JAJSCY9 March   2017 LM25141

PRODUCTION DATA.  

  1. 特長
  2. アプリケーション
  3. 概要
  4. 改訂履歴
  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 Switching Characteristics
    7. 6.7 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  High Voltage Start-up Regulator
      2. 7.3.2  VCC Regulator
      3. 7.3.3  Oscillator
      4. 7.3.4  Synchronization
      5. 7.3.5  Frequency Dithering (Spread Spectrum)
      6. 7.3.6  Enable
      7. 7.3.7  Power Good
      8. 7.3.8  Output Voltage
        1. 7.3.8.1 Minimum Output Voltage Adjustment
      9. 7.3.9  Current Sense
      10. 7.3.10 DCR Current Sensing
      11. 7.3.11 Error Amplifier and PWM Comparator
      12. 7.3.12 Slope Compensation
      13. 7.3.13 Hiccup Mode Current Limiting
      14. 7.3.14 Standby Mode
      15. 7.3.15 Soft-Start
      16. 7.3.16 Diode Emulation
      17. 7.3.17 High and Low Side Drivers
  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 Custom Design With WEBENCH® Tools
      3. 8.2.3 Inductor Calculation
      4. 8.2.4 Current Sense Resistor
      5. 8.2.5 Output Capacitor
      6. 8.2.6 Input Filter
        1. 8.2.6.1 EMI Filter Design
        2. 8.2.6.2 MOSFET Selection
        3. 8.2.6.3 Driver Slew Rate Control
        4. 8.2.6.4 Frequency Dithering
      7. 8.2.7 8.9 Control Loop
        1. 8.2.7.1 Feedback Compensator
      8. 8.2.8 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Layout Procedure
    2. 10.2 Layout Examples
  11. 11デバイスおよびドキュメントのサポート
    1. 11.1 WEBENCH®ツールによるカスタム設計
    2. 11.2 ドキュメントの更新通知を受け取る方法
    3. 11.3 コミュニティ・リソース
    4. 11.4 商標
    5. 11.5 静電気放電に関する注意事項
    6. 11.6 Glossary
  12. 12メカニカル、パッケージ、および注文情報

パッケージ・オプション

メカニカル・データ(パッケージ|ピン)
サーマルパッド・メカニカル・データ
発注情報

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.

Application Information

The LM5141 is a synchronous buck controller used to convert a higher input voltage to a lower output voltage. The following design procedure can be used to select external component values. Alternately, the WEBENCH® software may be used to generate a complete design. The WEBENCH software uses an iterative design procedure and accesses a comprehensive database of components when generating a design. This section presents a simplified design process. In addition to the WEBENCH software the LM5141ADESIGN-CALC.xls quick start Excel calculator is available at www.ti.com.

Typical Application

LM25141 simplified_schematic_snvsaj6.gif Figure 28. Typical Application Schematic

Design Requirements

For this design example, the intended input, output, and performance parameters are shown in Table 2.

Table 3. Design Requirements

DESIGN PARAMETER EXAMPLE VALUE
Input voltage range (Steady State) 8 V to 18 V
VIN maximum (Transient) 42 V
VIN minimum (Cold Crank) 3.8 V
Output voltage 3.3 V
Output current 6 A
Operating frequency 2.2 MHz
Output voltage regulation ±1%
Standby current, one output enabled, no-load < 35 µA
Shutdown Current 10 µA

Detailed Design Procedure

Custom Design With WEBENCH® Tools

Click here to create a custom design using the LM25141 device with the WEBENCH® Power Designer.

  1. Start by entering the input voltage (VIN), output voltage (VOUT), and output current (IOUT) requirements.
  2. Optimize the design for key parameters such as efficiency, footprint, and cost using the optimizer dial.
  3. Compare the generated design with other possible solutions from Texas Instruments.

The WEBENCH Power Designer provides a customized schematic along with a list of materials with real-time pricing and component availability.

In most cases, these actions are available:

  • Run electrical simulations to see important waveforms and circuit performance
  • Run thermal simulations to understand board thermal performance
  • Export customized schematic and layout into popular CAD formats
  • Print PDF reports for the design, and share the design with colleagues

Get more information about WEBENCH tools at www.ti.com/WEBENCH.

  • Buck Inductor value
  • Calculate the peak inductor current
  • Current Sense resistor value
  • Output capacitor value
  • Input filter
  • MOSFET selection
  • Control Loop design

Inductor Calculation

For peak current mode control, sub-harmonic oscillation occurs with a duty cycle greater than 50% and is characterized by alternating wide and narrow pulses at the SW pin. By adding a slope compensating ramp equal to at least one-half the inductor current down-slope, any tendency toward sub-harmonic oscillation is damped within one switching cycle. For design simplification, the LM5141 has an internal slope compensation ramp added to the current sense signal.

For the slope compensation ramp to dampen sub-harmonic oscillation, the inductor value should be calculated based on the following guidelines (equation 15 assumes an inductor ripple current 30%):

Equation 15. LM25141 equation_14_snvsaj6.gif
  • Lower inductor values increase the peak-to-peak inductor current, which minimizes size and cost and improves transient response at the expense of reduced efficiency due to higher peak currents.
  • Higher inductance values decrease the peak-to-peak inductor current, which typically increases efficiency by reducing the RMS current but requires larger output capacitors to meet load-transient specifications.
Equation 16. LM25141 equation_15_snvsaj6.gif
LM25141 equation_15.1_snvsaj6.gif

A standard inductor value of 1.5 µH was selected

Equation 17. LM25141 equation_16_snvsaj6.gif
Equation 18. LM25141 equation_17_snvsaj6.gif

The peak-to-peak inductor current is:

Equation 19. LM25141 equation_18_snvsaj6.gif
Equation 20. LM25141 equation_19_snvsaj6.gif
Equation 21. LM25141 equation_20_snvsaj6.gif
Equation 22. LM25141 equation_21_snvsaj6.gif

Current Sense Resistor

When calculating the current sense resistor, the maximum output current capability (IOUT(MAX)) should be at least 20% higher than the required full load current to account for tolerances, ripple current, and load transients. For this example, 120% of the 6.41 A peak inductor current calculated in the previous section (Ipk) is 7.69 A. The current sense resistor value can be calculated using:

Equation 23. LM25141 equation_22_snvsaj6.gif
Equation 24. LM25141 equation_23_snvsaj6.gif

where

  • V(CS) is the 75 mV current limit threshold

The RSENSE value selected is 9 mΩ

Carefully observe the PCB layout guidelines to ensure that noise and DC errors do not corrupt the differential current sense signals between the CS and VOUT pins. Place the sense resistor close to the devices with short, direct traces, creating Kelvin-sense connections between the current-sense resistor and the LM5141.

The propagation delays through the current limit comparator, logic, and external MOSFET gate drivers allow the peak current to increase above the calculated current limit threshold. For a propagation delay of tdly, the worst case peak current through the inductor with the output shorted can be calculated from:

Equation 25. LM25141 equation_24_snvsaj6.gif

From the Electrical Characterization Table, tdly is typically 40 ns.

Equation 26. LM25141 equation_25_snvsaj6.gif

Once the peak current and the inductance parameters are known, the inductor can be chosen. An inductor with a saturation current greater than IpkSCKT (8.81 Apk) should be selected.

Output Capacitor

In a switch mode power supply, the minimum output capacitance is typically selected based on the capacitor ripple current rating and the load transient requirements. The output capacitor must be large enough to absorb the inductor energy and limit over voltage when transitioning from full-load to no-load, and to limit the output voltage undershoot during no-load to full load transients. The worst-case load transient from zero to full load occurs when the input voltage is at the maximum value and a current switching cycle has just finished. The total output voltage drop ΔVOUT is the sum of the voltage drop while the inductor is ramping up to support the full load and the voltage drop before the next pulse can occur.

The output capacitance required to maintain the minimum output voltage drop (ΔVOUT) can be calculated as follows:

Equation 27. LM25141 equation_26_snvsaj6.gif
Equation 28. LM25141 equation_27_snvsaj6.gif

where

  • ISTEP = 4 A
  • ΔVOUT = 1% of 3.3 V, or 33 mV

For this example a total of 211 μF of capacitance is used, two 82-μF aluminum capacitors for energy storage and one 47 μF low ESR ceramic capacitor to reduce high frequency noise.

Generally, when sufficient capacitance is used to satisfy the undershoot requirement, the overshoot during a full-load to no-load transient will also be satisfactory. After the output capacitance has been selected, calculate the output ripple current and verify that the ripple current is within the capacitor ripple current ratings.

Equation 29. LM25141 equation_28_snvsaj6.gif
Equation 30. LM25141 equation_29_snvsaj6.gif

Input Filter

A power supply input typically has a relatively high source impedance at the switching frequency. Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current during the buck switch on-time. When the buck switch turns on, the current drawn from the input capacitor steps from zero to the valley of the inductor current waveform, then ramps up to the peak value, and then drops to the zero at turn-off.

Average input current can be calculated from the total input power required to support the load at VOUT:

Equation 31. LM25141 equation_30_snvsaj6.gif

The efficiency (η) is assumed to be 83% for this design example, yielding a total input power:

Equation 32. LM25141 equation_31_snvsaj6.gif
Equation 33. LM25141 equation_32_snvsaj6.gif
Equation 34. LM25141 equation_33_snvsaj6.gif

The input capacitors should be selected with sufficient RMS current rating and the maximum voltage rating.

Equation 35. LM25141 equation_34_snvsaj6.gif
Equation 36. LM25141 equation_35_snvsaj6.gif

EMI Filter Design

EMI Filter Design Steps:

  • Calculate the required attenuation
  • Capacitor CIN represents the existing capacitor at the input of the switching converter (10 µF was used for this application)
  • Inductor LF is usually selected between 1 μH and 10 μH (1.8 µH was used for this application), but can be smaller to reduce losses in a high current design
  • Calculate capacitor CF
LM25141 input_emi_filter_snvsaj6.gif Figure 29. Input EMI Filter

By calculating the first harmonic current from the Fourier series of the input current waveform and multiplying it by the input impedance (the impedance is defined by the existing input capacitor CIN), a formula can be derived to obtain the required attenuation:

Equation 37. LM25141 equation_36_snvsaj6.gif
Equation 38. LM25141 equation_37_snvsaj6.gif

VMAX is the allowed dBμV noise level for the particular EMI standard. CIN is the existing input capacitors of the Buck converter, for this application 10 µF was selected. DMAX is the maximum duty cycle, Ipk is the inductor current, the current at the input can be modeled as a square wave, FSW is the switching frequency.

Equation 39. LM25141 equation_38_snvsaj6.gif
Equation 40. LM25141 equation_39_snvsaj6.gif

For this application, CF was chosen to be 1 μF. Adding an input filter to a switching regulator modifies the control-to output transfer function. The output impedance of the filter must be sufficiently small such that the input filter does not significantly affect the loop gain of the buck converter. The impedance of the filter peaks at the filter resonant frequency.

Equation 41. LM25141 equation_40_snvsaj6.gif
Equation 42. LM25141 equation_41_snvsaj6.gif

Referring to Figure 29, the purpose of RD is to reduce the peak output impedance of the filter at the cutoff frequency. The capacitor CD blocks the dc component of the input voltage, and avoids excessive power dissipation on RD. The capacitor CD should have lower impedance than RD at the resonant frequency, with a capacitance value greater than 5 times the filter capacitor CIN. This will prevent it from interfering with the cutoff frequency of the main filter. Added damping is needed when the output impedance is high at the resonant frequency (Q) of filter formed by CIN and LF is too high):

An electrolytic cap CD can be used as damping device, with value:

Equation 43. LM25141 equation_42_snvsaj6.gif

For this design CD = 47 µF was selected

Equation 44. LM25141 equation_43_snvsaj6.gif

MOSFET Selection

The LM5141 gate drivers are powered by the internal 5-V VCC bias regulator. To reduce power dissipation in the controller and improve efficiency, the VCCX pin should be connected to the 5-V output or an external 5 V bias supply. The MOSFETs used with the LM5141 require a logic-level gate threshold with RDS(ON) specified with VGS = 4.5 V or lower.

The MOSFETs must be chosen with a VDS rating to withstand the maximum VIN voltage plus supply voltage transients and spikes (ringing). In addition, the N-channel MOSFETs must be capable of delivering the load current plus peak ripple current during switching.

The high-side MOSFET losses are associated with the RDS(ON) of the MOSFET and the switching losses.

Equation 45. LM25141 equation_44_snvsaj6.gif
Equation 46. LM25141 equation_45_snvsaj6.gif

where

  • tr = ts = 17 ns

The losses in the low side MOSFET include: RDS(ON) losses, dead time losses, and losses in the MOSFETs internal body diode. The body diode conducts the inductor current during the dead time before the rising edge of the switch node; minority carriers are injected into and stored in the diode PN junction when forward biased. As the high side FET starts to turn-on, a negative current must first flow through the diode to remove the stored charge before the diode can block a reverse voltage. During this time, the high side drain-source voltage remains at VIN until all the diode minority carriers are removed. Then, the diode begins to block negative voltage and the reverse current continues to flow to charge the body diode depletion capacitance. The total charge involved in this period is called reverse-recovery charge Qrr.

Equation 47. LM25141 equation_46_snvsaj6.gif
Equation 48. LM25141 equation_47_snvsaj6.gif

where

  • tdr and tdf are the switch node voltage rise and fall times (20 ns)
  • VD(FET) is the forward voltage drop across the low-side MOSFET internal body diode (0.8 V)
  • DQrr is the internal body diodes reverse recovery charge (105 nC)
  • RDS(ON) is the on resistance of the MOSFETs (26 mΩ at TJ = 125ºC)

Table 4 provides parameters for several MOSFETs that have tested in the LM5141 evaluation module.

Table 4. EVM MOSFETs

Manufacture Part Number VDS (V) ID (A) Qg(MAX)
(nC)
VGS = 4.5 V
RDS(ON)
VGS = 4.5 V (Ω)
COSS(MAX) (pF) Application
VISHAY SQJ850EP 60 24 30 32 215 Automotive High Power
VISHAY SQ7414EN 60 5.6 25 36 175 Automotive Low Power
Texas Instruments CSD18534Q5A 60 13 11.1 12.4 217 Industrial

Driver Slew Rate Control

Figure 30 shows the high current driver outputs with independent source and current sink pins for slew rate control. Slew rate control enables the user to adjust the switch node rise and fall times which can reduce the conducted EMI in the FM radio band (30 MHz to 108 MHz). Figure 31 shows the measured results without slew rate control.

The conducted EMI results with slew rate control are shown in Figure 32, a 10-dB reduction in conduction emissions in the FM band is attained by using slew rate control. This can help reduce the size and cost of the EMI filters.

LM25141 drivers_snvsaj6.gif Figure 30. Drivers with Slew Rate Control
LM25141 emi_measurements_cispr_25_class_5_without_slew_rate_control_snvsaj6.png Figure 31. Conducted EMI Measurement, Without Slew Rate Control
LM25141 emi_measurements_cispr_25_witt_slew_rate_control_snvsaj6.png Figure 32. Conducted EMI Measurements, With Slew Rate Control

Frequency Dithering

Figure 33 shows the conducted emission test run on the LM5141EVM, without the Dither feature enabled. The first harmonic (peak measurement) is 48 dBµV, Figure 34 shows the conducted emissions test results with the Dither feature enabled. With the Dither featured enabled, the first harmonic (peak measurement) was lowered to 40 dBµV, an 8 dB reduction.

LM25141 cispr_25_class_5_conducted_emi_without_dither_snvsaj6.png Figure 33. Measured Conducted EMI, Without Dither
LM25141 cispr_25_class_5_with_dither_snvsaj6.png Figure 34. Measured Conducted EMI, With Dither

8.9 Control Loop

The open loop gain is defined as the product of modulator transfer function and feedback transfer function. When plotted on a dB scale, the open loop gain is shown as the sum of modulator gain and feedback gain.

DC modulator gain is:

Equation 49. LM25141 equation_48_snvsaj6.gif

The modulator gain plus power stage transfer function with an embedded current loop is show in Equation 50. The equation includes the sample gain at FSW /2 n), which is caused by sampling effect of current mode control.

Equation 50. LM25141 equation_49_snvsaj6.gif

where

  • s = 2 × π × FSW
  • LM25141 equation_50_snvsaj6.gif
  • LM25141 equation_51_snvsaj6.gif
  • LM25141 equation_52_snvsaj6.gif

    LM25141 equation_53_snvsaj6.gif
  • K = 1
  • GCS is the current sense amplifier gain which is 12

Because the loop cross over frequency is well below sample gain effects, Equation 50 can be simplified as one pole and a one zero system as shown in Equation 51.

Equation 51. LM25141 equation_54_snvsaj6.gif

RLOAD is the load resistance

RDCR is the dc resistance on the output inductor which is 8.1 mΩ

RSENSE is the current sense resistance which is 9 mΩ

Feedback Compensator

A type II compensator using an transconductance error amplifier (EA), Gm, is shown in Figure 35. The dominant pole of the EA open-loop gain is set by the EA output resistance, RAMP, and effective bandwidth-limiting capacitance, CO, as follows:

Equation 52. LM25141 equation_55_snvsaj6.gif

The EA high frequency pole is neglected in the above expression. The compensator transfer function from output voltage to COMP, including the gain contribution from the feedback resistor divider network is:

Equation 53. LM25141 equation_56_snvsaj6.gif
Equation 54. LM25141 equation_57_snvsaj6.gif

where

Equation 55. LM25141 equation_58_snvsaj6.gif

Which simplifies to:

Equation 56. LM25141 equation_59_snvsau1.gif
LM25141 transconductance_amplifier_snvsaj6.gif Figure 35. Transconductance Amplifier
Equation 57. LM25141 equation_60_snvsaj6.gif
Equation 58. LM25141 equation_61_snvsaj6.gif
Equation 59. LM25141 equation_62_snvsaj6.gif

Typically RCOMP << RAMP and CCOMP >> (CHF + CO) so the approximations are valid.

where

VREF is the feedback voltage reference (1.2 V)

Gm is the error amplifier gain transconductance (1200 µS)

RAMP is the error amplifier output impedance (2.5 MΩ)

The error amplifier compensation components create a pole at the origin, a zero, and a high frequency pole.

The procedure for choosing compensation components for a stable closed loop is:

  • Select the desired open loop gain crossover frequency (fc); for this application 30 kHz was chosen
  • Calculate the RCOMP resistor for the gain crossover frequency at 30 kHz
  • Equation 60. LM25141 equation_63_snvsaj6.gif
    Equation 61. LM25141 equation_64_snvsaj6.gif

    The value selected for RCOMP is 22.6 kΩ.

    where

    RDCR = 0.0081 Ω

  • Calculate the CCOMP capacitor value to create a zero that cancels the pole ωp ( ωp = 1/RLOAD × COUT)
  • Equation 62. LM25141 equation_65_snvsaj6.gif
    Equation 63. LM25141 equation_66_snvsaj6.gif

    The value selected for CCOMP is 10nF.

Application Curves

The Bode Plots of the modulator and plus power stage are shown in refer to Figure 36. The results of the total loop gain crossover frequency are 40 kHz with 112º of phase margin, (see Figure 37).

LM25141 gain_and_phase_modulator_vout_vc_snvsaj6.png Figure 36. (VOUT/VC) Modulator Gain and Phase
LM25141 gain_and_phase_loop_snvsaj6.png Figure 37. Loop Gain and Phase