LP8552_14 Datasheet, PDF (19/49 Pages) Texas Instruments – LP8552 High-Efficiency LED Backlight Driver For Notebooks

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VBOOST[4:0]
11111
31
LP8552
SNVS693E – SEPTEMBER 2011 – REVISED SEPTEMBER 2014
VOLTAGE (typical)
40
8.3.3.4 Adaptive Boost Control
Adaptive boost control function adjusts the boost output voltage to the minimum sufficient voltage for proper LED
driver operation. The output with highest V_F LED string is detected and boost output voltage adjusted
accordingly. Driver headroom can be adjusted with <DRIVER_HEADR[2:0]> EEPROM bits from approximately
300 mV to 1200 mV. Boost adaptive control voltage step size is 125 mV. Boost adaptive control operates
similarly with and without PSPWM.
V_BOOST
V_BOOST
Driver
headroom
Time
Figure 18. Boost Adaptive Control Principle With PSPWM
8.3.4 Fault Detection
The LP8552 has fault detection for LED fault, low-battery voltage, overcurrent, and thermal shutdown. The open
drain output pin (FAULT) can be used to indicate occurred fault. The cause for the fault can be read from status
register. Reading the fault register will also reset the fault. Setting the EN pin low will also reset the faults, even if
an external 5V line is used to power VLDO pin.
8.3.4.1 LED Fault Detection
With LED fault detection, the voltages across the LED drivers are constantly monitored. Shorted or open LED
string is detected.
If LED fault is detected:
• The corresponding LED string is taken out of boost adaptive control loop;
• Fault bits are set in the fault register to identify whether the fault has been open/short and how many strings
are faulty; and
• Fault open-drain pin is pulled down.
LED fault sensitivity can be adjusted with <LED_FAULT_THR> EEPROM bit which sets the allowable variation
between LED output voltage to 3.3 V or 5.3 V. Depending on application, and how much variation there can be in
normal operation between LED string forward voltages, this setting can be adjusted.
Fault is cleared by setting EN pin low or by reading the fault register.
By default the LED fault detection is active only in automatic PWM & current dimming mode. If LED fault
detection is needed in PWM dimming mode, please contact a Texas Instruments representative for guidance.
8.3.4.2 Undervoltage Detection
LP8552 has detection for too-low VIN voltage. Threshold level for the voltage is set with EEPROM register bits
as seen in Table 3:
Table 3. Threshold Level for Voltage Set with EEPROM Register Bits
UVLO[1:0]
00
01
10
11
THRESHOLD (V)
OFF
2.7
5.4
8.1
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