Bootstrap Capacitor (C14)

The bootstrap capacitor maintains power to the high-side driver during the high-side switch ON time. Per the requirements of the integrated MOSFET, the value of CBOOT is 100 nF with a minimum 10-V rating.

8.2.2.7 Bootstrap Resistor (R2)

The bootstrap resistor slows the rising edge of the SW voltage to reduce ringing and improve EMI. Per the datasheet recommendation a 5.1-Ω resistor is selected.

8.2.2.8 RC Snubber (R9 and C18)

To effectively limit the switch node ringing, select a 1.0-Ω resistor and a 1000-pF capacitor

8.2.2.9 VDD Bypass Capacitor (C11)

Per the data sheet recommended pin terminations, bypass VDD to GND with a 1.0-µF capacitor.

8.2.2.10 BP5 Bypass Capacitor (C12)

Per the data sheet recommended pin functions, bypass BP5 to GND with a capacitor with a value of at least 1.0-µF. For additional filtering and noise immunity, select a 4.7-µF capacitor.

8.2.2.11 Soft-Start Capacitor (C13)

The soft-start capacitor provides a constant ramp voltage to the error amplifier to provide controlled, smooth start-up. The soft-start capacitor is sized using Equation 14.

Equation 14. TPS56121 deq_css_lusah4.gif

8.2.2.12 Current Limit (R1)

The TPS56221 uses the negative drop across the internal low-side FET at the end of the OFF-time to measure the valley of the inductor current. Allowing for a minimum 20-A, or 30% over maximum load, the programming resistor is selected using Equation 15.

Equation 15. TPS56121 deq_rocset_lusah4.gif

Select a standard 1.78-kΩ resistor from the E-48 series.

8.2.2.13 Feedback Divider (R4, R7)

The TPS56121 converter uses a full operational amplifier with an internally fixed 0.600-V reference. R4 is selected between 10 kΩ and 50 kΩ for a balance of feedback current and noise immunity. With R4 set to 20.5 kΩ, program the output voltage with a resistor divider as calculated in Equation 16.

Equation 16. TPS56121 deq_r7_lusah4.gif

Select a standard 30.1-kΩ resistor from the E-48 series.

8.2.2.14 Compensation (C15, C16, C17, R3, R6)

Using the TPS40k Loop Stability Tool for 50 kHz of bandwidth and 60 degrees of phase margin with an R4 value of 20.5 kΩ, the design yields the following values.

  • C17 = C_1 = 680 pF
  • C15 = C_2 = 2200 pF
  • C16 = C_3 = 100 pF
  • R6 = R_2 = 1.00 kΩ
  • R3 = R_3 = 7.87 kΩ

8.2.3 Application Curves

Output voltage 12 V to 1.0 V at 0-A to 15-A input current.

TPS56121 deg_eff.png Figure 21. Efficiency vs Load Current
TPS56121 deq_waveform1_lusah4.gif Figure 23. Output Ripple 20 mV/div, 1.0 µs/div, 20 MHz Bandwidth, AC Coupled
TPS56121 loop_response_lusah4.png Figure 22. Loop Response, 47-kHz Bandwidth,
48° Phase Margin
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