SNVS602L March   2009  – June 2016 LM3409 , LM3409-Q1 , LM3409HV , LM3409HV-Q1

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Buck Current Regulators
      2. 8.3.2 Controlled Off-Time (COFT) Architecture
        1. 8.3.2.1 Adjustable Peak Current Control
        2. 8.3.2.2 Controlled Off-Time
      3. 8.3.3 Average LED Current
      4. 8.3.4 Inductor Current Ripple
      5. 8.3.5 Switching Frequency
      6. 8.3.6 PWM Dimming Using the EN Pin
      7. 8.3.7 High Voltage Negative BIAS Regulator
      8. 8.3.8 External Parallel FET PWM Dimming
    4. 8.4 Device Functional Modes
      1. 8.4.1 Low-Power Shutdown
      2. 8.4.2 Thermal Shutdown
  9. Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Input Undervoltage Lockout (UVLO)
      2. 9.1.2 Operation Near Dropout
      3. 9.1.3 LED Ripple Current
      4. 9.1.4 Buck Converters without Output Capacitors
      5. 9.1.5 Buck Converters With Output Capacitors
      6. 9.1.6 Output Overvoltage Protection
      7. 9.1.7 Input Capacitors
      8. 9.1.8 P-Channel MOSFET (PFET)
      9. 9.1.9 Re-Circulating Diode
    2. 9.2 Typical Applications
      1. 9.2.1 EN PIN PWM Dimming Application for 10 LEDs
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
          1. 9.2.1.2.1  Nominal Switching Frequency
          2. 9.2.1.2.2  Inductor Ripple Current
          3. 9.2.1.2.3  Average LED Current
          4. 9.2.1.2.4  Output Capacitance
          5. 9.2.1.2.5  Input Capacitance
          6. 9.2.1.2.6  PFET
          7. 9.2.1.2.7  Diode
          8. 9.2.1.2.8  Input UVLO
          9. 9.2.1.2.9  IADJ Connection Method
          10. 9.2.1.2.10 PWM Dimming Method
        3. 9.2.1.3 Application Curve
      2. 9.2.2 Analog Dimming Application for 4 LEDs
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
          1. 9.2.2.2.1  Nominal Switching Frequency
          2. 9.2.2.2.2  Inductor Ripple Current
          3. 9.2.2.2.3  Average LED Current
          4. 9.2.2.2.4  Output Capacitance
          5. 9.2.2.2.5  Input Capacitance
          6. 9.2.2.2.6  PFET
          7. 9.2.2.2.7  Diode
          8. 9.2.2.2.8  Input UVLO
          9. 9.2.2.2.9  IADJ Connection Method
          10. 9.2.2.2.10 PWM Dimming Method
        3. 9.2.2.3 Application Curve
      3. 9.2.3 LM3409 Buck Converter Application
        1. 9.2.3.1 Design Requirements
        2. 9.2.3.2 Detailed Design Procedure
          1. 9.2.3.2.1  Nominal Switching Frequency
          2. 9.2.3.2.2  Inductor Ripple Current
          3. 9.2.3.2.3  Average LED Current
          4. 9.2.3.2.4  Output Capacitance
          5. 9.2.3.2.5  Input Capacitance
          6. 9.2.3.2.6  PFET
          7. 9.2.3.2.7  Diode
          8. 9.2.3.2.8  Input UVLO
          9. 9.2.3.2.9  IADJ Connection Method
          10. 9.2.3.2.10 PWM Dimming Method
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Related Links
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

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

9 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.

9.1 Application Information

9.1.1 Input Undervoltage Lockout (UVLO)

Undervoltage lockout is set with a resistor divider from VIN to GND and is compared against a 1.24V threshold as shown in Figure 28. Once the input voltage is above the preset UVLO rising threshold (and assuming the part is enabled), the internal circuitry becomes active and a 22µA current source at the UVLO pin is turned on. This extra current provides hysteresis to create a lower UVLO falling threshold. The resistor divider is chosen to set both the UVLO rising and falling thresholds.

LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085613.gif Figure 28. UVLO Circuit

The turn-on threshold (VTURN-ON) is defined as follows:

Equation 17. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856c3.gif

The hysteresis (VHYS) is defined as follows:

Equation 18. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856c4.gif

9.1.2 Operation Near Dropout

Because the power MOSFET is a PFET, the LM3409/09HV can be operated into dropout which occurs when the input voltage is approximately equal to output voltage. Once the input voltage drops below the nominal output voltage, the switch remains constantly on (D=1) causing the output voltage to decrease with the input voltage. In normal operation, the average LED current is regulated to the peak current threshold minus half of the ripple. As the converter goes into dropout, the LED current is exactly at the peak current threshold because it is no longer switching. This causes the LED current to increase by half of the set ripple current as it makes the transition into dropout. Therefore, the inductor current ripple should be kept as small as possible (while remaining above the previously established minimum) and output capacitance should be added to help maintain good line regulation when approaching dropout.

9.1.3 LED Ripple Current

Selection of the ripple current through the LED array is analogous to the selection of output ripple voltage in a standard voltage regulator. Where the output voltage ripple in a voltage regulator is commonly ±1% to ±5% of the DC output voltage, LED manufacturers generally recommend values for ΔiLED-PP ranging from ±5% to ±20% of ILED. For a nominal system operating point, a larger ΔiLED-PP specification can reduce the necessary inductor size and/or allow for smaller output capacitors (or no output capacitors at all) which helps to minimize the total solution size and cost. On the other hand, a smaller ΔiLED-PP specification would require more output inductance, a higher switching frequency, or additional output capacitance.

9.1.4 Buck Converters without Output Capacitors

Because current is being regulated, not voltage, a buck current regulator is free of load current transients, therefore output capacitance is not needed to supply the load and maintain output voltage. This is very helpful when high frequency PWM dimming the LED load. When no output capacitor is used, the same design equations that govern ΔiL-PP also apply to ΔiLED-PP.

9.1.5 Buck Converters With Output Capacitors

A capacitor placed in parallel with the LED load can be used to reduce ΔiLED-PP while keeping the same average current through both the inductor and the LED array. With an output capacitor, the inductance can be lowered, making the magnetics smaller and less expensive. Alternatively, the circuit can be run at lower frequency with the same inductor value, improving the efficiency and increasing the maximum allowable average output voltage. A parallel output capacitor is also useful in applications where the inductor or input voltage tolerance is poor. Adding a capacitor that reduces ΔiLED-PP to well below the target provides headroom for changes in inductance or VIN that might otherwise push the maximum ΔiLED-PP too high.

LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856f2.png Figure 29. Calculating Dynamic Resistance rD

Output capacitance (CO) is determined knowing the desired ΔiLED-PP and the LED dynamic resistance (rD). rD can be calculated as the slope of the LED’s exponential DC characteristic at the nominal operating point as shown in Figure 29. Simply dividing the forward voltage by the forward current at the nominal operating point will give an incorrect value that is 5x to 10x too high. Total dynamic resistance for a string of n LEDs connected in series can be calculated as the rD of one device multiplied by n. The following equations can then be used to estimate ΔiLED-PP when using a parallel capacitor:

Equation 19. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085642.gif
Equation 20. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085663.gif

In general, ZC should be at least half of rD to effectively reduce the ripple. Ceramic capacitors are the best choice for the output capacitors due to their high ripple current rating, low ESR, low cost, and small size compared to other types. When selecting a ceramic capacitor, special attention must be paid to the operating conditions of the application. Ceramic capacitors can lose one-half or more of their capacitance at their rated DC voltage bias and also lose capacitance with extremes in temperature. Make sure to check any recommended de-ratings and also verify if there is any significant change in capacitance at the operating voltage and temperature.

9.1.6 Output Overvoltage Protection

Because the LM3409/09HV controls a buck current regulator, there is no inherent need to provide output overvoltage protection. If the LED load is opened, the output voltage will only rise as high as the input voltage plus any ringing due to the parasitic inductance and capacitance present at the output node. If a ceramic output capacitor is used in the application, it should have a minimum rating equal to the input voltage. Ringing seen at the output node should not damage most ceramic capacitors, due to their high ripple current rating.

9.1.7 Input Capacitors

Input capacitors are selected using requirements for minimum capacitance and RMS ripple current. The PFET current during tON is approximately ILED, therefore the input capacitors discharge the difference between ILED and the average input current (IIN) during tON. During tOFF, the input voltage source charges up the input capacitors with IIN. The minimum input capacitance (CIN-MIN) is selected using the maximum input voltage ripple (ΔvIN-MAX) which can be tolerated. ΔvIN-MAX is equal to the change in voltage across CIN during tON when it supplies the load current. A good starting point for selection of CIN is to use ΔvIN-MAX of 2% to 10% of VIN. CIN-MIN can be selected as follows:

Equation 21. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085664.gif

An input capacitance at least 75% greater than the calculated CIN-MIN value is recommended. To determine the RMS input current rating (IIN-RMS) the following approximation can be used:

Equation 22. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085665.gif

Because this approximation assumes there is no inductor ripple current, the value should be increased by 10-30% depending on the amount of ripple that is expected. Ceramic capacitors are the best choice for input capacitors for the same reasons mentioned in the Buck Converters With Output Capacitors section. Careful selection of the capacitor requires checking capacitance ratings at the nominal operating voltage and temperature.

9.1.8 P-Channel MOSFET (PFET)

The LM3409/09HV requires an external PFET (Q1) as the main power MOSFET for the switching regulator. Q1 should have a voltage rating at least 15% higher than the maximum input voltage to ensure safe operation during the ringing of the switch node. In practice all switching converters have some ringing at the switch node due to the diode parasitic capacitance and the lead inductance. The PFET should also have a current rating at least 10% higher than the average transistor current (IT):

Equation 23. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085666.gif

The power rating is verified by calculating the power loss (PT) using the RMS transistor current (IT-RMS) and the PFET on-resistance (RDS-ON):

Equation 24. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856c5.gif
Equation 25. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856c6.gif

It is important to consider the gate charge of Q1. As the input voltage increases from a nominal voltage to its maximum input voltage, the COFT architecture will naturally increase the switching frequency. The dominant switching losses are determined by input voltage, switching frequency, and PFET total gate charge (Qg). The LM3409/09HV must provide and remove charge Qg from the input capacitance of Q1 to turn it on and off. This occurs more often at higher switching frequencies which requires more current from the internal regulator, thereby increasing internal power dissipation and eventually causing the LM3409/09HV to thermally cycle. For a given range of operating points the only effective way to reduce these switching losses is to minimize Qg.

A good rule of thumb is to limit Qg < 30nC (if the switching frequency remains below 300kHz for the entire operating range then a larger Qg can be considered). If a PFET with small RDS-ON and a high voltage rating is required, there may be no choice but to use a PFET with Qg > 30nC.

When using a PFET with Qg > 30nC, the bypass capacitor (CF) should not be connected to the VIN pin. This will ensure that peak current detection through RSNS is not affected by the charging of the PFET input capacitance during switching, which can cause false triggering of the peak detection comparator. Instead, CF should be connected from the VCC pin to the CSN pin which will cause a small DC offset in VCST and ultimately ILED, however it avoids the problematic false triggering.

In general, the PFET should be chosen to meet the Qg specification whenever possible, while minimizing RDS-ON. This will minimize power losses while ensuring the part functions correctly over the full operating range.

9.1.9 Re-Circulating Diode

A re-circulating diode (D1) is required to carry the inductor current during tOFF. The most efficient choice for D1 is a Schottky diode due to low forward voltage drop and near-zero reverse recovery time. Similar to Q1, D1 must have a voltage rating at least 15% higher than the maximum input voltage to ensure safe operation during the ringing of the switch node and a current rating at least 10% higher than the average diode current (ID):

Equation 26. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085667.gif

The power rating is verified by calculating the power loss through the diode. This is accomplished by checking the typical diode forward voltage (VD) from the I-V curve on the product data sheet and calculating as follows:

Equation 27. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856c8.gif

In general, higher current diodes have a lower VD and come in better performing packages minimizing both power losses and temperature rise.

9.2 Typical Applications

9.2.1 EN PIN PWM Dimming Application for 10 LEDs

LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085618.gif Figure 30. EN PIN PWM Dimming Application for 10 LEDs Schematic

9.2.1.1 Design Requirements

fSW = 525 kHz

VIN = 48 V; VIN-MAX = 75 V

VO = 35 V

ILED = 2 A

ΔiLED-PP = ΔiL-PP = 1 A

ΔvIN-PP = 1.44 V

VTURN-ON = 10 V; VHYS = 1.1 V

η = 0.95

9.2.1.2 Detailed Design Procedure

Table 1. Design 1 Bill of Materials

QTY PART ID PART VALUE MANUFACTURER PART NUMBER
1 LM3409HV/LM3409QHV Buck controller TI LM3409HVMY/LM3409QHVMY
2 CIN1, CIN2 2 µF X7R 10% 100 V MURATA GRM43ER72A225KA01L
1 CF 1 µF X7R 10% 16 V TDK C1608X7R1C105K
1 COFF 470 pF X7R 10% 50 V TDK C1608X7R1H471K
1 Q1 PMOS 100 V 3.8 A ZETEX ZXMP10A18KTC
1 D1 Schottky 100 V 3 A VISHAY SS3H10-E3/57T
1 L1 15 µH 20% 4.2 A TDK SLF12565T-150M4R2
1 ROFF 24.9 kΩ 1% VISHAY CRCW060324K9FKEA
1 RUV1 6.98 kΩ 1% VISHAY CRCW06036K98FKEA
1 RUV2 49.9 kΩ 1% VISHAY CRCW060349K9FKEA
1 RSNS 0.1 Ω 1% 1W VISHAY WSL2512R1000FEA

9.2.1.2.1 Nominal Switching Frequency

Assume COFF = 470 pF and η = 0.95. Solve for ROFF:

Equation 28. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085684.gif

The closest 1% tolerance resistor is 24.9 kΩ; therefore, the actual tOFF and target fSW are:

Equation 29. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085685.gif
Equation 30. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085686.gif

The chosen components from step 1 are:

Equation 31. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085687.gif

9.2.1.2.2 Inductor Ripple Current

Solve for L1:

Equation 32. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085688.gif

The closest standard inductor value is 15 µH therefore the actual ΔiL-PP is:

Equation 33. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085689.gif

The chosen component from step 2 is:

Equation 34. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085690.gif

9.2.1.2.3 Average LED Current

Determine IL-MAX:

Equation 35. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085691.gif

Assume VADJ = 1.24 V and solve for RSNS:

Equation 36. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085692.gif

The closest 1% tolerance resistor is 0.1 Ω therefore the ILED is:

Equation 37. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085693.gif

The chosen component from step 3 is:

Equation 38. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085694.gif

9.2.1.2.4 Output Capacitance

No output capacitance is necessary.

9.2.1.2.5 Input Capacitance

Determine tON:

Equation 39. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085695.gif

Solve for CIN-MIN:

Equation 40. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085696.gif

Choose CIN:

Equation 41. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085697.gif

Determine IIN-RMS:

Equation 42. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085698.gif

The chosen components from step 5 are:

Equation 43. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085699.gif

9.2.1.2.6 PFET

Determine minimum Q1 voltage rating and current rating:

Equation 44. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856d0.gif
Equation 45. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856d1.gif

A 100 V, 3.8 A PFET is chosen with RDS-ON = 19 0mΩ and Qg = 20 nC. Determine IT-RMS and PT:

Equation 46. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856d3.gif
Equation 47. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856d4.gif

The chosen component from step 6 is:

Equation 48. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085609.gif

9.2.1.2.7 Diode

Determine minimum D1 voltage rating and current rating:

Equation 49. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856d6.gif
Equation 50. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856d7.gif

A 100-V, 3-A diode is chosen with VD = 750 mV. Determine PD:

Equation 51. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856d9.gif

The chosen component from step 7 is:

Equation 52. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085620.gif

9.2.1.2.8 Input UVLO

Solve for RUV2:

Equation 53. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085621.gif

The closest 1% tolerance resistor is 49.9 kΩ therefore VHYS is:

Equation 54. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085632.gif

Solve for RUV1:

Equation 55. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085633.gif

The closest 1% tolerance resistor is 6.98 kΩ therefore VTURN-ON is:

Equation 56. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085634.gif

The chosen components from step 8 are:

Equation 57. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085643.gif

9.2.1.2.9 IADJ Connection Method

The IADJ pin is left open forcing VADJ = 1.24 V.

9.2.1.2.10 PWM Dimming Method

PWM dimming signal pair is applied to the EN pin and GND at fDIM = 1 kHz.

9.2.1.3 Application Curve

Figure 31 shows the LED current versus EN pin PWM duty cycle for the application.

LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 D001_SNVS602.gif
Black = 200 Hz Red = 1 kHz Gray = 20 kHz
Figure 31. EN Pin PWM Dimming

9.2.2 Analog Dimming Application for 4 LEDs

LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085619.gif

9.2.2.1 Design Requirements

fSW = 500 kHz

VIN = 24 V; VIN-MAX = 42 V

VO = 14 V

ILED = 1 A

ΔiL-PP = 450 mA; ΔiLED-PP = 50 mA

ΔvIN-PP = 1 V

VTURN-ON = 10 V; VHYS = 1.1 V

η = 0.90

9.2.2.2 Detailed Design Procedure

Table 2. Design 2 Bill of Materials

QTY PART ID PART VALUE MANUFACTURER PART NUMBER
1 LM3409/LM3409Q Buck controller TI LM3409MY/LM3409QMY
2 CIN1 4.7-µF X7R 10% 50 V MURATA GRM55ER71H475MA01L
1 CF 1-µF X7R 10% 16 V TDK C1608X7R1C105K
1 CF2 0.1-µF X7R 10% 16 V TDK C1608X7R1C104K
1 COFF 470-pF X7R 10% 50 V TDK C1608X7R1H471K
1 CO 2.2-µF X7R 10% 50 V MURATA GRM43ER71H225MA01L
1 Q1 PMOS 70 V 5.7 A ZETEX ZXMP7A17KTC
1 D1 Schottky 60 V 5 A COMCHIP CDBC560-G
1 L1 22 µH 20% 4.2 A TDK SLF12575T-220M4R0
1 RF2 1 kΩ 1% VISHAY CRCW06031K00FKEA
1 ROFF 15.4 kΩ 1% VISHAY CRCW060315K4FKEA
1 RUV1 6.98 kΩ 1% VISHAY CRCW06036K98FKEA
1 RUV2 49.9 kΩ 1% VISHAY CRCW060349K9FKEA
1 RSNS 0.2 Ω 1% 1W VISHAY WSL2512R2000FEA

9.2.2.2.1 Nominal Switching Frequency

Assume COFF = 470 pF and η = 0.90. Solve for ROFF:

Equation 58. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085644.gif

The closest 1% tolerance resistor is 15.4 kΩ; therefore, the actual tOFF and target fSW are:

Equation 59. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085645.gif
Equation 60. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085646.gif

The chosen components from step 1 are:

Equation 61. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085647.gif

9.2.2.2.2 Inductor Ripple Current

Solve for L1:

Equation 62. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085648.gif

The closest standard inductor value is 22 µH; therefore, the actual ΔiL-PP is:

Equation 63. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085649.gif

The chosen component from step 2 is:

Equation 64. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085650.gif

9.2.2.2.3 Average LED Current

Determine IL-MAX:

Equation 65. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085651.gif

Assume VADJ = 1.24 V and solve for RSNS:

Equation 66. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085652.gif

The closest 1% tolerance resistor is 0.2 Ω therefore ILED is:

Equation 67. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085653.gif

The chosen component from step 3 is:

Equation 68. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085654.gif

9.2.2.2.4 Output Capacitance

Assume rD = 2 Ω and determine ZC:

Equation 69. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085655.gif

Solve for CO-MIN and :

Equation 70. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085656.gif

Choose CO:

Equation 71. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085657.gif

The chosen component from step 5 is:

Equation 72. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085658.gif

9.2.2.2.5 Input Capacitance

Determine tON:

Equation 73. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085659.gif

Solve for CIN-MIN:

Equation 74. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085660.gif

Choose CIN:

Equation 75. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085661.gif

Determine IIN-RMS:

Equation 76. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085662.gif

The chosen component from step 5 is:

Equation 77. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856a1.gif

9.2.2.2.6 PFET

Determine minimum Q1 voltage rating and current rating:

Equation 78. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856e1.gif
Equation 79. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856e2.gif

A 70V, 5.7 A PFET is chosen with RDS-ON = 190 mΩ and Qg = 20 nC. Determine IT-RMS and PT:

Equation 80. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856e4.gif
Equation 81. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856e5.gif

The chosen component from step 6 is:

Equation 82. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856a2.gif

9.2.2.2.7 Diode

Determine minimum D1 voltage rating and current rating:

Equation 83. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856e7.gif
Equation 84. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856e8.gif

A 60 V, 5 A diode is chosen with VD = 750 mV. Determine PD:

Equation 85. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856a0.gif

The chosen component from step 7 is:

Equation 86. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856a4.gif

9.2.2.2.8 Input UVLO

Solve for RUV2:

Equation 87. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085621.gif

The closest 1% tolerance resistor is 49.9 kΩ therefore VHYS is:

Equation 88. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085632.gif

Solve for RUV1:

Equation 89. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085633.gif

The closest 1% tolerance resistor is 6.98 kΩ therefore VTURN-ON is:

Equation 90. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085634.gif

The chosen components from step 8 are:

Equation 91. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085643.gif

9.2.2.2.9 IADJ Connection Method

The IADJ pin is connected to an external voltage source and varied from 0 – 1.24 V to dim. An RC filter (RF2 = 1 kΩ and CF2 = 0.1 µF) is used as recommended.

9.2.2.2.10 PWM Dimming Method

No PWM dimming is necessary.

9.2.2.3 Application Curve

Figure 32 shows the LED current versus IADJ voltage for the application.

LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 D002_SNVS602.gif Figure 32. Analog Dimming Profile

9.2.3 LM3409 Buck Converter Application

LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085601.gif Figure 33. LM3409 Buck Converter Simplified Schematic

9.2.3.1 Design Requirements

Nominal input voltage: VIN

Maximum input voltage: VIN-MAX

Nominal output voltage (number of LEDs x forward voltage): VO

LED string dynamic resistance: rD

Switching frequency (at nominal VIN, VO): fSW

Average LED current: ILED

Inductor current ripple: ΔiL-PP

LED current ripple: ΔiLED-PP

Input voltage ripple: ΔvIN-PP

UVLO characteristics: VTURN-ON and VHYS

Expected efficiency: η

9.2.3.2 Detailed Design Procedure

9.2.3.2.1 Nominal Switching Frequency

Calculate switching frequency (fSW) at the nominal operating point (VIN and VO). Assume a COFF value (from 470 pF to 1 nF) and a system efficiency (η). Solve for ROFF:

Equation 92. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085669.gif

9.2.3.2.2 Inductor Ripple Current

Set the inductor ripple current (ΔiL-PP) by solving for the appropriate inductor (L1):

Equation 93. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085670.gif

9.2.3.2.3 Average LED Current

Set the average LED current (ILED) by first solving for the peak inductor current (IL-MAX):

Equation 94. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085672.gif

Peak inductor current is detected across the sense resistor (RSNS). In most cases, assume the maximum value (VADJ = 1.24 V) at the IADJ pin and solve for RSNS:

Equation 95. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085673.gif

If the calculated RSNS is far from a standard value, the beginning of the process can be iterated to choose a new ROFF, L1, and RSNS value that is a closer fit. The easiest way to approach the iterative process is to change the nominal fSW target knowing that the switching frequency varies with operating conditions anyways.

Another method for finding a standard RSNS value is to change the VADJ value. However, this would require an external voltage source or a resistor from the IADJ pin to GND as explained in the Adjustable Peak Current Control section of this data sheet.

9.2.3.2.4 Output Capacitance

A minimum output capacitance (CO-MIN) may be necessary to reduce ΔiLED-PP below ΔiL-PP. With the specified ΔiLED-PP and the known dynamic resistance (rD) of the LED string, solve for the required impedance (ZC) for CO-MIN:

Equation 96. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085674.gif

Solve for CO-MIN:

Equation 97. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085675.gif

9.2.3.2.5 Input Capacitance

Set the input voltage ripple (ΔvIN-PP) by solving for the required minimum capacitance (CIN-MIN):

Equation 98. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085676.gif

The necessary RMS input current rating (IIN-RMS) is:

Equation 99. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085677.gif

9.2.3.2.6 PFET

The PFET voltage rating should be at least 15% higher than the maximum input voltage (VIN-MAX) and current rating should be at least 10% higher than the average PFET current (IT):

Equation 100. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085666.gif

Given a PFET with on-resistance (RDS-ON), solve for the RMS transistor current (IT-RMS) and power dissipation (PT):

Equation 101. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856c5.gif
Equation 102. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856c6.gif

9.2.3.2.7 Diode

The Schottky diode needs a voltage rating similar to the PFET. Higher current diodes with a lower forward voltage are suggested. Given a diode with forward voltage (VD), solve for the average diode current (ID) and power dissipation (PD):

Equation 103. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085667.gif
Equation 104. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 300856c8.gif

9.2.3.2.8 Input UVLO

Input UVLO is set with the turnon threshold voltage (VTURN-ON) and the desired hysteresis (VHYS). To set VHYS, solve for RUV2:

Equation 105. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085680.gif

To set VTURN-ON, solve for RUV1:

Equation 106. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085681.gif

9.2.3.2.9 IADJ Connection Method

The IADJ pin controls the high-side current sense threshold in three ways outlined in the Adjustable Peak Current Control section.

Method 1: Leave IADJ pin open and ILED is calculated as in the Average LED Current section of the Design Guide.

Method 2: Apply an external voltage (VADJ) to the IADJ pin from 0 to 1.24 V to analog dim or to reduce ILED as follows:

Equation 107. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085682.gif

Keep in mind that analog dimming will eventually push the converter in to DCM and the inductor current ripple will no longer be constant causing a divergence from linear dimming at low levels.

A 0.1 µF capacitor connected from the IADJ pin to GND is recommended when using this method. It may also be necessary to have a 1kΩ series resistor with the capacitor to create an RC filter. The filter will help remove high frequency noise created by other connected circuitry.

Method 3: Connect an external resistor or potentiometer to GND (REXT) and the internal 5 µA current source will set the voltage. Again, a 0.1 µF capacitor connected from the IADJ pin to GND is recommended. To set ILED, solve for REXT:

Equation 108. LM3409 LM3409-Q1 LM3409HV LM3409HV-Q1 30085683.gif

9.2.3.2.10 PWM Dimming Method

There are two methods to PWM dim using the LM3409/09HV:

Method 1:Apply an external PWM signal to the EN terminal.

Method 2: Perform external parallel FET shunt dimming as detailed in the External Parallel FET PWM Dimming section.