LP85571 datasheet(19/49 Pages) TI2

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Duty cycle D = 1 - VIN / VOUT

tSW = 1 / fSW

Slew rate control,

programmable

Spread spectrum scheme,

programmable pseudo

random duty cycle changes

minimize EMI

LP8557, LP85571

www.ti.com

SNVSA15B – DECEMBER 2013 – REVISED DECEMBER 2015

Feature Description (continued)

8.3.5 EMI Reduction Schemes

LP8557I features two EMI reduction schemes. By default, the schemes are disabled; however, the schemes can

be enabled by I2C writes to SSEN and SREN bits in COMMAND register. The schemes are unavailable on the

LP8557.

The first scheme, programmable slew rate control, uses a combination of three drivers for boost switch. Enabling

all three drivers allows boost switch on/off transition times to be the shortest. On the other hand, enabling just

one driver allows boost switch on/off transition times to be the longest. The longer the transition times, the lower

the switching noise on the SW terminal. It should also be noted that the shortest transition times bring the best

efficiency as the switching losses are the lowest. This scheme can be enabled by setting SREN=1 with an I2C

write.

The second EMI reduction scheme is the spread spectrum scheme. This scheme deliberately spreads the

frequency content of the boost switching waveform, which inherently has a narrow bandwidth, makes the

switching waveform's bandwidth wider and ultimately reduces its EMI spectral density. This scheme can be

enabled by setting SSEN = 1 with an I2C write.

Figure 32. EMI Reduction Schemes

8.3.6 Fault Detection

The LP8557 and LP8557I have fault detection for LED SHORT, UVLO, BST_OVP, BST_OCP, BST_UV, and

TSD. Additionally, the LP8557I can support LED OPEN* fault. Faults are recorded in the STATUS register. Each

time the STATUS register is read it is automatically cleared.

8.3.6.1 LED Short Detection

Voltages at the individual current sinks are constantly monitored for the LED SHORT fault. This fault may occur

when some LEDs in a string are electrically bypassed making that LED string shorter than the other LED strings.

The reduced forward voltage causes the current sink attached to that string to have a higher headroom voltage

than the other current sinks. When the headroom voltage is higher than the fault comparator threshold

(configured with the 0V field in the LEDEN register), that current sink is disabled, and the PWM phasing is

automatically adjusted. The fault comparator threshold is at 2 V, typical.

8.3.6.2 LED OPEN* Detection

When the auto-detect function is disabled, each current sink is also monitored for the LED OPEN* condition. The

condition is set when the headroom voltage on one or more current sinks is below the LOW comparator

threshold, and the boost voltage is at the maximum. This fault condition may be caused by one or more OPEN

LED strings or by one or more current sinks shorted to GND. The LP8557I immediately shuts down the backlight

whenever an LED OPEN* condition is detected on any enabled LED drivers. The backlight does not turn on

again (regardless of the COMMAND.ON bit) until the STATUS register is read.

8.3.6.3 Undervoltage Detection

The device continuously monitors the voltage on the VDD pin. When the VDD voltage drops below 2.5 V the

backlight is immediately shut down, and the UVLO bit is set in the STATUS register. The backlight automatically

starts again when the voltage has increased above 2.5 V + 50 mV hysteresis. Hysteresis is implemented to avoid

continuously triggering undervoltage.

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