SNVSA50C August   2015  – May 2017 LP8861-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  Internal LDO Electrical Characteristics
    7. 7.7  Protection Electrical Characteristics
    8. 7.8  Power Line FET Control Electrical Characteristics
    9. 7.9  Current Sinks Electrical Characteristics
    10. 7.10 PWM Brightness Control Electrical Characteristics
    11. 7.11 Boost/SEPIC Converter Characteristics
    12. 7.12 Logic Interface Characteristics
    13. 7.13 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Integrated Boost/SEPIC Converter
      2. 8.3.2 Internal LDO
      3. 8.3.3 LED Current Sinks
        1. 8.3.3.1 Current Sink Configuration
        2. 8.3.3.2 Current Setting
        3. 8.3.3.3 Brightness Control
      4. 8.3.4 Power-Line FET Control
      5. 8.3.5 LED Current Dimming With External Temperature Sensor
      6. 8.3.6 Protection and Fault Detection
        1. 8.3.6.1 Adaptive Boost Control and Functionality of LED Fault Comparators
        2. 8.3.6.2 Overview of the Fault/Protection Schemes
    4. 8.4 Device Functional Modes
      1. 8.4.1 Device States
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Typical Application for 4 LED Strings
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
          1. 9.2.1.2.1 Inductor Selection
          2. 9.2.1.2.2 Output Capacitor Selection
          3. 9.2.1.2.3 Input Capacitor Selection
          4. 9.2.1.2.4 LDO Output Capacitor
          5. 9.2.1.2.5 Diode
          6. 9.2.1.2.6 Power Line Transistor
          7. 9.2.1.2.7 Input Current Sense Resistor
        3. 9.2.1.3 Application Curves
      2. 9.2.2 High Output Current Application
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
        3. 9.2.2.3 Application Curves
      3. 9.2.3 SEPIC Mode Application
        1. 9.2.3.1 Design Requirements
        2. 9.2.3.2 Detailed Design Procedure
          1. 9.2.3.2.1 Diode
          2. 9.2.3.2.2 Inductor
        3. 9.2.3.3 Application Curves
      4. 9.2.4 Application with Temperature Based LED Current De-rating
        1. 9.2.4.1 Design Requirements
        2. 9.2.4.2 Detailed Design Procedure
        3. 9.2.4.3 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 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    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
      1. 13.1.2 Tape and Reel Information
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

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 LP8861-Q1 is designed for automotive applications, and an input voltage VIN is intended to be connected to the car battery. Device circuitry is powered from the internal LDO which, alternatively, can be used as external VDD voltage — in that case, external voltage must be in the 4.5-V to 5.5-V range.

The LP8861-Q1 uses a simple four-wire control:

  • VDDIO/EN for enable
  • PWM input for brightness control
  • SYNC pin for boost synchronisation (optional)
  • FAULT output to indicate fault condition (optional)

Typical Applications

Typical Application for 4 LED Strings

Figure 24 shows the typical application for LP8861-Q1 which supports 4 LED strings with maximum current 100 mA and boost switching frequency of 300 kHz.

LP8861-Q1 typ_app1_SNVSA50.gif Figure 24. Typical Application for Four Strings 100 mA/String Configuration

Design Requirements

DESIGN PARAMETER VALUE
VIN voltage range 4.5…28 V
LED string 4 x 8 LEDs (30 V)
LED string current 100 mA
Max boost voltage 37 V
Boost switching frequency 300 kHz
External boost sync not used
Boost spread spectrum enabled
L1 33 μH
CIN 10 µF 50 V
CIN BOOST 2 × 10-µF, 50-V ceramic + 33-µF, 50-V electrolytic
COUT 2 × 10-µF, 50-V ceramic + 33-µF, 50-V electrolytic
CLDO 1 µF 10 V
CFB 15 pF
RISET 24 kΩ
RFSET 210 kΩ
RISENSE 50 mΩ
R1 750 kΩ
R2 130 kΩ
R3 10 kΩ
RGS 20 kΩ

Detailed Design Procedure

Inductor Selection

There are two main considerations when choosing an inductor; the inductor must not saturate, and the inductor current ripple must be small enough to achieve the desired output voltage ripple. Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of application should be requested from the manufacturer. Shielded inductors radiate less noise and are preferred. The saturation current must be greater than the sum of the maximum load current and the worst-case average to peak inductor current. Equation 10 shows the worst-case conditions:

Equation 10. LP8861-Q1 inductor_select_eq.gif

where

  • IRIPPLE - peak inductor current
  • IOUTMAX - maximum load current
  • VIN - minimum input voltage in application
  • L - min inductor value including worst case tolerances
  • ƒ - minimum switching frequency
  • VOUT - output voltage
  • D - Duty Cycle for CCM Operation
  • VOUT - Output Voltage

As a result the inductor should be selected according to the ISAT. A more conservative and recommended approach is to choose an inductor that has a saturation current rating greater than the maximum current limit. A saturation current rating at least 3 A is recommended for most applications. See Table 2 for inductance recommendation for the different switch frequency ranges. The inductor’s resistance should be less than 300 mΩ for good efficiency.

See detailed information in Understanding Boost Power Stages in Switch Mode Power Supplies (SLVA061). Power Stage Designer™ Tools can be used for the boost calculation: http://www.ti.com/tool/powerstage-designer.

Output Capacitor Selection

A ceramic and electrolytic capacitors should have sufficient voltage rating. The DC-bias effect in ceramic capacitors can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Capacitance recommendation for different switching frequency range is shown in Table 2. To minimize audible of noise ceramic capacitors their geometric size is usually minimized.

Input Capacitor Selection

A ceramic and electrolytic capacitors should have sufficient voltage rating. The DC-bias effect in ceramic capacitors can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Capacitance recommendation for different switching frequency range is shown in Table 2. To minimize audible of noise ceramic capacitors their geometric size is usually minimized.

LDO Output Capacitor

A ceramic capacitor with at least 10-V voltage rating is recommended for the output capacitor of the LDO. The DC-bias effect in ceramic capacitors can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically a 1-µF capacitor is sufficient.

Diode

A Schottky diode should be used for the boost output diode. Ordinary rectifier diodes should not be used, because slow switching speeds and long recovery times degrade the efficiency and the load regulation. Diode rating for peak repetitive current should be greater than inductor peak current (up to 3 A) to ensure reliable operation. Average current rating should be greater than the maximum output current. Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing efficiency. Choose a reverse breakdown voltage of the Schottky diode significantly larger than the output voltage.

Power Line Transistor

A pFET transistor with necessary voltage rating (VDS at least 5 V higher than max input voltage) should be used. Current rating for the FET should be the same as input peak current or greater. Transfer characteristic is very important for pFET. VGS for open transistor must be less than VIN. A 20-kΩ resistor between pFET gate and source is sufficient.

Input Current Sense Resistor

A high-power 50-mΩ resistor should be used for sensing the boost input current. Power rating can be calculated from the input current and sense resistor resistance value. Increasing RISENSE decreases VIN OCP current proportionally.

Application Curves

LP8861-Q1 startup_app_SNVSA50.gif
ƒSW = 300 kHz VIN = 10 V
Brightness PWM 50% 100 Hz
Figure 25. Typical Start-up
LP8861-Q1 C011_SNVSA50.png
Load 4 strings, 8 LED per string ƒSW= 300 kHz
I = 100 mA/string for VIN = 12 V and 16 V
I = 60 mA/sting for VIN = 8 V
I = 50 mA/string for VIN = 5 V
Figure 27. Boost Efficiency
LP8861-Q1 open_led_fault_SNVSA50.gif
Figure 26. Open LED Fault
LP8861-Q1 C012_SNVSA50.png
Load 4 strings, 8 LED per string ƒSW = 300 kHz
I = 100 mA/string for VIN = 12 V and VIN = 16 V
I = 60 mA/sting for VIN = 8 V
I = 50 mA/string for VIN = 5 V
Figure 28. System Efficiency

High Output Current Application

The LP8861-Q1 current sinks can be tied together to drive LED with higher current. To drive 200 mA per string 2 outputs can be connected together. All 4 outputs connected together can drive an up to 400-mA LED string. Device circuitry is powered from external VDD voltage.

LP8861-Q1 typ_app2_SNVSA50.gif Figure 29. Two Strings 200 mA/String Configuration

Design Requirements

DESIGN PARAMETER VALUE
VIN voltage range 4.5…28 V
LED string 2 × 8 LEDs (30 V)
LED string current 200 mA
Max boost voltage 37 V
Boost switching frequency 2.2 MHz
External boost sync not used
Boost spread spectrum disabled
L1 4.7 μH
CIN 10 µF 50 V
CIN BOOST 2 × 10-µF, 50-V ceramic
COUT 3 × 10-µF, 50-V ceramic
CLDO 1 µF 10 V
CFB 4.7 pF
RISET 24 kΩ
RFSET 24 kΩ
R1 750 kΩ
R2 130 kΩ
R3 10 kΩ
RGS 20 kΩ

Detailed Design Procedure

See Detailed Design Procedure.

Application Curves

See Application Curves.

SEPIC Mode Application

When LED string voltage can be above or below VIN voltage, SEPIC configuration can be used. The SW pin voltage is equal to the sum of the input voltage and output voltage in SEPIC mode — this fact limits the maximum input voltage in this mode. LED current sinks not used should be connected to ground. External frequency can be used to synchronize boost/SEPIC switching frequency, and external frequency can be modulated to spread switching frequency spectrum.

LP8861-Q1 typ_app3_SNVSA50.gif Figure 30. SEPIC Mode, 4 Strings 100 mA/String Configuration

Design Requirements

DESIGN PARAMETER VALUE
VIN voltage range 4.5…30 V
LED string 4 × 4 LEDs (14.5 V)
LED string current 100 mA
Max boost voltage 17.5 V
Boost switching frequency 300 kHz
External boost sync used
Boost spread spectrum not available with external sync
L1 33 μH
CIN 10 µF 50 V
CIN SEPIC 2 × 10-µF, 50-V ceramic + 33-µF 50-V electrolytic
C1 10-µF 50-V ceramic
COUT 2 × 10-µF, 50-V ceramic + 33-µF 50-V electrolytic
CLDO 1 µF 10 V
RISET 24 kΩ
RFSET 210 kΩ
RISENSE 50 mΩ
R1 390 kΩ
R2 130 kΩ
R3 10 kΩ
RGS 20 kΩ

Detailed Design Procedure

See Detailed Design Procedure for external component recommendations. The Power Stage Designer™ Tools can be use for defining SEPIC component current and voltage ratings according to application: http://www.ti.com/tool/powerstage-designer

Diode

A Schottky diode with a low forward drop and fast switching speed should be used for the SEPIC output diode. Do not use ordinary rectifier diodes, because slow switching speeds and long recovery times degrade the efficiency and load regulation. The diode must be able to handle peak repetitive current greater than the integrated FET peak current (SW pin limit), thus 3 A or higher must be used to ensure reliable operation. Average current rating should be greater than the maximum output current. Choose a diode with reverse breakdown larger than the sum of input voltage and output voltage.

Inductor

Coupled or uncoupled inductors can be used in SEPIC mode. Coupled inductor typically provides better efficiency. Power Stage Designer™ Tools can be used for the SEPIC inductance calculation: http://www.ti.com/tool/powerstage-designer.

Application Curves

LP8861-Q1 C015_SNVSA50.png
Load 4 strings, 4 LED per string ƒSW = 300 kHz
I = 100 mA/string
Figure 31. SEPIC Efficiency
LP8861-Q1 C016_SNVSA50.png
Load 4 strings, 4 LED per string ƒSW = 300 kHz
I = 100 mA/string
Figure 32. System Efficiency

Application with Temperature Based LED Current De-rating

The LP8881-Q1 is able to protect connected LED strings from overheating. LED current versus temperature behavior can be adjusted with external resistor as described in LED Current Dimming With External Temperature Sensor.

LP8861-Q1 typ_app4_SNVSA50.gif Figure 33. Temperature Based LED Current De-rating

Design Requirements

DESIGN PARAMETER VALUE
VIN voltage range 4.5…30 V
LED string 4 × 9 LEDs (33 V)
LED string current 50 mA
Max boost voltage 43 V
Boost switching frequency 400 kHz
External boost sync not used
Boost spread spectrum enabled
L1 33 μH
CIN 10-µF 50-V ceramic
CIN BOOST 2 × 10-µF, 50-V ceramic + 33-µF, 50-V electrolytic
COUT 2 × 10-µF, 50-V ceramic + 33-µF, 50-V electrolytic
CLDO 1 µF 10 V
CFB 15 pF
RISET 48 kΩ
RFSET 160 kΩ
RISENSE 50 mΩ
R1 866 kΩ
R2 130 kΩ
R3 12 kΩ
R4 10 kΩ
R5 1.8 kΩ
R6 82 kΩ
R7 16 kΩ
R8 10 kΩ
RT 10 kΩ @ 25°C
RGS 20 kΩ

Detailed Design Procedure

See Detailed Design Procedure.

Application Curve

LP8861-Q1 C007_SNVSA50.png Figure 34. LED Current vs Temperature