SNVS656D September   2010  – October 2016 LM3492 , LM3492-Q1

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Description (continued)
  6. 6 Pin Configuration and Functions
  7. 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. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Switching Frequency
      2. 8.3.2 LDO Regulator
      3. 8.3.3 Enable and Disable
      4. 8.3.4 Current Limit
      5. 8.3.5 Thermal Protection
      6. 8.3.6 Dynamic Headroom Control, Over-Ride, and Soft-Start
      7. 8.3.7 Current Regulator
      8. 8.3.8 Output Voltage Feedback
      9. 8.3.9 Bidirectional Communication Pin
        1. 8.3.9.1 Power-Good Indication
        2. 8.3.9.2 Overtemperature Indication
        3. 8.3.9.3 Output Current Undervoltage Indication
        4. 8.3.9.4 Switching Frequency Tuning
    4. 8.4 Device Functional Modes
    5. 8.5 Programming
      1. 8.5.1 Output Current Overvoltage Indication
      2. 8.5.2 COMM Pin Bit Pattern
      3. 8.5.3 Channel 1 Disable
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 RFB1, RFB2, and CFB
        2. 9.2.2.2 L1
        3. 9.2.2.3 D1
        4. 9.2.2.4 CIN and COUT
        5. 9.2.2.5 CVCC
        6. 9.2.2.6 CCDHC
        7. 9.2.2.7 RRT and RIREF
        8. 9.2.2.8 RCOMM
      3. 9.2.3 Application Curve
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Related Links
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 Community Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

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

The LM3492/-Q1 device is ideal for automotive and marine GPS display and applications that require a high contrast ratio.

9.2 Typical Application

LM3492 LM3492-Q1 30116110.gif Figure 26. Typical Application Schematic

9.2.1 Design Requirements

The following procedures are to design an LED driver using the LM3492/-Q1 with an input voltage ranged from 9 V to 24 V and two LED strings consists of 10 LEDs each with a forward voltage of 3.8 V for each LED when running at 200 mA. The output power is 15.2 W. The switching frequency fSW is designed to be 300 kHz.

Equation 6. IL1(PEAK) = IL1 + ILR / 2

9.2.2 Detailed Design Procedure

9.2.2.1 RFB1, RFB2, and CFB

The nominal voltage of the LED string with 10 LEDs is 38 V, and the minimum voltage of the IOUTn pin (n = 1, 2) is 1 V when ILED is 200 mA. As a result, VOUT(nom) is 39 V. Design VOUT(max) to be 65 V. From Equation 5, VFB(nom) is approximately 1.5 V, which falls in the recommended operation range from 1.05 V to 2 V. Also, design RFB2 to be 16.2 kΩ. From Equation 3, RFB1 is calculated to be 405 kΩ, and a standard resistor value of 402 kΩ is selected. CFB is selected to be 10 pF as recommended.

9.2.2.2 L1

The main parameter affected by the inductor is the peak to peak inductor current ripple (ILR). To maintain a continuous conduction mode (CCM) operation, ensure that the average inductor current IL1 is larger than half of ILR. For a boost converter, IL1 equals to the input current IIN. Hence,

Equation 7. IIN = (VOUT(nom) × 2×ILED ) / VIN

Also,

Equation 8. ton = (1 – VIN/VOUT) / fSW
Equation 9. L1 = (VIN × ton) / 2IIN

If VIN is maximum, which is 24 V in this example, and only one LED string is turned on (because the two channels of the device are individually dimmable), IIN is minimum. From Equation 7 to Equation 9, it can be calculated that IIN(MIN), ton, and L1 are 0.325 A, 1.28 µs, and 47 µH. However, from Equation 7, IIN is maximum when VIN is minimum, which is 9 V in this example, and the two LED strings are turned on together. Hence IIN(max) is 1.73 A. Then, ILR is

Equation 10. ILR = (VIN x ton) / L1

From Equation 8, ton is 2.56 µs. From (9), ILR is 0.49 A. The steady-state peak inductor current IL1(PEAK) is

Equation 11. IL1(PEAK) = IL1 + ILR / 2

As a result, IL1(PEAK) is 1.98 A. A standard value of 47 µH is selected for L1, and its saturation current is larger than 1.98 A.

9.2.2.3 D1

The selection of the boost diode D1 depends on two factors. The first factor is the reverse voltage, which equals to VOUT for a boost converter. The second factor is the peak diode current at the steady state, which equals to the peak inductor current as shown in Equation 11. In this example, a 100-V, 3-A Schottky diode is selected.

9.2.2.4 CIN and COUT

The function of the input capacitor CIN and the output capacitor COUT is to reduce the input and output voltage ripples. Experimentation is usually necessary to determine their value. The rated DC voltage of capacitors used should be higher than the maximum DC voltage applied. Owing to the concern of product lifetime, TI recommends ceramic capacitors. But ceramic capacitors with high rated DC voltage and high capacitance are rare in general. Multiple capacitors connecting in parallel can be used for CIN and COUT. In this example, two 10-µF ceramic capacitor are used for CIN, and two 2.2-µF ceramic capacitor are used for COUT.

9.2.2.5 CVCC

The capacitor on the VCC pin provides noise filtering and stabilizes the LDO regulator. It also prevents false triggering of the VCC UVLO. CVCC is recommended to be a 1-µF, good quality and low ESR ceramic capacitor.

9.2.2.6 CCDHC

The capacitor at the CDHC pin not only affects the sensitivity of the DHC but also determines the soft-start time tSS, the time for the output voltage to rise until power good. tSS is determined from the following equation:

Equation 12. LM3492 LM3492-Q1 30170509.gif

In this example, CCDHC is recommended to be a 0.47-µF good quality and low ESR ceramic capacitor.

9.2.2.7 RRT and RIREF

The resistors RRT and RIREF set the switching frequency fSW of the boost converter and the LED current ILED respectively. From Figure 16, if fSW is 300 kHz, RRT is selected to be 442 kΩ. From Figure 21, if ILED is 200 mA, RIREF is selected to be 6.19 kΩ.

9.2.2.8 RCOMM

Because the COMM pin is open drain, a resistor RCOMM of 52.3 kΩ is used to connect the VCC and COMM pins to act as a pullup function.

9.2.3 Application Curve

LM3492 LM3492-Q1 30123309.png Figure 27. LED 50% Dimming, Both Channels Combined
(VIN = 12 V, ILED = 150 mA, 200 Hz)