LP85571 datasheet(32/49 Pages) TI2

32 / 49 page

LP8557, LP85571

SNVSA15B – DECEMBER 2013 – REVISED DECEMBER 2015

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9.1.1.2 Setting Boost Compensation

For stable LP8557 boost operation, appropriate boost compensation must be selected based on the selected

boost switching frequency and the boost inductance. Table 3 shows recommended boost compensation options

based on the boost switching frequency and selected boost circuit inductance.

Table 3. Recommended Boost Compensation Options Based

on the Boost Switching Frequency and Inductance

RECOMMENDED BOOST

ƒSW (kHz)

L (µH)

COMPENSATION OPTION

500

4.7

1000

6.8

The LP8557 boost converter compensation is set by placing an external resistor, RISET, from the ISET pin to

GND. Note that the ISET pin is shared for setting the full-scale LED current in addition to setting the boost

compensation. Setting the boost compensation and the full-scale LED current using an external resistor is shown

in Table 4.

9.1.1.3 Setting Full-Scale Led Current

The LP8557 full-scale current is set by placing an external resistor, RISET, from the ISET pin to GND. Note that

the ISET pin is shared for setting the boost compensation in addition to the full-scale LED current. Setting the

boost compensation and the full-scale LED current using an external resistor is shown in Table 4.

Table 4. Setting Full-Scale LED Current and Boost Compensation Using an

External Resistor

RISET [

Ω] (TOLERANCE)

BOOST COMPENSATION

ILED (mA)

OPTION

470k - 1M (±5%)

300k, 330k (±5%)

200k (±5%)

147k, 150k, 154k, 158k (±1%)

121k (±1%)

100k (±1%)

86.6k (±1%)

75.0k (±1%)

63.4k (±1%)

52.3k, 53.6k (±1%)

44.2k, 45.3k (±1%)

39.2k (±1%)

34.0k (±1%)

30.1k (±1%)

26.1k (±1%)

23.2k (±1%)

0 (grounded)

9.1.2 Designing With LP8557I

The LP8557I is intended for applications that can utilize an I2C master to control the device. Use of an external

PWM signal is allowed for controlling the brightness levels; however, I2C commands are required to turn the

backlight on or off. Boost switching frequency, boost compensation, PWM dimming frequency, and the maximum

LED current are set to default values. Re-configuration is possible with I2C writes; however, re-programming has

to be done every time power on the VDD pin is recycled.

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