SNVS307G September   2004  – April 2016 LM5010

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 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 Control Circuit Overview
      2. 7.3.2 Start-Up Regulator (VCC)
      3. 7.3.3 Regulation Comparator
      4. 7.3.4 Overvoltage Comparator
      5. 7.3.5 ON-Time Control
      6. 7.3.6 Current Limit
      7. 7.3.7 Soft Start
      8. 7.3.8 N-Channel Buck Switch and Driver
      9. 7.3.9 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown
  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 Component Selection
          1. 8.2.2.1.1  R1 and R2
          2. 8.2.2.1.2  RON, FS
          3. 8.2.2.1.3  L1
          4. 8.2.2.1.4  RCL
          5. 8.2.2.1.5  C2 and R3
          6. 8.2.2.1.6  D1
          7. 8.2.2.1.7  C1
          8. 8.2.2.1.8  C3
          9. 8.2.2.1.9  C4
          10. 8.2.2.1.10 C5
          11. 8.2.2.1.11 C6
        2. 8.2.2.2 Increasing The Current Limit Threshold
        3. 8.2.2.3 Ripple Configuration
      3. 8.2.3 Application Curves
    3. 8.3 Do's and Don'ts
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Community Resources
    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 LM5010 is a non-synchronous buck regulator converter designed to operate over a wide input voltage and output current range. Spreadsheet-based calculator tools, available on the TI product website at Quick-Start Calculator, can be used to design a single output non-synchronous buck converter.

Alternatively, online WEBENCH® software is available to create a complete buck design and generate the bill of materials, estimated efficiency, solution size, and cost of the complete solution.

8.2 Typical Application

The final circuit is shown in Figure 12, and its performance is shown from Figure 14 to Figure 17.

LM5010 20119933.gif Figure 12. LM5010 Example Circuit

8.2.1 Design Requirements

Table 1 lists the operating parameters for Figure 12.

Table 1. Design Parameters

PARAMETER EXAMPLE VALUE
Input voltage 15 V to 75 V
Output voltage 10 V
Load current 150 mA to 1 A
Soft-start time 5 ms

8.2.2 Detailed Design Procedure

The procedure for calculating the external components is illustrated with a design example. Configure the circuit in Figure 12 according to the components listed in Table 2.

Table 2. List of Components for LM5010 Example Circuit

COMPONENT DESCRIPTION VALUE
C1 Ceramic Capacitor 2.2 µF, 100 V
C2 Ceramic Capacitor 15 µF, 25 V
C3 Ceramic Capacitor 0.1 µF, 16 V
C4, C6 Ceramic Capacitor 0.022 µF, 16 V
C5 Ceramic Capacitor 0.1 µF, 100 V
D1 Ultra-fast diode 100 V, 2 A
L1 Inductor 100 µH
R1 Resistor 3 kΩ
R2 Resistor 1 kΩ
R3 Resistor 2.8 Ω
RON Resistor 137 kΩ
U1 Switching regulator

8.2.2.1 Component Selection

8.2.2.1.1 R1 and R2

Calculate the ratio of these resistors with Equation 7.

Equation 7. R1 / R2 = (VOUT / 2.5 V) - 1

R1 and R2 calculates to 3. The resistors should be chosen from standard value resistors in the range of 1 kΩ to 10 kΩ. Values of 3 kΩ for R1, and 1 kΩ for R2 are used.

8.2.2.1.2 RON, FS

RON sets the ON-time, and can be chosen using Equation 2 to set a nominal frequency, or from Equation 5 if the ON-time at a particular VIN is important. A higher frequency generally means a smaller inductor and capacitors (value, size and cost), but higher switching losses. A lower frequency means a higher efficiency, but with larger components. If PC board space is tight, a higher frequency is better. The resulting ON-time and frequency have a ±25% tolerance, rearranging Equation 2 to Equation 8.

Equation 8. LM5010 20119921.gif

The next larger standard value (137 kΩ) is chosen for RON, yielding a nominal frequency of 618 kHz.

8.2.2.1.3 L1

The inductor value is determined based on the load current, ripple current, and the minimum and maximum input voltage (VIN(min), VIN(max)). See Figure 13.

LM5010 20119922.gif Figure 13. Inductor Current

To keep the circuit in continuous conduction mode, the maximum allowed ripple current is twice the minimum load current, or 300 mAP-P. Using this value of ripple current, the inductor (L1) is calculated using Equation 9 and Equation 10.

Equation 9. LM5010 20119923.gif

where

  • FS(min) is the minimum frequency (FS - 25%)
Equation 10. LM5010 20119924.gif

Equation 10 provides the minimum value for inductor L1. When selecting an inductor, use a higher standard value (100 uH). L1 must be rated for the peak current (IPK+) to prevent saturation. The peak current occurs at maximum load current with maximum ripple. The maximum ripple is calculated by rearranging Equation 9 using VIN(max), FS(min), and the minimum inductor value, based on the manufacturer’s tolerance. Assume for Equation 11, Equation 12, and Equation 13 that the inductor’s tolerance is ±20%.

Equation 11. LM5010 20119925.gif
Equation 12. LM5010 20119926.gif
Equation 13. IPK+ = 1 A + 0.234 A / 2 = 1.117 A

8.2.2.1.4 RCL

Since it is obvious that the lower peak of the inductor current waveform does not exceed 1 A at maximum load current (see Figure 13), it is not necessary to increase the current limit threshold. Therefore RCL is not needed for this exercise. For applications where the lower peak exceeds 1 A, see Increasing The Current Limit Threshold.

8.2.2.1.5 C2 and R3

Since the LM5010 requires a minimum of 25 mVP-P of ripple at the FB pin for proper operation, the required ripple at VOUT1 is increased by R1 and R2. This necessary ripple is created by the inductor ripple current acting on C2’s ESR + R3. First, determine the minimum ripple current with Equation 14.

Equation 14. LM5010 20119928.gif

The minimum ESR for C2 is then equal to Equation 15.

Equation 15. LM5010 20119929.gif

If the capacitor used for C2 does not have sufficient ESR, R3 is added in series as shown in Figure 12. C2 should generally be no smaller than 3.3 µF, although that is dependent on the frequency and the allowable ripple amplitude at VOUT1. Experimentation is usually necessary to determine the minimum value for C2, as the nature of the load may require a larger value. A load which creates significant transients requires a larger value for C2 than a non-varying load.

8.2.2.1.6 D1

The important parameters are reverse recovery time and forward voltage drop. The reverse recovery time determines how long the current surge lasts each time the buck switch is turned on. The forward voltage drop is significant in the event the output is short-circuited as it is mainly this diode’s voltage (plus the voltage across the current limit sense resistor) which forces the inductor current to decrease during the OFF-time. For this reason, a higher voltage is better, although that affects efficiency. A reverse recovery time of ≊30 ns, and a forward voltage drop of ≊0.75 V are preferred. The reverse leakage specification is important as that can significantly affect efficiency. Other types of diodes may have a lower forward voltage drop, but may have longer recovery times, or greater reverse leakage. D1 should be rated for the maximum VIN, and for the peak current when in current limit (IPK in Figure 11) which is equal to Equation 16.

Equation 16. IPK = 1.5 A + IOR(max) = 1.734 A

where

  • 1.5 A is the maximum guaranteed current limit threshold
  • the maximum ripple current was previously calculated as 234 mAP-P

This calculation is only valid when RCL is not required.

8.2.2.1.7 C1

Assuming the voltage supply feeding VIN has a source impedance greater than zero, this capacitor limits the ripple voltage at VIN while supplying most of the switch current during the ON-time. At maximum load current, when the buck switch turns on, the current into VIN increases to the lower peak of the output current waveform, ramps up to the peak value, then drops to zero at turnoff. The average current into VIN during this ON-time is the load current. For a worst case calculation, C1 must supply this average load current during the maximum ON-time. The maximum ON-time is calculated using Equation 5, with a 25% tolerance added in Equation 17.

Equation 17. LM5010 20119930.gif

C1 is calculated with Equation 18.

Equation 18. LM5010 20119931.gif

where

  • IO is the load current
  • ΔV is the allowable ripple voltage at VIN (1 V for this example)

TI recommends quality ceramic capacitors with a low ESR for C1. To allow for capacitor tolerances and voltage effects, use a 2.2-µF capacitor.

8.2.2.1.8 C3

The capacitor at the VCC pin provides not only noise filtering and stability, but also prevents false triggering of the VCC UVLO at the buck switch ON and OFF transitions. For this reason, C3 should be no smaller than 0.1 µF, and should be a good quality, low ESR, ceramic capacitor. This capacitor also determines the initial start-up delay (t1 in Figure 7).

8.2.2.1.9 C4

TI recommends a value of 0.022 µF for C4. TI recommends a high-quality ceramic capacitor with low ESR, because C4 supplies the surge current to charge the buck switch gate at turnon. A low ESR also ensures a complete recharge during each OFF-time.

8.2.2.1.10 C5

This capacitor suppresses transients and ringing due to long lead inductance at VIN. TI recommends a low ESR, 0.1-µF ceramic chip capacitor, placed physically close to the LM5010.

8.2.2.1.11 C6

The capacitor at the SS pin determines the soft-start time (that is the time for the reference voltage at the regulation comparator and the output voltage), to reach their final value. Determine the time with Equation 19.

Equation 19. LM5010 20119932.gif

For a 5-ms soft-start time, C6 calculates to 0.022 µF.

8.2.2.2 Increasing The Current Limit Threshold

The current limit threshold is nominally 1.25 A, with a minimum guaranteed value of 1 A. If, at maximum load current, the lower peak of the inductor current (IPK– in Figure 13) exceeds 1 A, resistor RCL must be added between SGND and ISEN to increase the current limit threshold to be equal or exceed that lower peak current. This resistor diverts some of the recirculating current from the internal sense resistor so that a higher current level is needed to switch the internal current limit comparator. Calculate IPK– with Equation 20.

Equation 20. LM5010 20119938.gif

where

  • IO(max) is the maximum load current
  • IOR(min) is the minimum ripple current calculated using Equation 14

RCL is calculated with Equation 21.

Equation 21. LM5010 20119939.gif

where

  • 0.11 Ω is the minimum value of the internal resistance from SGND to ISEN

The next smaller standard value resistor should be used for RCL. With the addition of RCL it is necessary to check the average and peak current values to ensure they do not exceed the LM5010 limits. At maximum load current the average current through the internal sense resistor is calculated with Equation 22.

Equation 22. LM5010 20119940.gif

If IAVE is less than 2 A, no changes are necessary. If it exceeds 2 A, RCL must be reduced. The upper peak of the inductor current (IPK+), at maximum load current, is calculated using Equation 23.

Equation 23. LM5010 20119941.gif

where

If IPK+ exceeds 3.5 A , the inductor value must be increased to reduce the ripple amplitude. This necessitates recalculation of IOR(min), IPK–, and RCL.

When the circuit is in current limit, the upper peak current out of the SW pin is calculated with Equation 24.

Equation 24. LM5010 20119942.gif

The inductor L1 and diode D1 must be rated for this current.

8.2.2.3 Ripple Configuration

The LM5010 uses a constant-ON-time (COT) control scheme where the ON-time is terminated by a one-shot and the OFF-time is terminated by the feedback voltage (VFB) falling below the reference voltage. Therefore, for stable operation, the feedback voltage must decrease monotonically in phase with the inductor current during the OFF-time. Furthermore, this change in feedback voltage (VFB) during OFF-time must be large enough to dominate any noise present at the feedback node.

Table 3 presents three different methods for generating appropriate voltage ripple at the feedback node. Type 1 and type 2 ripple circuits couple the ripple from the output of the converter to the feedback node (FB). The output voltage ripple has two components:

  1. Capacitive ripple caused by the inductor current ripple charging or discharging the output capacitor.
  2. Resistive ripple caused by the inductor current ripple flowing through the ESR of the output capacitor and R3.

Table 3. Ripple Configuration

TYPE 1 TYPE 2 TYPE 3
Lowest cost Reduced ripple Minimum ripple
LM5010 Min_Cost_Ripple_Ckt_LM5160_SNVSA03.gif
LM5010 Cff_Ripple_Ckt_LM5160_SNVSA03.gif
LM5010 Min_Inj_Ripple_Ckt_LM5160_SNVSA03.gif
Equation 25. LM5010 eq22_low_cost_snvsa03.gif
Equation 26. LM5010 eq23_reduce_ripple_snvsa03.gif
Equation 27. LM5010 LM5160_CorrectMinRippleEq.gif

The capacitive ripple is out of phase with the inductor current. As a result, the capacitive ripple does not decrease monotonically during the OFF-time. The resistive ripple is in phase with the inductor current and decreases monotonically during the OFF-time. The resistive ripple must exceed the capacitive ripple at output (VOUT) for stable operation. If this condition is not satisfied, then unstable switching behavior is observed in COT converters with multiple ON-time bursts in close succession followed by a long OFF-time.

The type 3 ripple method uses a ripple injection circuit with RA, CA, and the switch node (SW) voltage to generate a triangular ramp. This triangular ramp is then AC-coupled into the feedback node (FB) using the capacitor CB. This circuit is suited for applications where low output voltage ripple is imperative because this circuit does not use the output voltage ripple. See AN-1481 Controlling Output Ripple and Achieving ESR Independence in Constant ON-Time (COT) Regulator Designs, (SNVA166) for more details on each ripple generation method.

8.2.3 Application Curves

LM5010 20119934.gif Figure 14. Efficiency vs VIN
LM5010 20119936.gif Figure 16. Output Voltage Ripple vs VIN
LM5010 20119935.gif Figure 15. Efficiency vs Load Current and VIN
LM5010 20119937.gif Figure 17. Frequency vs VIN

8.3 Do's and Don'ts

A minimum load current of 1 mA is required to maintain proper operation. If the load current falls below that level, the bootstrap capacitor can discharge during the long OFF-time and the circuit either shuts down or cycles ON and OFF at a low frequency. If the load current is expected to drop below 1 mA in the application, choose the feedback resistors to be low enough in value to provide the minimum required current at nominal VOUT.