®
RT8561A
High Voltage 8-CH LED Driver
General Description
Features
The RT8561A is a 40V 8-CH LED driver capable of delivering
30mA to each channel with 10 LEDs (3.6V per diode),
total of 80 LEDs with one driver. The RT8561A is a current
mode boost converter operated at 1MHz, wide VIN range
covers from 6V to 24V and the on-chip current switch is
rated at 2.5A.
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Applications
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UMPC and Notebook Computer Backlight
GPS, Portable DVD Backlight
Desk Lights and Room Lighting
Pin Configurations
Note :
Lead Plating System
G : Green (Halogen Free and Pb Free)
Richtek products are :
`
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
`
GND
SW
SW
GND
21
20
19
18
VCC
LED2
2
17
CREG
LED3
3
16
LED8
LED4
4
15
LED7
LED5
5
14
LED6
OVP
6
13
EN
GND
25
7
8
9
10
11
12
DCTL
Package Type
QW : WQFN-24L 4x4 (W-Type)
22
ACTL
RT8561A
23
1
RISET
Ordering Information
24
LED1
SS
The RT8561A is packaged with a tiny footprint package of
WQFN-24L 4x4 packages.
GND
(TOP VIEW)
GND
Other protecting features include programmable output
over voltage protection, LED current limit, PWM switch
current limit and thermal shutdown.
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VC
The 1.5% matched LED currents on all channels are simply
programmed with a resistor or a current sink. Both analog
dimming and digitally controlled PWM dimming are
supported by the RT8561A. Analog dimming is linearly
controlled by an external voltage. With an on-chip output
clamping amplifier and a 300kΩ resistor, PWM dimming
signal is easily low-pass filtered to an analog dimming
signal with one external capacitor for noise-free PWM
dimming. A very high contrast ratio true digital PWM
dimming can be achieved by driving ACTL pin with a PWM
signal.
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NC
The PWM output voltage loop regulates the LED pins to
0.6V with an auto-adjustment circuit allowing voltage
mismatches between LED strings. The RT8561A automatically detects and disconnects any unconnected and/or
broken strings during operation from PWM loop to prevent
VOUT from over voltage.
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High Voltage : VIN up to 24V, VOUT up to 40V, Driving
up to 80 x 3.6V LEDs (10 each channel)
Channel Current Programmable 10mA to 30mA and
Matched to 1.5%
Current Mode PWM 1MHz Boost Converter
Easy Analog and Digital Dimming Control
Programmable Soft Start
Automatic Detecting Unconnected and/or Broken
Channel
Programmable Over Voltage Protection
Disconnects LED in Shutdown
Providing High Accuracy Digital Dimming by PWM
at ACTL Pin
VIN Under Voltage Lockout
Over Temperature Protection
Current Limiting Protection
Small 24-Lead WQFN Package
RoHS Compliant and Halogen Free
WQFN-24L 4x4
Marking Information
CZ= : Product Code
CZ=YM
DNN
YMDNN : Date Code
Suitable for use in SnPb or Pb-free soldering processes.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8561A-02 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT8561A
Typical Application Circuit
VIN
6V to 24V
VOUT
40V MAX
L
10µH
D
COUT
10µF
CVIN1
10µF
RVIN1
10
:
:
:
:
RT8561A
19, 20 SW
18
CVIN1
2.2µF
1k
5V
R1
VOUT
LED1 1
2
LED2
VCC
OVP
LED7
... ::
:
:
:
: 10 LED String
:
:
.
.
.
13 EN
6
:
:
:
:
15
LED8 16
R2
17
4.7µF
0.1µF
RVC
1.8k
CREG
RISET
9 SS
7
VC
12
DCTL
GND
ACTL
RISET
4.75k
10
ILED (mA) = 20 x 4.75
RISET (kΩ)
21, 22, 23, 24, 25 (Exposed Pad)
1k
11
PWM Signal
CVC
3.9nF
Figure 1. 1MHz, 20mA Full Scale Current PWM Dimming Control
VIN
6V to 24V
VOUT
40V MAX
L
10µH
D
COUT
10µF
CVIN1
10µF
RVIN1
10
:
:
:
:
RT8561A
19, 20 SW
18
CVIN1
2.2uF
1k
5V
R1
VOUT
LED1 1
2
LED2
VCC
13 EN
6
OVP
LED7
17 CREG
RISET
9 SS
0.1µF
RVC
1.8k
CVC
3.9nF
... ::
:
:
:
: 10 LED String
:
:
.
.
.
LED8 16
R2
4.7µF
15
:
:
:
:
7
VC
12
DCTL
GND
ACTL
10
RISET
4.75k
ILED (mA) = 20 x 4.75
RISET (kΩ)
21, 22, 23, 24, 25 (Exposed Pad)
11
1k
Analog
Dimming
DCTL can be floating or
connected to ACTL
Figure 2. 1MHz, 20mA Full Scale Current Analog Dimming Control
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS8561A-02 April 2012
RT8561A
VIN
6V to 24V
VOUT
40V MAX
L
10µH
D
COUT
10µF
CVIN1
10µF
RVIN1
10
:
:
:
:
RT8561A
19, 20 SW
18
CVIN1
2.2µF
1k
5V
LED1 1
2
LED2
VCC
6
OVP
LED7
R2
... ::
:
: 10 LED String
:
:
:
:
.
.
.
13 EN
R1
VOUT
:
:
:
:
15
LED8 16
17
4.7µF
CREG
RISET
RISET
4.75k
10
ILED (mA) = 20 x 4.75
RISET (kΩ)
9 SS
0.1µF
7
RVC
1.8k
1k 12
VC
GND
DCTL
ACTL
21, 22, 23, 24, 25 (Exposed Pad)
11
0.1µF
CVC
3.9nF
PWM Signal
Figure 3. 1MHz, 20mA Full Scale Current PWM to Analog Dimming Control
VIN
4V to 36V
VOUT
40V MAX
L
10µH
D
COUT
10µF
CVIN1
10µF
VAVDD
6V to 24V
19, 20 SW
RVIN1
10
18
CVIN1
2.2µF
VOUT
1k
5V
R1
VCC
6
OVP
CREG
9 SS
RVC
1.8k
LED1 1
2
LED2
LED7
15
:
:
:
:
... ::
:
:
:
: 10 LED String
:
:
.
.
.
LED8 16
17
0.1µF
RT8561A
13 EN
R2
4.7µF
:
:
:
:
7
12
VC
DCTL
CVC
3.9nF
RISET
GND
ACTL
10
RISET
4.75k
ILED (mA) = 20 x 4.75
RISET (kΩ)
21, 22, 23, 24, 25 (Exposed Pad)
11
PWM Signal
Note :
1. Due to the limitation of maximum duty 5V input can support typically to VOUT = 33V.
2. Due to the limitation of maximum duty 4V input can support typically to VOUT = 26V.
Figure 4. Wide Range VIN Application by Connecting VCC Pin to LCD Driver Power AVDD
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8561A-02 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
3
RT8561A
Function Block Diagram
SW
OSC
VCC
-
5.6V
LED1
S
+
+
R
OVP
1.2V
-
+
-
EN
LED2
-
1.6V
CREG
-
R
+
+
+
Shutdown
-
5V
LDO
VOUT
Regulation
Unit
VC
LED7
+
5V
-
5uA
SS
LED8
+
+
1.2V
DCTL
+
-
-
300k
-
ACTL
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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4
RISET
is a registered trademark of Richtek Technology Corporation.
DS8561A-02 April 2012
RT8561A
Functional Pin Description
Pin No.
Pin Name
Pin Function
1, 2, 3, 4, 5
LED1, LED2,
LED3, LED4,
LED5
Channel 1 to Channel 5 LED current sink. Leave the pin unconnected if not
used.
6
OVP
Over Voltage Protection. PWM boost converter turns off when VOVP goes
higher than 1.2V.
7
VC
PWM boost converter loop compensation node.
8
NC
No Internal Connection.
9
SS
Soft Start Pin, a capacitor of at least 10nF is required for soft start.
10
RISET
A resistor or a current from DAC on this pin programs the full LED current.
11
ACTL
Analog/Digital dimming control. When using analog dimming,
20 x 4.75
ILED (mA) =
for VACTL ≥ 1.2V.
RISET (kΩ)
12
DCTL
By adding a 0.1μF filtering capacitor on the ACTL pin, the PWM dimming
signal on the DCTL pin will be averaged out and converted into analog
dimming signal on the ACTL pin.
13
EN
Chip enable pin, when pulled low, chip is in shutdown mode.
LED6, LED7,
LED8
Channel 6 to Channel 8 LED current sink. Leave the pin unconnected if not
used.
17
CREG
1μF capacitor should be placed on this pin to stabilize the 5V output of the
internal regulator. This regulator is for chip internal use only.
18
VCC
Power supply of the chip. For good bypass, a low ESR capacitor is required.
19, 20
SW
PWM boost converter switch node.
GND
Ground pin of the chip. The exposed pad must be soldered to a large PCB
and connected to GND for maximum power dissipation.
14, 15, 16
21, 22, 23, 24,
25 (Exposed Pad)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8561A-02 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
RT8561A
Absolute Maximum Ratings
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(Note 1)
Supply Voltage, VCC ----------------------------------------------------------------------------------------------------SW Pin Voltage at Switching Off -------------------------------------------------------------------------------------LED1 to LED8 Pin --------------------------------------------------------------------------------------------------------DCTL, ACTL, EN, OVP Pin Voltage ---------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
WQFN-24L 4x4 -----------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
WQFN-24L 4x4, θJA ------------------------------------------------------------------------------------------------------WQFN-24L 4x4, θJC -----------------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Mode) ---------------------------------------------------------------------------------------------MM (Machine Mode) ------------------------------------------------------------------------------------------------------
Recommended Operating Conditions
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−0.3V to 28V
−0.3V to 45V
−0.3V to 35V
−0.3V to 5.5V
1.923W
52°C/W
7°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------- 6V to 24V
Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VCC = 17V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Supply Current
I VCC
VIN Under Voltage Lockout
Threshold
VUVLO
Shutdown Current
I SHDN
EN Threshold
Voltage
Test Conditions
Min
Typ
Max
Unit
VC ≤ 0.2V (Switching off)
--
3
5
mA
VIN Rising
--
5.6
5.95
VIN Falling
--
4.9
--
VEN ≤ 0.7V
--
--
10
Logic-High
VEN_H
1.6
--
5
Logic-Low
VEN_L
--
--
1
V
μA
V
PWM Threshold Logic-High
Voltage
Logic-Low
VPWM_H
1.3
--
5
VPWM_L
--
--
0.15
EN Pin Input Current
I EN
VEN ≤ 5V
--
--
0.1
μA
I LED
2V > VLED > 0.6V, RISET = 4.75kΩ,
VACTL > 1.2V
19
20
21
mA
2V > VLED > 0.6V, RISET = 4.75kΩ,
VACTL > 1.2V, Calculating
(I(MAX) − I(MIN)) / I Average x 100%
--
--
1.5
%
1.17
1.2
1.23
V
--
1
2
μA
V
LED Current Programming
LED Current
LEDs Current Matching
RISET Pin Voltage
VRISET
3.6kΩ ≤ RISET ≤ 9.6kΩ, VACTL > 1.2V
Input Current of ACTL
I ACTL
0.3V ≤ VACTL ≤ 1.2V
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS8561A-02 April 2012
RT8561A
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Threshold of ACTL
VACTL
LED Current Off
--
0.2
--
V
Input Current of DCTL
IDCTL
0.3V ≤ VDCTL ≤ 5.5V
--
1
2
μA
Un-connection
--
0.1
--
V
0.8
1
1.2
MHz
--
100
--
ns
0.5
0.6
0.7
V
VLED Threshold
PWM Boost Converter
Switching Frequency
Minimum On Time
Regulated VLED
Highest Voltage LED String
Amplifier (gm) Output Current
2.4V > VC > 0.2V
--
±15
--
μA
VC Threshold
PWM Switch Off
0.1
0.2
--
V
--
0.3
0.5
Ω
2.5
--
--
A
80
88
--
%
1.1
1.2
1.3
V
SW RDS(ON)
SW Current Limit
ILIM
SW Maximum Duty
OVP & Soft-Start
OVP Threshold
VOVP
OVP Input Current
IOVP
VOVP ≤ 3V
--
--
50
nA
Soft-Start Current
ISS
VSS ≤ 2.5V
3
5
8
μA
--
150
--
°C
--
20
--
°C
Thermal Shutdown
Temperature
Thermal Shutdown Hysteresis
TSD
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. θJC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8561A-02 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
7
RT8561A
Typical Operating Characteristics
LED Current vs. Input Voltage
Efficiency vs. Input Voltage
26
100
90
24
80LEDs
LED Current (mA)
Efficiency (%)
80
70
60
50
40
30
20
22
20
LED1
LED2
LED3
LED4
LED5
LED6
LED7
LED8
18
16
14
12
10
10
0
8
10
12
14
16
18
20
22
8
24
10
12
16
18
20
22
24
VRISET vs. Temperature
LED Current vs. Temperature
24
1.24
23
1.22
22
1.20
21
VRISET (V)
LED Current (mA)
14
Input Voltage (V)
Input Voltage (V)
20
19
1.18
1.16
1.14
18
1.12
17
VIN = 12V
16
-50
-25
0
25
50
75
100
VIN = 12V
1.10
-50
125
-25
Temperature (°C)
0
25
50
75
100
125
Temperature (°C)
VRISET vs. Input Voltage
LED Current vs. ACTL PWM Duty Cycle
25
1.25
1.24
20
LED Current (mA)
V RISET (V)
1.23
1.22
1.21
1.20
1.19
15
ACTL = 200Hz
ACTL = 1kHz
ACTL = 10kHz
PWM = 30kHz
10
5
1.18
1.17
ACTL = 0V to 3V, VIN = 12V
0
8
10
12
14
16
18
20
22
Input Voltage (V)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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8
24
0
0
0.1
10
0.2
1
20 0.3
30 0.4
40 0.5
50 0.6
60 0.7
70 0.8
80 0.9
90 100
Duty Cycle (%)
is a registered trademark of Richtek Technology Corporation.
DS8561A-02 April 2012
RT8561A
LED Current vs. ACTL Analog Voltage
LED Current vs. DCTL PWM Duty Cycle
28
25
24
LED Current (mA)
LED Current (mA)
20
DCTL = 200Hz
DCTL = 1kHz
DCTL = 10kHz
15
10
20
16
12
8
5
4
DCTL = 0V to 3V, VIN = 12V
0
0
10
20
30
40
50
60
70
80
90
VIN = 12V
0
0
100
0.25
Frequency vs. Input Voltage
1
1.25
1.5
Shutdown Current (µA)
10
1060
1020
980
940
8
6
4
2
VEN = 0V
0
900
8
10
12
14
16
18
20
22
8
24
10
12
14
16
18
20
22
24
Input Voltage (V)
Input Voltage (V)
SS Current vs. Temperature
Supply Current vs. Input Voltage
4.0
8.0
3.9
7.6
3.8
7.2
3.7
6.8
SS Current (µA)
Switch Off Current (mA)
0.75
Shutdown Current vs. Input Voltage
1100
Frequency (kHz)
0.5
ACTL Analog Voltage (V)
Duty Cycle (%)
3.6
3.5
3.4
3.3
6.4
6.0
5.6
5.2
4.8
3.2
3.1
VC = 0V
3.0
8
10
12
14
16
18
20
22
Input Voltage (V)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8561A-02 April 2012
24
4.4
VIN = 12V, CSS = 0.1μF
4.0
-50
-25
0
25
50
75
100
125
Temperature (°C)
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9
RT8561A
SS Current vs. Input Voltage
OVP Voltage vs. Temperature
1.30
9
1.28
1.26
7
OVP Voltage (V)
SS Current (µA)
8
6
5
4
1.24
1.22
1.20
1.18
1.16
1.14
3
1.12
CSS = 0.1μF
2
8
10
12
14
16
18
20
22
VIN = 12V
1.10
-50
24
-25
0
50
75
100
125
Temperature (°C)
Input Voltage (V)
ACTL Off Threshold Voltage vs. Temperature
OVP Voltage vs. Input Voltage
0.22
ACTL Off Threshold Voltage (V)
1.3
1.2
OVP Voltage (V)
25
1.1
1.0
0.9
0.8
0.21
0.20
0.19
0.18
0.17
0.16
0.15
VIN = 12V
0.14
8
10
12
14
16
18
20
22
24
-50
-25
0
25
50
75
100
Input Voltage (V)
Temperature (°C)
Power On from EN
Line Transient Response
125
VIN = 10.8V to 13.2V
VEN
(2V/Div)
VIN
(5V/Div)
VOUT
(20V/Div)
I IN
(500mADiv)
VIN = 12V, CSS = 0.1μF
Time (10ms/Div)
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10
IOUT
(100mA/Div)
Time (50ms/Div)
is a registered trademark of Richtek Technology Corporation.
DS8561A-02 April 2012
RT8561A
OVP
R1 = 3.3MΩ,
R2 = 100kΩ
VIN
(10V/Div)
VOUT
(20V/Div)
SW
(20V/Div)
VIN = 12V, All LED Pin Open
Time (2.5ms/Div)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8561A-02 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
RT8561A
Applications Information
The RT8561A is a current mode boost converter operated
at 1MHz to power up to 80 white LEDs with a
programmable current for uniform intensity. The part
integrates current sources, soft-start, and easy analog
and digital dimming control. The protection block provides
the circuitry for over temperature, over-voltage and current
limit protection features.
Abnormal Power
on sequence
Power off
sequence
Power on
sequence
VIN
UVLO
PWM
EN
VIN must be turned
off early than EN and
PWM signal
VIN must be
turned on late
than EN and
PWM signal
Input UVLO
The input operating voltage range of the RT8561A is 6V to
24V. An input capacitor at the VIN pin can reduce ripple
voltage. It is recommended to use a ceramic 10μF or larger
capacitance as the input capacitor. This IC provides an
under voltage lockout (UVLO) function to enhance the
stability when startup. The UVLO threshold of input voltage
rising is set at 5.6V and falling is set at 4.9V typically.
No Soft-Start
If PWM turns
on late
Soft-Start
VOUT
Figure 6. Power On Sequence Control by VIN
EN and/or VIN should be
pulled low once PWM pull low
for over 10 ms
EN/VIN
Power Sequence
Please refer to the below Figure 5 and Figure 6. The
recommended power on sequence is that the PWM ready
before EN and/or VIN ready. If not, the soft-start function
will be disabled. As to power off sequence, the EN/VIN
must be pulled low within 10ms to prevent “Hard-Start”
shown as Figure 7.
Power on
sequence
Power off
sequence
Abnormal Power
on sequence
VIN
UVLO
10ms
Figure 7. To Prevent “Hard-Start” Sequence
Soft-Start
The RT8561A employs a soft-start feature to limit the inrush
current. The soft-start circuit prevents the excessive inrush
current and input voltage droop. The soft-start time is
determined by capacitor CSS connected to SS with 5μA
constant current to charge CSS. The value of capacitor
CSS is user-defined to satisfy the designer' requirement.
PWM
EN
PWM
EN must be turned
on late than VIN
and PWM signal
Soft-Start
VOUT
EN must be turned
off early than VIN
and PWM signal
LED Connection
No Soft-Start
If PWM turns
on late
Figure 5. Power On Sequence Control by EN
The RT8561A equips 8-CH LED drivers and each channel
supports up to 10 LEDs. The 8 LED strings are connected
from VOUT to pin 1, 2, 3, 4, 5, 14, 15, and 16 respectively.
If one of the LED channel is not used, the LED pin should
be opened directly.
Setting and Regulation of LED Current
The LED current can be calculated by the following
equation :
ILED (mA) = 20 x 4.75
RISET (kΩ)
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is a registered trademark of Richtek Technology Corporation.
DS8561A-02 April 2012
RT8561A
Where, the RISET is the resister between RISET pin and
GND.
This setting is the reference for the LED current at
LED1-8 and represents the sensed LED current for each
string. The DC/DC converter regulates the LED current
according to the setting.
Brightness Control
The RT8561A features both analog and digital dimming
control. Analog dimming is linearly controlled by an
external voltage (0.3V < VACTL < 1.2V). With an on-chip
output clamping amplifier and a 300kΩ resistor, PWM
dimming signal is easily low-pass filtered to an analog
dimming signal with one external capacitor for noise-free
PWM dimming. A very high contrast ratio true digital PWM
dimming can be achieved by driving ACTL pin with a PWM
signal and the suggested PWM frequency is from 100Hz
to 10kHz. Dimming frequency can be sufficiently adjusted
from 100Hz to 30kHz.However, LED current cannot be
100% proportional to duty cycle especially for high
frequency and low duty ratio because of physic limitation
caused by inductor rising time. Refer to Figure 8 and Table
1.
LED Current vs. ACTL PWM Duty Cycle
25
Table 1.
Dimming Frequency (Hz)
100 < fPWM ≤ 200
200 < fPWM ≤ 500
500 < fPWM ≤ 1k
1k < fPWM ≤ 2k
2k < fPWM ≤ 5k
5k < fPWM ≤ 10k
10k < f PWM ≤ 20k
Duty (Min.)
0.16%
0.40%
0.80%
1.60%
4.00%
8.00%
16.00%
Duty (Max.)
100%
100%
100%
100%
100%
100%
100%
Note : The minimum duty in Table 1 is based on the application
circuit and does not consider the deviation of current linearity.
Over Voltage Protection
The RT8561A equips over voltage protection (OVP)
function. When the voltage at the OVP pin reaches a
threshold of approximately1.2V, the MOSFET drive output
(SW) will turn “OFF”. The MOSFET drive output (SW)
will turn “ON” again once the voltage at OVP drops below
the threshold of approximately 1.2V.
So, the output voltage can be clamped at a certain voltage
level and it can be calculated by the following equation :
VOUT, OVP = VOVP × ⎛⎜ 1+ R1 ⎞⎟
⎝ R2 ⎠
Where
R1 and R2 are the voltage divider connected to OVP pin.
VOVP is typically 1.2V.
LED Current (mA)
20
If at least one string is in normal operation, the controller
will automatically ignore the open strings and continue to
regulate the current for the string(s) in normal operation.
15
ACTL = 200Hz
ACTL = 1kHz
ACTL = 10kHz
ACTL = 30kHz
10
5
ACTL = 0V to 3V, VIN = 12V
0
0
0
0.1
10
0.2
1
20 0.3
30 0.4
40 0.5
50 0.6
60 0.7
70 0.8
80 0.9
90 100
Duty Cycle (%)
Figure 8. LED Current vs. ACTL PWM Dimming Duty Cycle
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8561A-02 April 2012
Current Limit Protection
The RT8561A can limit the peak current to achieve over
current protection. The RT8561A senses the inductor
current through SW pin in the switch-on period. The duty
cycle depends on the current sense signal summing with
the internal slope compensation compared to the VC
signal. The internal N-MOSFET will be turned off when
the current signal is larger than the VC signal. In the off
period, the inductor current will descend. The internal
MOSFET is turned on by the oscillator in the next
beginning cycle.
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RT8561A
Over Temperature Protection
Capacitor Selection
The RT8561A has over temperature protection (OTP)
function to prevent the excessive power dissipation from
overheating. The OTP will shut down switching operation
if the junction temperature exceeds 150°C. Main converter
starts switching after junction temperature cools down
approximately 20°C.
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most applications, a 10uF ceramic capacitor is sufficient.
A value higher or lower may be used depending on the
noise level from the input supply and the input current to
the converter.
Inductor Selection
The value of the output inductor (L), where the transition
from discontinuous to continuous mode occurs is
approximated by the following equation :
L=
(VOUT − VIN ) × VIN2
It is recommended to choose a ceramic capacitor bases
on the output voltage ripple requirements. The minimum
value of the output capacitor COUT is approximately given
by the following equation :
COUT =
2 × IOUT × f × VOUT 2
(VOUT − VIN ) × IOUT
η × VRIPPLE × VOUT × f
Where,
Layout Guideline
VOUT = maximum output voltage.
PCB layout is very important for designing power switching
converter circuits. The following layout guide lines should
be strictly followed for best performance of the RT8561A.
VIN = minimum input voltage.
f = operating frequency.
IOUT = sum of current from all LED strings.
`
The power components L, D, CVIN1, COUT1 and COUT2
must be placed as close as possible to reduce the ac
current loop. The PCB trace between power components
must be short and wide as possible due to large current
flow through these trace during operation.
`
Place L and D connected to SW pin as close as possible.
The trace should be short and wide as possible.
`
It is recommend to place CVIN2 close to VIN pin.
`
Pin7 is the compensation point to adjust system
stability. Place the compensation components to pin7
as close as possible, no matter the compensation is
RC or capacitance.
η is the efficiency of the power converter.
The boost converter operates in discontinuous mode over
the entire input voltage range when the L1 inductor value
is less than this value L. With an inductance greater than
L, the converter operates in continuous mode 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 provided by
the following equation :
IPEAK = VOUT × IOUT + VIN × T ⎛⎜ VOUT − VIN ⎞⎟
η × VIN
2 × L ⎝ VOUT
⎠
Diode Selection
Schottky diode is a good choice for an asynchronous
Boost converter due to the small forward voltage. However,
for power dissipation, reverse voltage rating and pulsating
peak current are the important parameters of Schottky
diode selection. Choose a suitable diode whose reverse
voltage rating is greater than the maximum output voltage.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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14
is a registered trademark of Richtek Technology Corporation.
DS8561A-02 April 2012
RT8561A
Place these components C
OUT1
as close as possible.
D
VOUT
COUT2
GND
CVIN1
VIN
GND
GND
GND
GND
SW
SW
L
24
23
22
21
20
19
GND
CVIN2
LED1
1
18
VCC
LED2
2
17
CREG
LED3
3
LED4
4
LED5
5
OVP
6
GND
9
10
11
12
SS
RISET
ACTL
DCTL
VC
CVC2
8
NC
25
7
RVC
CVC1
RVIN
16
Locate the C VIN2 as
close to VCC pin as
LED8 possible.
15
LED7
14
LED6
13
EN
Locate the compensation
components to VC pin as
close as possible.
GND
Figure 9. PCB Layout Guide
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8561A-02 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
15
RT8561A
Outline Dimension
D2
D
SEE DETAIL A
L
1
E
E2
e
b
1
2
2
DETAIL A
Pin #1 ID and Tie Bar Mark Options
A
A3
A1
1
Note : The configuration of the Pin #1 identifier is optional,
but must be located within the zone indicated.
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
0.700
0.800
0.028
0.031
A1
0.000
0.050
0.000
0.002
A3
0.175
0.250
0.007
0.010
b
0.180
0.300
0.007
0.012
D
3.950
4.050
0.156
0.159
D2
2.300
2.750
0.091
0.108
E
3.950
4.050
0.156
0.159
E2
2.300
2.750
0.091
0.108
e
L
0.500
0.350
0.020
0.450
0.014
0.018
W-Type 24L QFN 4x4 Package
Richtek Technology Corporation
5F, No. 20, Taiyuen 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.
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DS8561A-02 April 2012
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