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DS8555-01 August 2016 www.richtek.com
RT8555
©
Copyright 2016 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Brightness Control Signal Selection
The RT8555 integrates a dimming control signal selection. The dimming control signal source could be set by the
second bit of register 00h. If the bit equals to 0, it means the dimming control signal source just depends on the input
signal of the PWM pin. Otherwise, if the bit equals to 1, the dimming control signal is controlled by the command of
register 04/05h. The option is shown in Table 3 below.
Table 3. Dimming Control Signal Selection
Address Bit Name Default Value Description R/W
00h [1]
ILED Brightness
Selection
PWM Pin
(B0)
B0 : depend on the status of PWM pin
B1 : depend on address:04/05h data
R/W
Switching Frequency
The LED driver switching frequency is adjusted by the I
2
C, the switching frequency setting range, Spread Spectrum, LX
Slew Rate and resolutions are shown in the Table 4 below.
Table 4. Switching Frequency Setting
Address Bit Name Default Value Description Resolution R/W
00h [4]
Spread
Spectrum
w/o
(B0)
B0 : w/o
B1 : w/i
R/W
00h [6:5]
Edge Rate
Control
Fast (B10)
B00 : Slow
B01 : Normal
B10 : Fast
R/W
01h [3:0]
Boost
Switching
Frequency
900kHz
(0x06h)
0x00h : 300kHz
0x07h : 1MHz
0x0Ch : 2MHz
100kHz (0x00h to 0x07h)
200kHz (0x07h to 0x0Ch)
R/W
If the switching frequency command is below to register 0x01. The switching frequency is from 300kHz to 1MHz and
resolution is 100kHz. The switching frequency is from 1MHZ to 2MHz and resolution is 200kHz.
Current Limit Protection
The RT8555 integrates current limit protection, and the current of current limit protection could be set by I
2
C, which is
shown in the Table 5 below.
Table 5. Current Limit Protection Setting
Address Bit Name Default Value Description Resolution R/W
01h [4]
Switching Current
Limitation Selection
2.5A
(B1)
B0 : 1.5A
B1 : 2.5A
-- R/W
The RT8555 can limit the peak current to achieve over current protection. The RT8555 senses the inductor current during
the “ON” period that flows through the LX pin. The duty cycle depends on the current signal and internal slope
compensation in comparison with the error signal. The internal switch of Boost converter will be turned off when the peak
current value of inductor current is larger than the over current protection setting. In the “OFF” period, the inductor
current will be decreased until the internal switch is turned on by the oscillator.
