SLUS774C AUGUST   2007  – December 2014

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  Oscillator
      3. 7.3.3  Input Undervoltage Lockout (UVLO) and Startup
      4. 7.3.4  Enable and Timed Turn On of the Outputs
      5. 7.3.5  Output Voltage Sequencing
      6. 7.3.6  Soft-Start
      7. 7.3.7  Output Voltage Regulation
      8. 7.3.8  Feedback Loop and Inductor-Capacitor (L-C) Filter Selection
      9. 7.3.9  Inductor-Capacitor (L-C) Selection
      10. 7.3.10 Maximum Output Capacitance
      11. 7.3.11 Minimum Output Capacitance
      12. 7.3.12 Modifying The Feedback Loop
        1. 7.3.12.1 Using High-ESR Output Capacitors
        2. 7.3.12.2 Using All Ceramic Output Capacitors
      13. 7.3.13 Example: TPS54386 Buck Converter Operating at 12-V Input, 3.3-V Output and 400-mA(P-P) Ripple Current
      14. 7.3.14 Bootstrap for the N-Channel MOSFET
      15. 7.3.15 Light Load Operation
      16. 7.3.16 SW Node Ringing
        1. 7.3.16.1 SW Node Snubber
        2. 7.3.16.2 Bootstrap Resistor
      17. 7.3.17 Output Overload Protection
      18. 7.3.18 Operating Near Maximum Duty Cycle
      19. 7.3.19 Dual Supply Operation
      20. 7.3.20 Cascading Supply Operation
      21. 7.3.21 Multiphase Operation
      22. 7.3.22 Bypass and FIltering
      23. 7.3.23 Overtemperature Protection and Junction Temperature Rise
      24. 7.3.24 Power Derating
    4. 7.4 Device Functional Modes
      1. 7.4.1 Minimum Input Voltage
      2. 7.4.2 ENx Control
  8. 8 Applications and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 12-V to 5-V and 3.3-V Converter
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1  Duty Cycle Estimation
          2. 8.2.1.2.2  Inductor Selection
          3. 8.2.1.2.3  Rectifier Diode Selection
          4. 8.2.1.2.4  Output Capacitor Selection
          5. 8.2.1.2.5  Voltage Setting
          6. 8.2.1.2.6  Compensation Capacitors
          7. 8.2.1.2.7  Input Capacitor Selection
          8. 8.2.1.2.8  Boot Strap Capacitor
          9. 8.2.1.2.9  ILIM
          10. 8.2.1.2.10 SEQ
          11. 8.2.1.2.11 Power Dissipation
        3. 8.2.1.3 Application Curves
      2. 8.2.2 24-V to 12-V and 24-V to 5-V
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
        3. 8.2.2.3 Application Curves
      3. 8.2.3 5-V to 3.3V and 5-V to 1.2 V
        1. 8.2.3.1 Design Requirements
        2. 8.2.3.2 Detailed Design Procedure
        3. 8.2.3.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 PowerPAD Package
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Definition of Symbols
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Related Links
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8 Applications 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 TPS5438x is a dual 28-V, 3-A, step down regulator with an integrated high-side MOSFETs. This device is typically used to convert a higher DC voltage to a lower DC voltage with a maximum available output current of 3 A on each channel. Example applications are: High Density Point-of-Load Regulators for Set-top Box, Digital TV, Power for DSP and other Consumer Electronics.

8.2 Typical Applications

8.2.1 12-V to 5-V and 3.3-V Converter

The following example illustrates a design process and component selection for a 12-V to 5-V and 3.3-V dual non-synchronous buck regulator using the TPS54383 converter.

des_ex_1_lus774.gifFigure 37. Design Example Schematic

8.2.1.1 Design Requirements

PARAMETER NOTES AND CONDITIONS MIN NOM MAX UNIT
INPUT CHARACTERISTICS
VIN Input voltage 6.9 12.0 13.2 V
IIN Input current VIN = nom, IOUT = max 1.6 2.0 A
No load input current VIN = nom, IOUT = 0 A 12 20 mA
OUTPUT CHARACTERISTICS
VOUT1 Output voltage 1 VIN = nom, IOUT = nom 4.8 5.0 5.2 V
VOUT2 Output voltage 2 VIN = nom, IOUT = nom 3.2 3.3 3.4
Line regulation VIN = min to max 1%
Load regulation IOUT = min to max 1%
VOUT(ripple) Output voltage ripple VIN = nom, IOUT = max 50 mVPP
IOUT1 Output current 1 VIN = min to max 0 2.0 A
IOUT2 Output current 2 VIN = min to max 0 2.0
IOCP1 Output overcurrent channel 1 VIN = nom, VOUT = VOUT1 = 5% 2.4 3 3.5
IOCP2 Output overcurrent channel 2 VIN = nom, VOUT = VOUT2 = 5% 2.4 3 3.5
Transient response ΔVOUT from load transient ΔIOUT = 1 A @ 3 A/μs 200 mV
Transient response settling time 1 ms
SYSTEM CHARACTERISTICS
fSW Switching frequency 250 310 370 kHz
η Full load efficiency 85%
TJ Operating temperature range 0 25 60 °C

Table 3. Design Example List of Materials

QTY REFERENCE
DESIGNATOR 		VALUE DESCRIPTION SIZE PART NUMBER MANUFACTURER
1 C1 100 μF Capacitor, Aluminum, 25V, 20% E-can EEEFC1E101P Panasonic
2 C10, C11 10 μF Capacitor, Ceramic, 25V, X5R 20% 1210 C3216X5R1E106M TDK
1 C12 4.7 μF Capacitor, Ceramic, 10V, X5R 20% 0805 Std Std
2 C14, C16 470 pF Capacitor, Ceramic, 25V, X7R, 20% 0603 Std Std
1 C15 6.8 nF Capacitor, Ceramic, 25V, X7R, 20% 0603 Std Std
1 C17, C5 100 μF Capacitor, Aluminum, 10V, 20%, FC Series F-can EEEFC1A101P Panasonic
4 C3, C4, C18, C19 10 μF Capacitor, Ceramic, 6.3V, X5R 20% 0805 C2012X5R0J106M TDK
1 C8 10 nF Capacitor, Ceramic, 25V, X7R, 20% 0603 Std Std
2 C9, C13 0.033 μF Capacitor, Ceramic, 25V, X7R, 20% 0603 Std Std
2 D1, D2 MBRS320 Diode, Schottky, 3-A, 30-V SMC MBRS330T3 On Semi
2 L1, L2 22 μH Inductor, Power, 6.8A, 0.038 Ω 0.484 x 0.484 MSS1278-153ML Coilcraft
2 R2, R9 20 kΩ Resistor, Chip, 1/16W, 1% 0603 Std Std
1 R5 422 Ω Resistor, Chip, 1/16W, 1% 0603 Std Std
2 R6, R10 10 Ω Resistor, Chip, 1/16W, 5% 0603 Std Std
1 R8 698 Ω Resistor, Chip, 1/16W, 1% 0603 Std Std
1 R4 3.83 kΩ Resistor, Chip, 1/16W, 1% 0603 Std Std
1 R7 6.34 kΩ Resistor, Chip, 1/16W, 1% 0603 Std Std
1 U1 TPS54383 DC-DC Switching Converter w/ FET HTSSOP-14 TPS54383PWP TI

8.2.1.2 Detailed Design Procedure

Use the following design procedure to select component values for the TPS5438x.

8.2.1.2.1 Duty Cycle Estimation

The first step is to estimate the duty cycle of each switching FET.

Equation 21. deq_dmax_lus774.gif
Equation 22. deq_dmin_lus774.gif

Using an assumed forward drop of 0.5 V for a schottky rectifier diode, the Channel 1 duty cycle is approximately 40.1% (minimum) to 48.7% (maximum) while the Channel 2 duty cycle is approximately 27.7% (minimum) to 32.2% (maximum).

8.2.1.2.2 Inductor Selection

The peak-to-peak ripple is limited to 30% of the maximum output current. This places the peak current far enough from the minimum overcurrent trip level to ensure reliable operation.

For both Channel 1 and Channel 2, the maximum inductor ripple current is 600 mA. The inductor size is estimated in Equation 23.

Equation 23. deq_lmin_lus774.gif

The inductor values are

  • L1 = 18.3 μH
  • L2 = 15.3 μH

The next higher standard inductor value of 22 μH is used for both inductors.

The resulting ripple currents are :

Equation 24. deq_iripple_lus774.gif

Peak-to-peak ripple currents of 0.498 A and 0.416 A are estimated for Channel 1 and Channel 2 respectively.

The RMS current through an inductor is approximated by Equation 25.

Equation 25. deq_ilrms_lus774.gif

and is approximately 2.0 A for both channels.

The peak inductor current is found using:

Equation 26. deq_ilpeak_lus774.gif

An inductor with a minimum RMS current rating of 2.0 A and minimum saturation current rating of 2.25 A is required. A Coilcraft MSS1278-223ML 22-μH, 6.8-A inductor is selected.

8.2.1.2.3 Rectifier Diode Selection

A schottky diode is selected as a rectifier diode for its low forward voltage drop. Allowing 20% over VIN for ringing on the switch node, the required minimum reverse break-down voltage of the rectifier diode is:

Equation 27. deq_vbr_lus774.gif

The diode must have reverse breakdown voltage greater than 15.8 V, therefore a 20-V device is used.

The average current in the rectifier diode is estimated by Equation 28.

Equation 28. deq_idavg_lus774.gif

For this design, 1.2-A (average) and 2.25 A (peak) is estimated for Channel 1 and 1.5-A (average) and 2.21-A (peak) for Channel 2.

An MBRS320, 20-V, 3-A diode in an SMC package is selected for both channels. This diode has a forward voltage drop of 0.4 V at 2 A.

The power dissipation in the diode is estimated by Equation 29.

Equation 29. deq_pdmax_lus774.gif

For this design, the full load power dissipation is estimated to be 480 mW in D1, and 580 mW in D2.

8.2.1.2.4 Output Capacitor Selection

The TPS54383's internal compensation limits the selection of the output capacitors. From Figure 25, the internal compensation has a double zero resonance at about 3 kHz. The output capacitor is selected by Equation 30.

Equation 30. deq_cout_lus774.gif

Solving for COUT using

  • fRES = 3 kHz
  • L = 22 μH

The resulting is COUT = 128 μF. The output ripple voltage of the converter is composed of the ripple voltage across the output capacitance and the ripple voltage across the ESR of the output capacitor. To find the maximum ESR allowable to meet the output ripple requirements the total ripple is partitioned, and the equation manipulated to find the ESR.

Equation 31. q_esrmax_lus749.gif

Based on 128 μF of capacitance, 300-kHz switching frequency and 50-mV ripple voltage plus rounding up the ripple current to 0.5 A, and the duty cycle to 50%, the capacitive portion of the ripple voltage is 6.5 mV, leaving a maximum allowable ESR of 87 mΩ.

To meet the ripple voltage requirements, a low-cost 100-μF electrolytic capacitor with 400 mΩ ESR (C5, C17) and two 10-μF ceramic capacitors (C3 and C4; and C18 and C19) with 2.5-mΩ ESR are selected. From the datasheets for the ceramic capacitors, the parallel combination provides an impedance of 28 mΩ @ 300 kHz for 14 mV of ripple.

8.2.1.2.5 Voltage Setting

The primary feedback divider resistors (R2, R9) from VOUT to FB should be between 10 kΩ and 50 kΩ to maintain a balance between power dissipation and noise sensitivity. For this design, 20 kΩ is selected.

The lower resistors, R4 and R7 are found using the following equations.

Equation 32. deq_r4_lus774.gif
Equation 33. deq_r7_lus774.gif
  • R2 = R9 = 20 kΩ
  • VFB = 0.80 V
  • R4= 3.80 kΩ (3.83 kΩ standard value is used)
  • R7= 6.40 kΩ (6.34 kΩ standard value is used)

8.2.1.2.6 Compensation Capacitors

Checking the ESR zero of the output capacitors:

Equation 34. q_de_fesrzero_lus774.gif

where

  • C = 100 μF
  • ESR = 400 mΩ
  • ESR(zero) = 3980 Hz

Since the ESR zero of the main output capacitor is less than 20 kHz, an R-C filter is added in parallel with R4 and R7 to compensate for the electrolytic capacitors' ESR and add a zero approximately 40 kHz.

Equation 35. q_de_r5_lus774.gif

where

  • fESR(zero) = 4 kHz
  • fESR(desired) = 40 kHz
  • R4 = 3.83 kΩ
  • R5 = 424Ω (422Ω selected)
  • R7 = 6.34 kΩ
  • R8 = 702Ω (698Ω selected)
Equation 36. q_de_req2_lus774.gif

where

  • R2 = R9 = 20 kΩ
  • REQ1 = 3.63 kΩ
  • REQ2 = 5.51 kΩ
Equation 37. q_de_c8_lus774.gif

where

  • C8 = 10.9 nF (10 nF selected)
  • C15 = 7.22 nF (6800 pF selected)

8.2.1.2.7 Input Capacitor Selection

The TPS54383 datasheet recommends a minimum 10-μF ceramic input capacitor on each PVDD pin. These capacitor must be capable of handling the RMS ripple current of the converter. The RMS current in the input capacitors is estimated by Equation 38.

Equation 38. q_irms_lus749.gif
  • IRMS(CIN) = 0.43 A

One 1210 10-μF, 25 V, X5R ceramic capacitor with 2-mΩ ESR and a 2-A RMS current rating are selected for each PVDD input. Higher voltage capacitors are selected to minimize capacitance loss at the DC bias voltage to ensure the capacitors maintain sufficient capacitance at the working voltage.

8.2.1.2.8 Boot Strap Capacitor

To ensure proper charging of the high-side FET gate and limit the ripple voltage on the boost capacitor, a 33-nF boot strap capacitor is used.

8.2.1.2.9 ILIM

Current limit must be set above the peak inductor current IL(peak). Comparing IL(peak) to the available minimum current limits, ILIM is connected to BP for the highest current limit level.

8.2.1.2.10 SEQ

The SEQ pin is left floating, leaving the enable pins to function independently. If the enable pins are tied together, the two supplies start-up ratiometrically. Alternatively, SEQ could be connected to BP or GND to provide sequential start-up.

8.2.1.2.11 Power Dissipation

The power dissipation in the TPS54383 is composed of FET conduction losses, switching losses and internal regulator losses. The RMS FET current is found using Equation 39.

Equation 39. q_irms_lus749.gif

This results in 1.05-A RMS for Channel 1 and 0.87-A RMS for Channel 2.

Conduction losses are estimated by:

Equation 40. deq_pcon_lus774.gif

Conduction losses of 198 mW and 136 mW are estimated for Channel 1 and Channel 2 respectively.

The switching losses are estimated in Equation 41.

Equation 41. deq_psw_lus774.gif

From the data sheet of the MBRS320, the junction capacitance is 658 pF. Since this is large compared to the output capacitance of the TPS54x8x the FET capacitance is neglected, leaving switching losses of 17 mW for each channel.

The regulator losses are estimated in Equation 42.

Equation 42. deq_preg_lus774.gif

With no external load on BP (IBP=0) the regulator power dissipation is 66 mW.

Total power dissipation in the device is the sum of conduction and switching for both channels plus regulator losses.

The total power dissipation is PDISS=0.198+0.136+0.017+0.017+.066 = 434 mW.

8.2.1.3 Application Curves

The following results are from the TPS54383-001 EVM.

swnode_lus774.gifFigure 38. Switching Node Waveforms
eff_33v_lus774.gifFigure 40. 3.3-V Output Efficiency vs. Load Current
voio_ch2_lus774.gifFigure 42. 3.3-V Output Voltage vs. Load Current
eff_5v_lus774.gifFigure 39. 5.0-V Output Efficiency vs. Load Current
voio_ch1_lus774.gifFigure 41. 5.0-V Output Voltage vs. Load Current
ex1loop_lus774.gifFigure 43. Example 1 Loop Response

8.2.2 24-V to 12-V and 24-V to 5-V

For a higher input voltage, both a snubber and bootstrap resistors are added to reduce ringing on the switch node and a 30 V schottky diode is selected. A higher resistance feedback network is chosen for the 12 V output to reduce the feedback current.

deschem_2_lus774.gifFigure 44. 24-V to 12-V and 24-V to 5-V Using the TPS54383

8.2.2.1 Design Requirements

PARAMETER NOTES AND CONDITIONS MIN NOM MAX UNIT
INPUT CHARACTERISTICS
VIN Input voltage 22 24 26 V
OUTPUT CHARACTERISTICS
VOUT1 Output voltage 1 VIN = nom, IOUT = nom 12.0 V
VOUT2 Output voltage 2 VIN = nom, IOUT = nom 5.0
IOUT1 Output current 1 VIN = min to max 0 2.0 A
IOUT2 Output current 2 VIN = min to max 0 2.0
SYSTEM CHARACTERISTICS
fSW Switching frequency 250 310 370 kHz

8.2.2.2 Detailed Design Procedure

See the previous Detailed Design Procedure.

8.2.2.3 Application Curves

wo_snub_lus749.gifFigure 45. Switch Node Ringing Without Snubber and Boost Resistor
eff_2412x_lus774.gifFigure 47. Efficiency vs. Load Current
w_snub_lus749.gifFigure 46. Switch Node Ringeing With Snubber and Boost Resistor

8.2.3 5-V to 3.3V and 5-V to 1.2 V

For a low input voltage application, the TPS54386 is selected for reduced size and all ceramic output capacitors are used. 22-μF input capacitors are selected to reduce input ripple and lead capacitors are placed in the feedback to boost phase margin.

deschem_3_lus774.gifFigure 48. 5-V to 3.3V and 5-V to 1.2 V

8.2.3.1 Design Requirements

PARAMETER NOTES AND CONDITIONS MIN NOM MAX UNIT
INPUT CHARACTERISTICS
VIN Input voltage 4.75 5 5.25 V
OUTPUT CHARACTERISTICS
VOUT1 Output voltage 1 VIN = nom, IOUT = nom 1,2 V
VOUT2 Output voltage 2 VIN = nom, IOUT = nom 3.3
IOUT1 Output current 1 VIN = min to max 0 3 A
IOUT2 Output current 2 VIN = min to max 0 1
SYSTEM CHARACTERISTICS
fSW Switching frequency 510 630 750 kHz

8.2.3.2 Detailed Design Procedure

See the pervious Detailed Design Procedure and Using All Ceramic Output Capacitors.

8.2.3.3 Application Curves

eff_533_lus774.gifFigure 49. Efficiency vs. Load Current
ex3loop_lus774.gifFigure 50. Example 3 Loop Response