PSPWM cycle

OUT1

Boost voltage

(dotted line)

String Vf

Boost

advance

adjust

1

2

4

3

5

1

Phase #

OUT2

OUT3

OUT4

OUT5

LP8543

SNVS604D – AUGUST 2009 – REVISED MARCH 2013

www.ti.com

Adaptive Boost Control

Adaptive boost control function adjusts the boost voltage to the minimum sufficient voltage for proper LED driver

operation. When PSPWM is used the output voltage can be adjusted for every phase shift step separately except

in 19.5 kHz PSPWM mode due to timing constraints. To enable PSPWM to each phase, the <BOOST_MODE>

voltage for each string is stored and used in predicting when the boost has to start increasing voltage for the next

step. The boost setup time can be defined with two EEPROM bits. Principle of the boost voltage adjustment with

PSPWM is illustrated below. If higher PWM value is used then more strings are on at the same time, and voltage

Figure 17. Boost Adaptive Voltage Control for 5–String PSPWM

When adaptive boost mode is selected the voltages across the LED drivers are constantly monitored. There are

three voltage thresholds used, Low, Mid and High. Low and High thresholds are adjustable with 3 EEPROM bits.

Low threshold range is from 0.5V to 2.25V and High threshold range is from 3 to 10V. Mid threshold is set 0.5V

above Low threshold. Threshold levels are listed in the table below. Adjustability is provided to enable adaptation

recommended to maintain good efficiency. Fault detection chapter describes how these thresholds are used also

for fault detection.

Table 8. LED Voltage Comparator Thresholds

EEPROM bits

Threshold (V)

LED_FAULT_THR[5:3] (HIGH comparator)

Low

High

Mid

DRV_HEADR_CTRL[2:0] (LOW comparator)

000

0.50

3

001

0.75

4

010

1.00

5

011

1.25

6

Low + 0.5V

100

1.50

7

101

1.75

8

110

2.00

9

111

2.25

10

If only one string is on at a time (Brightness value lower than 100% divided by number of strings) the voltage for

each string is adjusted so that the voltage across the driver will fall between Low and Mid threshold. If more

strings are on at the same time (high PWM value, or PSPWM not used) the situation looks like in the following

diagram. In this diagram 6 outputs are on at the same time. In normal operation voltages across all LED driver

outputs are between high and low threshold.

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