®
RT8555
36V High Efficiency Boost Converter with I2C Controlled
6-CH LED Driver
General Description
Features
The RT8555 is a high efficiency driver for white LEDs. It is
suitable for single/two cell battery input to drive LED light
bars which contains six strings in parallel and up to
10 WLEDs per string. The internal current sinks support
a maximum of ±2% current mismatching for excellent
brightness uniformity in each string of LEDs. To provide
enough headroom for current sink operation, the Boost
controller monitors the minimum voltage of the feedback
pins and regulates an optimized output voltage for power
efficiency.
The RT8555 has a wide input voltage operating range from
2.7V to 24V and contains I2C interface for controlling the
dimming mode, operating frequency and the LED current.
The internal 100mΩ, 36V power switch with current-mode
control provides over-current protection. The switching
frequency of the RT8555 is adjustable from 300kHz to
2MHz, which allows flexibility between efficiency and
component size.
The RT8555 is available in the WL-CSP-20B 1.65 x 2.05
(BSC), with pitch 0.4mm package.
Ordering Information
RT8555
Wide Operating Input Voltage : 2.7V to 24V
High Output Voltage : Up to 36V
Programmable Channel Current : 10mA to 35mA
Channel Current Regulation with Accuracy ±3%
and Matching ±2%
Dimming Controls
Direct PWM Dimming up to 20kHz and Minimum
On-Time to 400ns
PWM to Analog Dimming up to 20kHz with 8-bit
Resolution
I2C Programs LED Current, Switching Frequency,
Dimming Mode
Switching Frequency : 300kHz to 2MHz
Protections
LED Strings Open Detection
Current Limit
Programmable Over Voltage Protection
Over-Temperature Protection
20-Ball WL-CSP, with pitch 0.4mm Package
RoHS Compliant and Halogen Free
Applications
Tablet and Notebook Backlight
Package Type
WSC : WL-CSP-20B 1.65x2.05 (BSC)
Pin Configurations
Note :
(TOP VIEW)
Richtek products are :
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
A1
A2
A3
A4
LX PGND SDA SCL
B1
B2
B3
B4
LX PGND PWM EN
C1
C2
C3
C4
VIN VOUT GND FB3
Marking Information
1X : Product Code
1XW
W : Date Code
D1
D2
D3
D4
VIN VCP GND FB2
E1
E2
FB6 FB5
E3
E4
FB4
FB1
WL-CSP-20B 1.65 x 2.05 (BSC)
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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June 2015
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RT8555
Functional Pin Description
Pin No.
Pin Name.
Pin Function
A1, B1
LX
Switch Node of Boost Converter.
A2, B2
PGND
Power Ground.
A3
SDA
Data Signal Input of I2C Interface.
A4
SCL
Clock Signal Input of I2C Interface.
B3
PWM
PWM Dimming Control Input.
B4
EN
Enable Control Input (Active High).
C1, D1
VIN
Power Input.
C2
VOUT
Output of Boost Converter.
C3, D3
GND
Ground.
C4
FB3
Current Sink for LED3.
D2
VCP
Output of Internal Regulator.
D4
FB2
Current Sink for LED2.
E1
FB6
Current Sink for LED6.
E2
FB5
Current Sink for LED5.
E3
FB4
Current Sink for LED4.
E4
FB1
Current Sink for LED1.
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RT8555
Function Block Diagram
VCP
VIN
EN
VOUT
LDO
1.2V
OSC
LX
+
OTP
PWM
Controller
+
-
S
Q
R
Q
OCP
+
PGND
+
-
MUX
PWM
SCL
6
Mini LED
Selection
LED Open
Detection
2
SDA
VDS
I C
>
DAC
……
FB1
FB2
FB6
+
-
+
-
+
-
……
+
-
GND
Operation
Enable Control
OVP
When VIN is higher than the UVLO voltage and the EN pin
input voltage is higher than rising threshold, the VDC will
be regulated around 3.2V if VIN is higher than 3.2V.
The RT8555 integrates over voltage protection. The over
voltage protection could be set by the I2C, When the OVP
pin voltage is higher than 36V, the LX N-MOSFET is turned
off immediately to protect the LX N-MOSFET.
Switching Frequency
The LED driver switching frequency is adjusted by the
I2C. The switching frequency is from 300kHz to 1.9MHz.
PWM Controller
This controller includes some logic circuit to control LX
N-MOSFET on/off. This block controls the minimum ontime and max duty of LX.
Minimum LED Selection
This block detects all LEDx voltage and select a minimum
voltage to EA (Error Amplifier). This function can guarantee
the lowest of the LED pin voltage is around 500mV and
Vout can be Boost to the highest forward voltage of LED
strings.
LED Open Detection
OCP & OTP
When LX N-MOSFET peak current is higher than
2.5A(typically), the LX N-MOSFET is turned off
immediately and resumed again at next clock pulse. When
the junction temperature is higher than 150°C (typically),
the LX N-MOSFET will be turned off until the temperature
is lower than the 130°C (typically).
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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If the voltage at LEDx pin is lower than 100mV, this
channel is defined as open channel and the Minimum LED
Selection function will discard it to regulate other used
channels in proper voltage.
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RT8555
Absolute Maximum Ratings
(Note 1)
Supply Voltage, VIN to GND --------------------------------------------------------------------------------------------LX, VOUT, FB1, FB2, FB3, FB4, FB5, FB6 to GND --------------------------------------------------------------EN, PWM, SDA, SCL, VCP to GND ----------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
WL-CSP-20B 1.65 x 2.05 (BSC) --------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
WL-CSP-20B 1.65 x 2.05 (BSC), θJA ---------------------------------------------------------------------------------Junction Temperature -----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------------Storage Temperature Range --------------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Model) ----------------------------------------------------------------------------------------------MM (Machine Model) ------------------------------------------------------------------------------------------------------
Recommended Operating Conditions
−0.3V to 26.4V
−0.3V to 40V
−0.3V to 6V
2.72W
36.7°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 4)
Junction Temperature Range --------------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range --------------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VIN = 3.8V, CIN = 1μF, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
2.7
3.8
24
V
Input Power Supply
Input Supply Voltage
VIN
Quiescent Current
IQ
LX no switching
--
2.7
--
mA
Shutdown Current
ISHDN
VIN = 3.8V, EN = 0V
--
--
1
A
Under-Voltage Lockout Threshold
Under-Voltage Lockout Hysteresis
Over-Temperature Protection
Threshold
VUVLO
VUVLO
VIN Rising
---
2.3
200
---
V
mV
TOTP
--
150
--
C
TOTP_HYS
--
20
--
C
Over-Temperature Protection
Hysteresis
Interface Characteristic
Logic-High
VIH
1.4
--
--
V
Logic-Low
VIL
--
--
0.8
V
Internal Pull-Low Current for EN,
PWM
IIH_1
--
--
10
A
Internal Pull-Low Current for SCL,
SDA
IIH_2
--
0.01
1
A
Output Low Level for SDA
VOL
External Pull High Current = 3mA
--
0.3
0.5
V
Output Leakage Current for SDA
ILK_DIO
SDA Pin Voltage = 3.3V
--
--
1
A
--
400
--
kHz
EN, PWM, SCL, SDA
Input Voltage
2
I C Interface Timing
Maximum I2C Clock Frequency
f SC_MAX
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RT8555
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Hold Time for START And
Repeated START Condition
tHD,STA
0.6
--
--
s
SCL Clock Low Time
tLOW
1.3
--
--
s
SCL Clock High Time
tHIGH
600
--
--
ns
Setup Time for A Repeated
START Condition
tSU,STA
600
--
--
ns
SDA Data Hold Time
tHD,DAT
50
--
--
ns
SDA Data Setup Time
tSU,DAT
100
--
--
ns
Rising Time of SDA, SCL
tR
--
--
300
ns
Falling Time of SDA, SCL
tF
--
--
300
ns
Setup Time for STOP Condition
tSU,STO
600
--
--
ns
I C Bus Free Time Between a
STOP and a START
tBUF
1.3
--
--
s
Capacitive Load for I2C Bus
CB
--
--
400
pF
2
Boost Converter
Switching Frequency Accuracy
f SW_ACC
Boost Operates at PWM Mode,
f SW = 600kHz
10
--
10
%
Switching Frequency Setting
Range
f SW_RG
Boost Operates at PWM Mode
0.3
--
2
MHz
Maximum Duty Cycle
DMAX
f SW = 600kHz
--
95
--
%
Boost Switch RDS(ON)
RDS(ON)
--
0.1
0.3
Switching Current Limitation
IOCP
2
2.5
3
A
--
--
1
A
LED Current
Leakage Current of FBx
ILK_FB
VFBx = 36V, IFBx = 0mA
Minimum FBx Regulation Voltage VFB(MIN)
IFBx = 20mA
0.3
--
--
V
Maximum LED Current Setting
IFB(MAX)
LED 100% Setting
10
--
35
mA
Minimum LED Current Setting
IFB(MIN)
Setting By Dimming
0.2
--
--
mA
LED Current Accuracy
IFB_ACC
PWM Duty = 100%, IFBx = 20mA
3
--
3
%
LED Current Matching
IFB_MAT
PWM Duty = 100%, IFBx = 20mA
2
--
2
%
FBx Channel Unused Threshold
VFB_UNUSE
--
0.1
--
V
Light Bar Open Threshold
VFB_OPEN
--
0.1
--
V
PWM Minimum On Duty
DPWM_MIN
0.8
--
--
%
PWM Dimming Frequency = 20kHz
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may
affect device reliability.
Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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RT8555
Typical Application Circuit
VIN
2.7V to 24V
L1
6.8µH
CIN
4.7µF
R1
10
C1, D1
C1
1µF
VIN
D1
A1, B1
LX
A2, B2
PGND
RT8555
D2 VCP
VOUT C2
COUT
6.8µF
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
CCP
1µF
B4 EN
B3 PWM
MCU
A4 SCL
A3 SDA
FB1
FB2
FB3
FB4
FB5
FB6
E4
D4
C4
E3
E2
E1
GND
C3, D3
Note : For unused channels (FBx), please connect FBx pin to GND.
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RT8555
Timing Diagram
I2C Interface
SDA
VIH (MIN)
VIL (MAX)
tSU, DAT
tSU, STO
tHD, DAT
tHIGH
tLOW
tBUF
SCL VIH (MIN)
VIL (MAX)
tHD, STA
tf
tr
S
P
S
RT8555 I2C slave address = 7'b0110_001. I2C interface support fast mode (bit rate up to 400kb/s). The write or read bit
stream (N ≥ 1) is shown below
Read N bytes from RT8555
Slave Address
Register Address
S
0
1
Data 2
A
Data for Address = m
LSB
MSB
Data N
LSB
A
A
Register Address
S
0
R/W
A
MSB
Data 1
LSB
A
Assume Address = m
P
Data for Address = m + N - 1
Data for Address = m + 1
Write N bytes to RT8555
Slave Address
LSB
A
Assume Address = m
MSB
Data 1
MSB
A Sr
A
R/W
Slave Address
MSB
Data 2
LSB
A
Data for Address = m
MSB
A
Data for Address = m + 1
Data N
LSB
A P
Data for Address = m + N - 1
Driven by Master,
Driven by Slave (RT8555),
P Stop,
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S Start,
Sr Repeat Start
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RT8555
Typical Operating Characteristics
Efficiency vs. Input Voltage
LED Current vs. Input Voltage
100
26
LED1
LED2
LED3
LED4
LED5
LED6
90
24
Output Current (mA)
Efficiency (%)
80
70
60
50
40
30
20
22
20
18
16
10
9LEDs per Channel, fSW = 900kHz, PWM = 3.3V
54LEDs, fSW = 900kHz, PWM = 3.3V
0
14
4
8
12
16
20
24
4
8
12
16
20
24
Input Voltage (V)
Input Voltage (V)
LED Current vs. Temperature
VCP vs. Temperature
26
5
22
4
VCP (V)
LED Current (mA)
24
20
18
3
16
9LEDs per Channel, fSW = 900kHz, PWM = 3.3V
9LEDs per Channel, fSW = 900kHz
14
2
-50
-25
0
25
50
75
100
125
-50
-25
0
Temperature (°C)
25
50
75
100
125
Temperature (°C)
Frequency vs. Code
Frequency vs. Input Voltage
1000
2000
1800
Frequency (kHz)1
Frequency (kHz)1
1600
1400
1200
1000
800
600
400
800
600
400
9LEDs per Channel
9LEDs per Channel
200
200
0
4
8
12
Code
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8
16
4
8
12
16
20
24
Input Voltage (V)
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RT8555
OVP Threshold Voltage vs. Input Voltage
LED Current vs. PWM Duty Cycle
40
OVP Threshold Voltage (V)
20
LED Current (mA)
16
PWM 200Hz
PWM 1kHz
PWM 10kHz
PWM 20kHz
12
8
4
38
36
34
9LEDs per Channel, fSW = 900kHz
0
9LEDs per Channel, fSW = 900kHz
32
0
20
40
60
80
100
4
PWM Duty Cycle (%)
8
12
16
20
24
Input Voltage (V)
Line Transient Response
Quiescent Current vs. Input Voltage
Quiescunt Current (mA)
3.2
3.0
2.8
2.6
VIN
(10V/Div)
2.4
2.2
LX no Switching
I LED
(10mA/Div)
VIN = 5V to 21V, fSW = 900kHz, PWM = 3.3V
2.0
3
7
11
15
19
23
Time (20ms/Div)
27
Input Voltage (V)
Power On from EN
VEN
(3V/Div)
Power Off from EN
VEN
(3V/Div)
VIN = 5V, fSW = 900kHz,
PWM = 3.3V
VOUT
(20V/Div)
VOUT
(20V/Div)
LX
(20V/Div)
LX
(20V/Div)
I LED
(20mA/Div)
I LED
(20mA/Div)
Time (5ms/Div)
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VIN = 5V, fSW = 900kHz,
PWM = 3.3V
Time (10ms/Div)
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RT8555
Application Information
Table 1. Register Map
Slave Address : b0110001
Register
Address
bit7
0x00
MIX-26K
Edge Rate Control
Spread
Spectrum
0x01
10 bit
mode
selection
Over Voltage
Protection Selection
Switching
Current
Limitation
Selection
bit6
bit5
bit4
0x02
bit3
bit2
Mixed Mode
Change Duty
bit0
Default
Value
ILED
Brightness
Selection
Dimming
Mode
Selection
0x4c
Boost Switching Frequency
ILED Current Setting
LDO Regulation
Voltage Setting
0x03
0x04
ILED Brightness LSB Register 1
ILED Brightness LSB Register 2
0x07
0x08
Smart Dither Slope Time Control
0x09
Fade In / Out Time
Control
0x0A
26KHz Mode Division Frequency
0x0B
Smart
Dither
Enable
LED driver
headroom
PWM
DC
Dither Dither
Enable Enable
0x00
0x00
ILED Brightness MSB
Register 2
0x00
Advanced Brightness
Control
0x00
Dither Resolution
0x1C
Soft Start Time Control
0x04
Stop Compensation Duty
0x00
Control
CLK PFM
Function
Enable
0x0E
0x00
0x00
ILED Brightness MSB
Register 1
0x06
0x76
0x92
ILED Brightness Compensation Ratio
0x05
0x0D
bit1
0x00
LED Unused
Check
LED OVP Level
0x00
0x50
0x06
0x51
0x00
Note : Blank part in table is restricted register.
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RT8555
The RT8555 is a general purpose 6-CH LED driver and is
capable of delivering a maximum 35mA LED current. The
IC is a current mode Boost converter integrated with a
2.5A power switch and can cover a wide VIN range from
2.7V to 24V and contains I2C interface for controlling the
dimming mode, operating frequency and the LED current.
The internal 100mΩ, 36V power switch with current-mode
control provides over current protection. The switching
frequency of the RT8555 is adjustable from 300kHz to
2MHz, which allows flexibility between efficiency and
component size.
LED constant current
Boost switching frequency
Slope for brightness changes
Output Current Resolution
Dithering options
Start-up time
Current limit protection
Over voltage protection
LDO regulation voltage setting
Programmable functions include :
PWMO frequencies
Brightness Control by PWM Pin
The RT8555 provide three dimming modes for controlling the LED brightness. The three dimming modes include PWM
mode, DC mode and Mixed mode, and the dimming mode could be set by register 00h.
Table 2. Dimming Control Mode Selection
Address
00h
Bit
Name
Default Value
[0]
Dimming Mode Selection
PWM Mode (B0)
[3:2]
Mixed Mode Change Duty
25% (B11)
[7]
MIX-26K
PWM pin (B0)
Description
B0 : PWM mode
B1 : Mixed mode
B00 : 0%
B01 : 6.25%
B10 : 12.5%
B11 : 25%
R/W
R/W
R/W
B0 : follow PWM pin frequency
B1 : fixed 26kHz
R/W
Note : DC mode = Dimming Mode Selection (00h[0] = B1) + Mixed Mode Change Duty = 0% (00h[3:2] = B00)
PWM Mode
The ON/OFF of the current source is synchronized to the PWM signal. The frequency of LED current is equal to the
PWM input signal.
PWMI
time
ILED Max
ILED
PWMO
Duty
time
0
Figure 1. PWM Dimming
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RT8555
DC Mode
The LED current will have two cycle delay in this mode, while the delay cycles are for average current calculation.
50%75%25%10%50%
PWMI
time
ILED Max
75%
ILED
50%
25%
time
0
Figure 2. DC Dimming
Mixed Mode
In 25% Mixed mode, 25% the PWM input signal and LED current are both delayed by two cycles with additional
variations.
When 25% ≤ PWM duty ≤ 100%, the PWM duty modulated the amplitude of the current. (Same as DC mode)
PWM duty < 25%, the DC dimming will translate to PWM dimming, controlling the PWM duty instead by amplitude.
The LED current is fixed on quarter of LED current setting.
50%75%25%10%50%
PWMI
time
ILED
Max
ILED
75%
50%
25%
0
time
Figure 3. Mixed Mode Dimming
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
00h
[1]
Name
ILED Brightness
Selection
Default Value
PWM Pin
(B0)
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Description
R/W
B0 : depend on the status of PWM pin
B1 : depend on address:04/05h data
R/W
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RT8555
Switching Frequency
The LED driver switching frequency is adjusted by the I2C, 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
00h
[4]
Spread
Spectrum
w/o
(B0)
00h
[6:5]
Edge Rate
Control
Fast (B10)
01h
[3:0]
Boost
Switching
Frequency
900kHz
(0x06h)
Description
Resolution
R/W
B0 : w/o
B1 : w/i
R/W
B00 : Slow
B01 : Normal
B10 : Fast
R/W
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 I2C, which is
shown in the Table 5 below.
Table 5. Current Limit Protection Setting
Address
Bit
Name
Default Value
01h
[4]
Switching Current
Limitation Selection
2.5A
(B1)
Description
B0 : 1.5A
B1 : 2.5A
Resolution
R/W
--
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.
Over Voltage Protection
The RT8555 integrates over voltage protection. The over voltage protection could be set by the I2C, the voltage of over
voltage protection (VOVP) could be selected as the Table 6 below.
Table 6. OVP Voltage Setting
Address
01h
Bit
[6:5]
Name
Over Voltage
Protection
Selection
Default Value
36V
(B11)
Description
Boost output over
voltage protection.
B00 : 25V
B01 : 28V
B10 : 32V
B11 : 36V
Resolution
R/W
--
R/W
When the Boost output voltage rises above the VOVP, the internal switch will be turned off. Once the Boost output voltage
drop below the VOVP, the internal switch will be turned on again. The Boost output voltage can be clamped at the VOVP.
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RT8555
LED Current Setting
The LED current of each channel could be set by I2C command; it is shown in the Table 7.
Table 7. LED Current Setting
Address
02h
Bit
[7:0]
Name
ILED Current
Setting
Default Value
Description
Resolution
R/W
20.04mA
(0x92h)
Control the max current
0x00h : 0mA
0x01h to 0x49h : 10.02mA
0x49h : 10.02mA
0x92h : 20.04mA
0xFFh : 35mA
~0.137mA
(0x49h to 0xFFh)
R/W
When the LED current setting command is below 0x92h, the LED current will be kept at 20.04mA. When the command
is 0x00h, the LED current will be set to 0mA. the maximum LED current setting is 35mA. The one step of LED current
is approximately 0.137mA.
LDO Regulation Voltage Setting
The LDO regulation voltage could be set by the I2C, it is shown in the Table 8.
Table 8. LDO Regulation Voltage Setting
Address
03h
Bit
Name
[6:5]
LDO
Regulation
Voltage
Setting
Default Value
Description
LDO regulation voltage setting
B00: 3.2V
B01: 3.4V
B10: 3.6V
B11:4.6V
3.2V
(B00)
Resolution
R/W
--
R/W
When the LDO regulation voltage setting command is below B00, the LDO regulation voltage will be kept at 3.2V. The
maximum LDO setting is 4.6V. The setting condition is smaller than the input voltage.
Brightness Control by I2C Register
With brightness register control the output current is controlled with 8-bit resolution or 10-bit resolution register bits. It
is shown in the Table 9.
Table 9. Brightness Register
Address
Bit
Name
Default Value
01h
[7]
10 bit mode selection
8bit
(B0)
04h
[7:0]
ILED Brightness LSB
Register 1
0%
(0x00h)
0x00h : 0%
0xFFh : 100%
05h
[1:0]
ILED Brightness MSB
Register 1
0%
(0x00h)
If 01h[7] is 1, need 04h &
05h series write and then
ILED brightness change
06h
[7:0]
ILED Brightness LSB
Register 2
0%
(0x00h)
0x00h : 0%
0xFFh : 100%
07h
[1:0]
ILED Brightness MSB
Register 2
0%
(0x00h)
If 01h[7] is 1, need 06h &
07h series write and then
ILED brightness change
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Description
Resolution
R/W
B0 : 8 bit mode
B1 : 10 bit mode
R/W
~ 0.4%
R/W
R/W
~ 0.4%
R/W
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ILED Brightness Compensation
ILED Brightness compensation which is shown in the Table 10 below.
Table 10. ILED Brightness Compensation
Address
Bit
Name
Default Value
03h
[4:0]
ILED Brightness
compensation ratio
0Bh
[7:0]
Stop compensation
duty
Description
Resolution R/W
No
Compensation ratio.
compensation Formula : ILED / 1023 x {DAC [DAC
(0x00h)
x (1023 DAC) x k] / 1023 / 31}
0x00h
R/W
PWM duty compensation stop ratio
R/W
Note : ILED = ILED Current Setting (02h[7:0])
DAC = PWM pin Duty or ILED Brightness (04h[7:0], 05h[1:0] or 06h[7:0], 07h[1:0])
25
25
Mode : I2C-MIX-26kHz, Change Duty = 0%,
fSW = 900kHz, ILED = 21.55mA, no stop ratio.
20
ILED, K = 0
ILED, K = 02
ILED, K = 04
ILED, K = 08
ILED, K = 10
ILED, K = 1F
15
10
LED Current (mA)
LED Current (mA)
20
Mode : I2C-MIX-26kHz, Change Duty = 0%,
fSW = 900kHz, ILED = 21.55mA, stop ratio = 50%.
15
ILED, K = 0
10
5
5
0
0
ILED, K = 1F, Stop = 80
ILED, K = 1F
0
3F 7E BD FC 13B17A 1B91F8 237 276 2B5 2F4 333 372 3B13F0
0
3F 7E BD FC 13B17A 1B91F8 237 276 2B5 2F4 333 372 3B13F0
Register Code
Register Code
Figure 4. LED Current (Different Compensation Ratio) vs.
Register Code
Figure 5. LED Current vs. Register Code
Advanced Brightness Control
Dimming control is received either from PWM input pin or from I2C register bits which is shown in the Table 11 below.
Table 11. Advanced Brightness Control
Address
Bit
Name
Default Value
Description
Resolution R/W
2
08h
[1:0]
Advanced
Brightness
Control
B00
B00 : PWMO = PWMI or I C (04h, 05h)
2
B01 : PWMO = PWMI multiply I C(04h, 05h)
2
or I C(04h, 05h)
2
B10 : PWMO = PWMI multiply I C(06,07h)
2
2
or I C(04h, 05h) multiply I C (06, 07h)
B11 : same as B10
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RT8555
Table 12. Brightness Control Table
B
Control2
B
Control1
pwm/26K
Mix Mode
Duty
PWM/I2C
PWM/MIX
08h[1]
08h[0]
00[7]
00h[3:2]
00h[1]
00h[0]
PWM
0
0
0
Don’t care
0
PWM*I2C
0
1
0
Don’t care
I2C-PWM-26K
0
0
1
I2C-DC-26K
0
0
1
ILED Max
PWMO
Duty
0
PWMI
PWMI
0
0
PWMI*
04h, 05h
PWMI
Don’t care
1
0
04h, 05h
04h,
05h
00
1
1
04h, 05h
Register
MODE
LED Driver Headroom
The LED driver headroom could be set by the I2C, it is shown in the Table 13.
Table 13. LED Driver Headroom
Address
08h
Bit
[3:2]
Name
LED driver
headroom
Default Value
B00
Description
LED driver headroom
B00 : 500mV
B01 : 570mV
B10 : 600mV
B11 : 700mV
Resolution
R/W
--
R/W
The RT8555 detects all FBx voltage and selects a minimum voltage to EA (Error Amplifier). When the LED driver
headroom command is below B00, the LED driver headroom will be kept at 500mV and VOUT can be boost to the highest
forward voltage of LED strings. This function can guarantee the highest of FB pin voltage is 700mV.
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Fade IN / OUT Time Control
The fade in / out time control could be set by the I2C, it is shown in the Table 14.
Table 14. Fade In / Out Time Control
Address
Bit
Name
Default Value
09h
[6:5]
Fade IN /
OUT Time
Control
B00
Description
DC mode fade time control
B00 : 0.5s
B01 : 1s
B10 : 2s
B11 : 4s
Resolution
R/W
--
R/W
Fade in / out time can be control by address 09h[6:5], there are four brightness times that adjust range from 0.5μs to
4μs. When the fade in/out command is below B00, the brightness time of per step will be kept at 0.5μs. This function
can guarantee the highest of fade in/out time is 4μs. The Figure 6 shows the fade in time at 10 bit resolution. The
Figure 7 shows the fade out time at 10 bit resolution.
Fade Out Time (Dimming Down)
50.3
50.2
50.2
LED Current (%)
LED Current (%)
Fade In Time (Dimming Up)
50.3
50.1
50.0
49.9
50.0
49.9
49.8
49.8
0
1
2
3
4
Time (µs)
Figure 6. LED Current (Dimming Up) vs. Fade IN Time
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5
0
1
2
3
4
5
Time (µs)
Figure 7. LED Current (Dimming Down) vs. Fade Out
Time
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RT8555
Soft-Start Time Control
The soft-start time control could be set by the I2C, it is shown in the Table 15.
Table 15. Soft Start Time Control
Address
Bit
0Ah
[1:0]
Name
Default Value
Soft-Start
Time Control
Description
Resolution
R/W
--
R/W
Soft start time control
B00 : x1
B01 : x2
B10 : x4
B11 : x8
B00
Soft-start time can be control by address 0Ah[1:0], there are four soft start times that adjust range from 1 time to 8 time.
When the command is below B00, the soft start time will be kept at 1 time. This function can guarantee the highest of
soft start time is 8 time. The Figure 8 shows the soft start time at power on. The Figure 9 shows the soft start time at
power off.
Soft-Start Time (Dimming Up)
Soft Start Time (Dimming Down)
100
100
1 Time
2 Time
90
90
80
4 Time
70
LED Current (%)
LED Current (%)
80
8 Time
60
50
40
30
70
60
50
40
30
20
20
10
10
4 Time
8 Time
1 Time
2 Time
0
0
0
1280
2560
3840
5120
6400
7680
8960
Time (µs)
Figure 8. LED Current (Dimming Up) vs. Soft Start Time
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0
1280
2560
3840
5120
6400
7680
8960
Time (µs)
Figure 9. LED Current (Dimming Down) vs. Soft Start
Time
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Smart Dither Slope Time Control
The smart dither slope time control could be set by the I2C, it is shown in the Table 16.
Table 16. Smart Dither Slop Time Control
Address
Bit
Name
Default Value
08h
[6:4]
Smart Dither
Slope Time
Control
B000
09h
[4:4]
Smart Dither
Enable
B1
Description
Resolution
R/W
Slope time control
B000 : 7.8ms
B001 : 7.8ms
B010 : 15.625ms
B011 : 31.25ms
B100 : 62.5ms
B101 : 125ms
B110 : 250ms
B111 : 500ms
--
R/W
Smart dither enable
B0 : Disable
B1 : Enable
--
R/W
Smart dither slope time can be control by address 08h[6:4], there are many difference brightness times that adjust
range from 7.8ms to 500ms. When the smart dither slope command is below B000, the slope time that is the dimming
duty from 0% to 100% will be kept at 7.8ms. This function can guarantee the highest of slope time is 500ms. The
resolution is shown in Table 17.
Table 17. Smart Dither Resolution
Time (period T)
10 bit mode
0T~1T
1T~2T
2T~3T
3T~4T
Dimming Duty (%)
0~
1.5625%
1.5625%
~3.125%
3.125%
~6.25%
6.25%
~12.5%
Resolution (step)
1024
512
512
512
512
512
511
Slope Time Setting
7.8ms
1.114ms
1.114ms
1.114ms
1.114ms
1.114ms
1.114ms
1.114ms
Smart Dither Slope Control (Dimming Up)
12.5%
~25%
90
80
80
70
60
50
40
30
50%
~100%
0% to 100% total 7T
70
60
50
40
30
20
20
10
10
0
6T~7T
25%
~50%
100
0% to 100% total 7T
90
0
0
1
2
3
4
5
6
Time (period T)
Figure 10. LED Current (Dimming Up) vs. Time
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5T~6T
Smart Dither Slope Control (Dimming Down)
LED Current (%)
LED Current (%)
100
4T~5T
June 2015
7
0
1
2
3
4
5
6
7
Time (period T)
Figure 11. LED Current (Dimming Down) vs. Time
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RT8555
Dither Resolution Control
The dither resolution control could be set by the I2C, it is shown in the Table 18.
Table 18. Dither Slop Time Control
Address
Bit
Name
Default Value
09h
[1:0]
Dither
Resolution
B00
09h
[2:2]
PWM Dither
Enable
B1
09h
[3:3]
DC Dither
Enable
B1
Description
Dither resolution
B00 : 0 bit
B01 : 1 bit
B10 : 2 bit
B11 : 3 bit
PWM dither enable
B0 : Disable
B1 : Enable
DC dither enable
B0 : Disable
B1 : Enable
Resolution
R/W
--
R/W
--
R/W
--
R/W
Dither resolution can be control by address 09h[1:0], there are four kind of dither resolution that are from 0 bit to 3 bit.
When the command is below B00, the dither resolution that is the dimming duty from 0% to 100% will be kept at 0 bit.
This function can guarantee the highest of dither resolution is 3 bit. The dither resolution is shown in Table 19.
Table 19. Dither Resolution
Dimming Duty (%)
0~
1.5625%~ 3.125%
6.25%
Resolution
1.5625% 3.125%
~6.25% ~12.5%
(step) x Step Time(s)
No DC dither
16 x 1s 16 x 1s 32 x 1s 64 x 1s
09h[3:3] = 0
128 x
0bit dither
32 x 16s 32 x 16s 64 x 8s
4s
256 x
1bit dither
64 x 16s 64 x 16s 128 x 8s
4s
512 x
2bit dither
128 x 16s 128 x 16s 256 x 8s
4s
512 x
3bit dither
256 x 16s 256 x 16s 512 x 8s
8s
12.5%
~25%
128 x 1s
256 x 2s
512 x 1s
512 x 4s
512 x 8s
25%
~50%
50%
~100%
256 x
1s
512 x
1s
512 x
2s
512 x
4s
512 x
8s
Total Time (s)
(0%~100%)
511 x
1s
511 x
1s
511 x
2s
511 x
4s
511 x
8s
1023s
3583s
7166s
14332s
28664s
Note : Fade Time is 1s (09h[6:5] = B01)
Dither Resolution (Dimming Up)
100
No Dither
0Bit
1Bit
80
90
80
70
60
50
40
2Bit
30
3Bit
LED Current (%)
90
LED Current (%)
Dither Resolution (Dimming Down)
100
70
60
50
30
20
20
10
10
0
No Dither
0Bit
1Bit
40
2Bit
3Bit
0
0
4200
8400
12600 16800 21000 25200 29400
Time (µs)
Figuer 12. LED Current (Dimming Up) vs. Time
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0
4200
8400
12600 16800 21000 25200 29400
Time (µs)
Figuer 13. LED Current (Dimming Down) vs. Time
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26KHz Mode Division Frequency
The 26KHz mode division frequency could be set by the I2C, it is shown in the Table 20.
Table 20. 26KHz Mode Division Frequency
Address
Bit
0Ah
[7:5]
Name
26KHz mode
division
frequency
Default Value
B000
Description
PWMO frequency
B000 : 26KHz
B001 : 26KHz/2
B010 : 26KHz/4
B011 : 26KHz/8
B1xx : 26KHz/16
Resolution
R/W
--
R/W
The 26kHz mode division frequency can be control by address 0Ah[7:5], there are five kind of division frequency that
contain 26KHz, 26KHz/2, 26KHz/4, 26KHz/8 and 26KHz/16. When the command is below B000, the PWMO frequency
that is the 26kHz mode will be kept at 26KHz. This function can guarantee the most of division frequency is division 16
time.
Control CLK PFM Function Enable
The CLK PFM function enable could be set by the I2C, it is shown in the Table 21.
Table 21. Control CLK PFM Function Enable
Address
Bit
0Dh
[7:7]
Name
Control CLK
PFM function
enable
Default Value
B0
Description
Resolution
R/W
--
R/W
Control CLK PFM function
enable
B0 : off
B1 : on
The CLK PFM function enable can be control by address 0Dh[7:7]. If the bit equals to 0, it means the boost switching
frequency just depends on the switching frequency setting. Otherwise, if the bit equals to 1, the boost switching
frequency will be decreased, when the boost on time is lower than the minimum on time.
LED Protection
RT8555 has LED protection for LED OVP level, LED unused. The LED protection could be set by the I2C, it is shown in
the Table 22.
Table 22. LED Protection
Address
Bit
Name
Default Value
0Eh
[3:2]
LED OVP
level
B00
0Eh
[5:5]
LED unused
check
B0
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Description
LED OVP level
B00 : 2.1V
B01 : 2.52V
B10 : 2.8V
B11 : 3.5V
LED unused check
B0 : Disable
B1 : Enable
Resolution
R/W
--
R/W
--
R/W
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RT8555
LED OVP level
Over Temperature Protection
The LED OVP level can be control by address 0Eh[3:2],
there are four kind of LED OVP level that is from 2.1V to
3.5V. When the command is below B00, the LED OVP
level that is the minimum FBx voltage up to the target
level will be kept at 2.1V. This function can guarantee the
highest of LED OVP level is 3.5V. When the minimum
FBx voltage rises above the LED OVP level setting, the
internal switch will be turned off. Once the minimum FBx
voltage drops below the LED OVP level setting, the internal
switch will be turned on again. The minimum FBx voltage
can be clamped at the LED OVP level setting.
The RT8555 has over temperature protection function to
prevent the IC from overheating due to excessive power
dissipation. The OTP function will shutdown the IC when
junction temperature exceeds 150°C (typ.). When junction
temperature is cool down to 130°C (TOTP_hys = 20°C ), the
LED driver will return to normal work.
LED unused check
The LED unused check can be control by address
0Eh[5:5]. If the bit equals to 0, it means the function
disable. Otherwise, if the bit equals to 1, the function
enable, and the internal pulled current of the FBx pin will
be turned off. The FBx pin should be connected to GND.
This channel is detected as unused channel and latch. If
the un-used channel is not connected to GND, and the
FBx level is low to 100mV. It means open LED protection.
LED Connection
The RT8555 equips 6-CH LED drivers and each channel
supports up to 10 LEDs. The LED strings are connected
from the output of the boost converter to pin FBx (x = 1 to
6) respectively. If one of the current sink channels is not
used, the FBx pin should be connected to GND. If the unused channel is not connected to GND, it will be
considered that the LED string is opened; the channel
will turn light when the LED string is recovering connected.
Open LED Protection
If the FBx pin voltage is low to 0.1V, the LED driver will
judge the channel to be open. The FBx pin voltage will not
be regulated and not latch, until the FBx pin is recovery
connected, the FBx pin will normal work again. If all FBx
pin are open (floating), the output voltage will be clamped
to the setting voltage of OVP (VOUT(OVP)).
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Inductor Selection
The value of the inductance, L, can be approximated by
the following equation, where the transition is from
Discontinuous Conduction Mode (DCM) to Continuous
Conduction Mode (CCM) :
L
D (1 D)2 VOUT
2 fOSC IOUT
The duty cycle, D, can be calculated as the following
equation :
V
VIN
D OUT
VOUT
Where VOUT is the maximum output voltage, VIN is the
minimum input voltage, fOSC is the operating frequency,
and IOUT is the sum of current from all LED strings. The
boost converter operates in DCM over the entire input
voltage range when the inductor value is less than this
value, L. With an inductance greater than L, the converter
operates in CCM at the minimum input voltage and may
be discontinuous at higher voltages.
The inductor must be selected with a saturated current
rating that is greater than the peak current as provided by
the following equation :
IPEAK
VOUT IOUT VIN D TOSC
VIN
2L
where η is the efficiency of the power converter.
Diode Selection
Schottky diodes are recommended for most applications
because of their fast recovery time and low forward voltage.
Power dissipation, reverse voltage rating, and pulsating
peak current are important parameters for consideration
when making a Schottky diode selection. Make sure that
the diode's peak current rating exceeds IPEAK and reverse
voltage rating exceeds the maximum output voltage.
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Input Capacitor Selection
The ceramic capacitors are recommended for input
capacitor applications. Low ESR will effectively reduce
the input voltage ripple caused by switching operation.
Two 10μF/25V capacitors are sufficient for most
applications. Nevertheless, this value can be decreased
for lower output current requirement. Another consideration
is the voltage rating of the input capacitor must be greater
than the maximum input voltage.
ΔIL
Input Current
Output Current
Time
(1-D)TS
Output Capacitor Selection
Output ripple voltage is an important index for estimating
the performance. This portion consists of two parts, one
is the ESR voltage of output capacitor, another part is
formed by charging and discharging process of output
capacitor. Refer to Figure 14, evaluate ΔVOUT1 by ideal
energy equalization. According to the definition of Q, the
Q value can be calculated as following equation :
1
1
1
Q IIN IL IOUT IIN IL IOUT
2
2
2
VIN
1
COUT VOUT1
VOUT fOSC
where fOSC is the switching frequency, and ΔIL is the
inductor ripple current. Move COUT to the left side to
estimate the value of ΔVOUT1 as the following equation :
D IOUT
VOUT1
COUT fOSC
Then, take the ESR into consideration, the ESR voltage
can be determined as the following equation :
V D TOSC
I
VESR OUT IN
RESR
2L
1 D
Finally, the total output ripple ΔVOUT is combined from the
ΔVOUT1 and ΔVESR. In the general application, the output
capacitor is recommended to use a 4.7μF/50V electrolytic
capacitor.
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Inductor Current
Output Ripple
Voltage (ac)
Time
ΔVOUT1
Figure 14. The Output Ripple Voltage without the
Contribution of ESR
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
WL-CSP-20B 1.65x2.05 (BSC) package, the thermal
resistance, θJA, is 36.7°C/W on a standard JEDEC 51-7
four-layer thermal test board. The maximum power
dissipation at TA = 25°C can be calculated by the following
formula :
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PD(MAX) = (125°C − 25°C) / (36.7°C/W) = 2.72W for
WL-CSP-20B 1.65x2.05 (BSC) package
Maximum Power Dissipation (W)1
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 15 allows
the designer to see the effect of rising ambient temperature
on the maximum power dissipation.
Layout Consideration
For good regulation, place the power components as
close to the IC as possible. The traces should be wide
and short, especially for the high current output loop.
The input and output bypass capacitor should be placed
as close to the IC as possible and connected to the
ground plane of the PCB.
Minimize the size of the L nodes and keep traces wide
and short. Care should be taken to avoid running traces
that carry any noise-sensitive signals near LX or highcurrent traces.
Separate power ground (PGND) and ground
(GND).Connect the GND and the PGND islands at a
single end. Make sure that there are no other
connections between these separate ground planes.
Connect the exposed pad to a strong ground plane for
maximum thermal dissipation.
3.0
Four-Layer PCB
2.5
2.0
1.5
1.0
0.5
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
CIN
Figure 15. Derating Curve of Maximum Power
Dissipation
D1
RIN
CPUMP
FB6
VIN
VIN
LX
LX
FB5
VCP
VOUT
PGND
PGND
FB4
GND
GND
PWM
SDA
FB1
FB2
FB3
EN
L1
COUT1
I2C, Data
I2C, Clock
PWM dimming
LED light bar
Chip enable
CIN1
CIN2
SCL
COUT2
Battery/Adapter
Figure 16. PCB Layout Guide
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
www.richtek.com
24
is a registered trademark of Richtek Technology Corporation.
DS8555-00
June 2015
RT8555
Outline Dimension
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min.
Max.
Min.
Max.
A
0.500
0.600
0.020
0.024
A1
0.170
0.230
0.007
0.009
b
0.220
0.280
0.009
0.011
D
2.000
2.100
0.079
0.083
D1
E
1.600
1.600
0.063
1.700
0.063
0.067
E1
1.200
0.047
e
0.400
0.016
20B WL-CSP 1.65x2.05 Package (BSC)
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
DS8555-00
June 2015
www.richtek.com
25