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  • LP886x-Q1 Feedback Resistor Design Considerations

    • SNVAA19 May   2021 LP8863-Q1 , LP8864-Q1 , LP8864S-Q1 , LP8866-Q1 , LP8866S-Q1

       

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  • LP886x-Q1 Feedback Resistor Design Considerations
  1.   Trademarks
  2. 1Introduction
  3. 2Boost Output Voltage Range Design Consideration
  4. 3Boost UVP Design Consideration
  5. 4Boost OVP Design Consideration
  6. 5Calculation of Feedback Resistor Values
  7. 6Feedback Resistor Calculation Example
  8. 7References
  9. IMPORTANT NOTICE
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APPLICATION NOTE

LP886x-Q1 Feedback Resistor Design Considerations

Trademarks

All trademarks are the property of their respective owners.

1 Introduction

Unlike the conventional constant voltage boost controller that uses feedback resistor network to set one regulated voltage, the feedback network of LP886x-Q1 is used to determine the minimum and maximum voltages at boost output. In the actual application, the boost output voltage is adjusted dynamically by an internal 11-bit current DAC to the minimum necessary level needed by the output channels to achieve optimum efficiency. Boost UVP and OVP levels are also defined by the feedback network and appropriate feedback resistor values need to be chosen to prevent false triggering of UVP and OVP faults.

2 Boost Output Voltage Range Design Consideration

Feedback resistors are selected to ensure that the minimum boost voltage with ISEL = 0 μA is below the minimum LED string voltage, as shown in Equation 4. The maximum boost voltage with ISEL = 38.7 μA need to be at least 2 V greater than the maximum LED string voltage + the maximum headroom at LED sink pins as shown in Equation 2.

Equation 1. GUID-20210423-CA0I-8F7V-XGLK-NGMK8MPMPJZM-low.gif
Equation 2. GUID-20210423-CA0I-R2MS-BQVC-TND7Z6JVDPWD-low.gif

Where:

    • VMINBOOST is the minimum boost voltage when ISEL = 0 μA (minimum)
    • VMAXBOOST is the maximum boost voltage when ISEL = 38.7 μA (maximum)
    • VSTRING is the voltage across LED string
    • VHEADROOM is the headroom voltage of LED current sink and can be substituted with 1 V in Equation 2
    • 2 V in Equation 2 is an empirical design margin considering the variations of internal reference and the tolerance of feedback resistors.

3 Boost UVP Design Consideration

When VBOOST_UVP is reached, in other words, when VFB is below VUVP of 0.886 V, it is a sign that the boost is no longer in regulation. During normal operation, VFB is regulated at VBG of 1.21 V. If undervoltage condition lasts longer than 110 ms, LP8863-Q1 (using LP8863-Q1 as an example and it is similar for LP886x-Q1) will report boost overcurrent, by generating fault interrupt and asserting BSTOCP_STATUS bit in INTERRUPT_STATUS_1 register, as shown in Table 3-1. The device will go to standby for 200 ms before restart attempt. If VBOOST_UVP,MAX (when ISEL is at maximum value) level is lower than (VSTRING + VHEADROOM,MAX), open LED fault may be reported unintentionally instead of overcurrent fault during an overcurrent event. Therefore, we recommend to design the maximum boost UVP threshold VBOOST_UVP,MAX when the current DAC ISEL is at maximum value, higher than VSTRING,MAX + VHEADROOM,MAX, as shown in Equation 4. The aim is to prevent false triggering of LED open fault during an overcurrent event when boost under voltage is detected.

Equation 3. GUID-20210423-CA0I-BPK2-XL8K-M4TNCDFDMFPL-low.gif
Table 3-1 Boost OCP, OVP and Open LED Fault Table
FAULT NAMECONDITIONACTION
Boost overcurrentFB pin voltages falls below VUVP level for > 110 ms.Device goes to standby and then attempts to restart 200 ms after fault occurs.
Open LED stringHeadroom voltage on one or more channels is below minimum level and boost has adapted to maximum level.Faulted LED string is disabled and removed from adaptive boost control loop. String is re-enabled next power cycle.
Boost OVP lowFB pin voltage rises above VFB_OVPL level.Boost stops switching until boost voltage level falls. The device remains in normal mode with LED drivers operational.
Boost OVP highFB pin voltage rises above VFB_OVPH level or DISCHARGE pin voltage rises above VBST_OVPH.Device goes to Fault Recovery and waits until output voltage falls below threshold before restarting.

4 Boost OVP Design Consideration

When the voltage of DISCHARGE pin is higher than VBOOST_OVPH (typically 50 V, minimum 48.5 V for LP8866-Q1), boost OVP high fault will be reported and device will go to fault recovery until boost voltage falls below threshold before restarting, as shown in Table 3-1. Be careful when designing boost OVP low threshold of initial voltage considering the overshoot during start-up. The boost OVP low threshold of initial voltage should be designed to be triggered first before DISCHARGE pin reaching VBOOST_OVPH. Otherwise, boost OVP high fault will be reported if DISCHARGE pin reaches 48.5 V during start-up. Then the device will go to fault recovery mode and restart again and again. Therefore, we recommend to design VBOOST_OVP_LOW_INITIAL < 48 V, as shown in Equation 4.

Equation 4. GUID-20210423-CA0I-H27F-LFSB-KRKNGGVS5Z7D-low.gif

Where:

  • VBOOST_OVP_LOW_INITIAL is the boost OVP Llow threshold for initial voltage during start up
  • 48 V is 0.5 V (additional margin) below the minimum VBOOST_OVPH.

5 Calculation of Feedback Resistor Values

The resistive divider (RFB1, RFB2, RFB3) defines both the minimum and maximum adaptive boost voltage levels, as well as UVP and OVP levels. For feedback network using two-resistor method as shown in Figure 5-1, VMINBOOST, VMAXBOOST, VBOOST_UVP,MAX, VBOOST_OVP_LOW and VSTRING can be expressed in Equation 5, Equation 6, Equation 7, Equation 8, and Equation 9:

Equation 5. GUID-20210423-CA0I-81BV-7GM7-XXKBJJPHLBK6-low.gif
Equation 6. GUID-20210423-CA0I-04KV-NTTZ-K3GQD93H8XJL-low.gif
Equation 7. GUID-20210423-CA0I-BR4S-QVPR-NGBBDPKWVM7V-low.gif
Equation 8. GUID-20210423-CA0I-RLJQ-8PSW-0GVVMMLMNBL6-low.gif
Equation 9. GUID-20210423-CA0I-JR0R-BDR3-GBPQSHPKB7GK-low.gif

where:

  • VBG is the band-gap voltage = 1.21 V
  • RFB1 is the upper feedback resistor
  • RFB2 is the lower feedback resistor
  • ISEL_MAX is the maximum internal 11-bit DAC current ISEL = 38.7 μA
  • VUVP is the undervoltage threshold at feedback pin = 0.886 V
  • VOVPL is the overvoltage low level at feedback pin = 1.423 V
  • 0.886 is the ratio of initial ISEL DAC current with respect to ISEL_MAX to set initial boot voltage during start-up
  • VLED is the voltage cross an LED
  • n is the number of LEDs in a string

GUID-5D30CCFF-5D6C-4FAE-87A4-169990555870-low.gif

Figure 5-1 Two-Resistor Feedback Network

For feedback network using three-resistor method as shown in Figure 5-2, VMINBOOST, VMAXBOOST, VBOOST_UVP,MAX and VBOOST_OVP_LOW can be expressed in Equation 10, Equation 11, Equation 12, and Equation 13:

Equation 10. GUID-20210423-CA0I-W2SZ-DG7K-WT2HZHGBTCXB-low.gif
Equation 11. GUID-20210423-CA0I-GSQR-Z0B7-7TPKRFKX9DB5-low.gif
Equation 12. GUID-20210423-CA0I-RGNB-KSXX-K2QJGDDSF3NH-low.gif
Equation 13. GUID-20210423-CA0I-STFJ-5NDS-TBBGZHK7GF5J-low.gif

where:

  • RFB3 is the third feedback resistor can be used in applications where less than 200-kΩ resistors are required.

GUID-A019C145-E422-4D23-AF27-7AD2393EF69B-low.gif

Figure 5-2 Three-Resistor Feedback Network

6 Feedback Resistor Calculation Example

Using the 2-resistor network as an example, the feedback resistors calculation can be found from the product folder. The same calculator can also be used for 3-resistor network calculation by simply setting RFB3 = 0 Ω.

In this example, there are 8 LEDs in a string, with forward voltage drop varies from 3.1 V to 3.4 V. The worst-case VSTRING,MIN and (VSTRING + VHEADROOM,MAX) + Design Margin can be calculated as 24.8 V and 30.2 V, respectively, in Table 6-1.

Table 6-1 LED String Calculation
DESIGN PARAMETERVALUEUNIT
Number of LEDs in one string8S
Min LED Forward Voltage VF,MIN3.1V
Max LED Forward Voltage VF,MAX3.4V
Min Headroom VHEADROOM,MIN0.4V
Max Headroom VHEADROOM,MAX1.0V
Design Margin for reference and feedback resistor variations2.0V
VSTRING,MIN24.8V
VSTRING,MAX28.2V
(VSTRING,MAX + VHEADROOM,MAX) + Design Margin30.2V

Assuming 1% feedback resistors are used, select feedback resistor values to meet the worst-case conditions listed in Equation 4, Equation 2, Equation 4, and Equation 4. In this example, with RFB1 = 620 kΩ and RFB2 = 56 kΩ, the calculated VMINBOOST = 14.61 V (< 24.8 V) and VMAXBOOST = 38.60 V (> 30.2 V) are able to cover the voltage range of LED string, as shown in Table 6-2.

Table 6-2 VBoost Calculation
DESIGN PARAMETERVALUEUNIT
RFB1 (1%)620kΩ
RFB2 (1%)56kΩ
RFB3 (1%)0kΩ
ISEL_MAX38.7μA
Vbg1.21V
VMINBOOST (V)14.61V
VMAXBOOST (V)38.60V

With VUVP of 0.886 V, the calculated VBOOST_UVP,MAX = 34.69 V as shown in Table 6-3, which is above 28.2 V so that LED open fault will not get false triggered during boost UVP event.

Table 6-3 VBoost_UVP,MAX Calculation
DESIGN PARAMETERVALUEUNIT
VUVP0.886V
VBOOST_UVP,MAX34.69V

With VOVPL of 1.423 V, the calculated VBOOST_OVP_LOW = 38.44 V as shown in Table 6-4, which is below 48 V so that boost OVP low fault will always be reported before boost OVP High Fault.

Table 6-4 VBoost_OVP_LOW Calculation
DESIGN PARAMETERVALUEUNIT
VOVPL1.423V
VBOOST_OVP_LOW38.44V

7 References

  • Texas Instruments, LP8863-Q1 Automotive Display LED-Backlight Driver With Six 150-mA Channels Data Sheet
  • Texas Instruments, LP8864-Q1 Automotive Display LED-backlight Driver with Four 200-mA Channels Data Sheet
  • Texas Instruments, LP8864S-Q1 Automotive Display LED-backlight Driver with Four 150-mA Channels Data Sheet
  • Texas Instruments, LP8866-Q1 Automotive Display LED-backlight Driver with Six 200-mA Channels Data Sheet
  • Texas Instruments, LP8866S-Q1 Automotive Display LED-backlight Driver with Six 150-mA Channels Data Sheet
  • Texas Instruments, LP8863-Q1 External Component Selection Guide Application Report

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