Richtek RT8561AGQW Schematic

®

High Voltage 8-CH LED Driver

RT8561A

General Description

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.

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 automati-

cally detects and disconnects any unconnected and/or

broken strings during operation from PWM loop to prevent

V

from over voltage.

OUT

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.

Other protecting features include programmable output

over voltage protection, LED current limit, PWM switch

current limit and thermal shutdown.

The RT8561A is packaged with a tiny footprint package of

WQFN-24L 4x4 packages.

Ordering Information

RT8561A

Package Type QW : WQFN-24L 4x4 (W-Type)

Lead Plating System G : Green (Halogen Free and Pb Free)

Note :

Richtek products are :

` RoHS compliant and compatible with the current require-

ments of IPC/JEDEC J-STD-020.

` Suitable for use in SnPb or Pb-free soldering processes.

Features

z High V oltage : V

up to 80 x 3.6V LEDs (10 each channel)

z Channel Current Progra mmable 10mA to 30mA and

Matched to 1.5%

z Current Mode PWM 1MHz Boost Converter z Easy Analog and Digital Dimming Control z Programmable Soft Start z Automatic Detecting Unconnected and/or Broken

Channel

z Programmable Over Voltage Protection z Disconnects LED in Shutdown z Providing High Accuracy Digital Dimming by PWM

at ACTL Pin

z V

Under Voltage Lockout

IN

zz

z Over Temperature Protection

zz

z Current Limiting Protection

zz

z Small 24-Lead WQFN Package

zz

z RoHS Compliant and Halogen Free

zz

up to 24V , V

IN

up to 40V , Driving

OUT

Applications

z UMPC and Notebook Computer Backlight

z GPS, Portable DVD Backlight

z Desk Lights and Room Lighting

Pin Configurations

(TOP VIEW)

SW

21 20 19

25

11

ACTL

RISET GND

SW

18

17

16

15

14

13

DCTL

VCC

CREG

LED8

LED7

LED6

EN

LED1

LED2

LED3

LED4

LED5

OVP

GND

24 2223

1

2

3

4

5

6

GND

7

8910 12

SS

VC

NC

WQFN-24L 4x4

Marking Information

CZ= : Product Code

CZ=YM

DNN

YMDNN : Date Code

©

DS8561A-02 April 2012 www.richtek.com

1

RT8561A

Typical Application Circuit

V

IN

6V to 24V

R 10

VIN1

C

VIN1

2.2µF

V

OUT

4.7µF

0.1µF

10µH

C

VIN1

10µF

5V

R1

L

R2

R

VC

1.8k

C

VC

3.9nF

1k

19, 20

18

13

17

12

D

6

9

7

SW

VCC

EN

OVP

CREG

SS

VC

DCTL

C

OUT

10µF

RT8561A

V

OUT

40V MAX

:

...

:

.

LED1

LED2

: : : :

1

2

. .

15

LED7

16

LED8

RISET

GND

ACTL

R

ISET

4.75k

10

21, 22, 23, 24, 25 (Exposed Pad)

1k

11

PWM Signal

: :

10 LED String

: :

LED

I

(mA) =

20 x 4.75

Ω)

R(k

ISET

Figure 1. 1MHz, 20mA Full Scale Current PWM Dimming Control

V

IN

6V to 24V

R 10

VIN1

C

VIN1

2.2uF

V

OUT

4.7µF

0.1µF

10µH

C

VIN1

10µF

5V

R1

R

1.8k

C

3.9nF

L

R2

VC

D

19, 20

SW

18

VCC

1k

13

EN

6

OVP

17

CREG

9

SS

7

VC

12

DCTL

DCTL can be floating or connected to ACTL

C

OUT

10µF

RT8561A

1

LED1

2

LED2

15

LED7

16

LED8

10

RISET

21, 22, 23, 24, 25 (Exposed Pad)

GND

11

ACTL

: : : :

. . .

1k

40V MAX

:

...

: : :

R

ISET

4.75k

V

OUT

: : : :

: :

10 LED String

: :

LED

I

(mA) =

Analog Dimming

20 x 4.75

Ω)

R(k

ISET

Figure 2. 1MHz, 20mA Full Scale Current Analog Dimming Control

2

©

DS8561A-02 April 2012www.richtek.com

RT8561A

)

V

IN

6V to 24V

R 10

4.7µF

VIN1

C

VIN1

2.2µF

V

OUT

0.1µF

R

VC

1.8k

C

VC

3.9nF

L

10µH

C

VIN1

10µF

1k

5V

R1

R2

PWM Signal

19, 20

1k

18

13

6

17

9

7

12

D

SW

VCC

EN

OVP

CREG

SS

VC

DCTL

C

OUT

10µF

RT8561A

1

LED1

2

LED2

15

LED7

16

LED8

10

RISET

21, 22, 23, 24, 25 (Exposed Pad)

GND

11

ACTL

: : : :

. . .

V

OUT

40V MAX

:

...

: : :

R

ISET

4.75k

:

10 LED String

:

LED

I

0.1µF

(mA) =

Figure 3. 1MHz, 20mA Full Scale Current PWM to Analog Dimming Control

20 x 4.75

Ω)

R(k

ISET

V

IN

V

AVDD

6V to 24V

R 10

C

2.2µF

VIN1

4.7µF

4V to 36V

V

OUT

10µH

C

VIN1

10µF

5V

0.1µF

L

R1

R2

R

VC

1.8k

C

VC

3.9nF

1k

19, 20

18

13

17

12

D

6

9

7

SW

VCC

EN

OVP

CREG

SS

VC

DCTL

C

OUT

10µF

RT8561A

LED1

LED2

LED7

LED8

RISET

GND

ACTL

Note :

1. Due to the limitation of maximum duty 5V input can support typically to V

2. Due to the limitation of maximum duty 4V input can support typically to V

V

OUT

40V MAX

: : : :

1

2

:

: : : :

...

:

10 LED String

:

. . .

15

16

R

ISET

4.75k

10

21, 22, 23, 24, 25 (Exposed Pad)

11

PWM Signal

= 33V.

OUT

= 26V.

OUT

I

LED

(mA) =

20 x 4.75

R(k

ISET

Ω

Figure 4. Wide Range VIN Application by Connecting VCC Pin to LCD Driver Power AVDD

DS8561A-02 April 2012 www.richtek.com

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3

RT8561A

Function Block Diagram

SW

VCC

OVP

EN

CREG

VC

SS

DCTL

5.6V

1.2V

1.6V

5V

LDO

5uA

+

-

+

1.2V

+

-

OSC

Shutdown

300k

+

-

V

Regulation

S

R

OUT

Unit

+

-

+

-

+

5V

+

-

+

-

LED1

LED2

LED7

LED8

ACTL

©

RISET

DS8561A-02 April 2012www.richtek.com

4

Functional Pin Description

Pin No. Pin Name Pin Function

LED1, LED2,

1, 2, 3, 4, 5

6 OVP

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.

LED3, LED4, LED5

Channel 1 to Channel 5 LED current sink. Leave the pin unconnected if not used.

Over Voltage Protection. PWM boost converter turns off when V higher than 1.2V.

RT8561A

goes

OVP

11 ACTL

12 DCTL

13 EN Chip enable pin, when pulled low, chip is in shutdown mode.

14, 15, 16

17 CREG

18 VCC Power supply of the chip. For good bypass, a low ESR capacitor is required.

19, 20 SW PWM boost converter switch node.

21, 22, 23, 24,

25 (Exposed Pad)

LED6, LED7, LED8

GND

Analog/Digital dimming control. When using analog dimming,

I (mA) = for V 1.2V.

LED ACTL

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.

Channel 6 to Channel 8 LED current sink. Leave the pin unconnected if not used.

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.

Ground pin of the chip. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation.

20 x 4.75

R(kΩ)

ISET

≥

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5

RT8561A

Absolute Maximum Ratings (Note 1)

z Supply Voltage, VCC ----------------------------------------------------------------------------------------------------- −0.3V to 28V

z SW Pin Voltage at Switching Off -------------------------------------------------------------------------------------- −0.3V to 45V

z LED1 to LED8 Pin--------------------------------------------------------------------------------------------------------- −0.3V to 35V

z DCTL, ACTL, EN, OVP Pin Voltage ---------------------------------------------------------------------------------- −0.3V to 5.5V

z Power Dissipation, P

WQFN-24L 4x4 ------------------------------------------------------------------------------------------------------------ 1.923W

z Package Thermal Resistance (Note 2)

WQFN-24L 4x4, θJA------------------------------------------------------------------------------------------------------- 52°C/W

WQFN-24L 4x4, θJC------------------------------------------------------------------------------------------------------ 7°C/W

z Junction Temperature ----------------------------------------------------------------------------------------------------- 150°C

z Lead Temperature (Soldering, 10 sec.)------------------------------------------------------------------------------- 260°C

z Storage Temperature Range -------------------------------------------------------------------------------------------- −65°C to 150°C

z ESD Susceptibility (Note 3)

HBM (Human Body Mode) ---------------------------------------------------------------------------------------------- 2kV

MM (Machine Mode) ------------------------------------------------------------------------------------------------------ 200V

Recommended Operating Conditions (Note 4)

@ TA = 25°C

D

z Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------- 6V to 24V

z Junction Temperature Range --------------------------------------------------------------------------------------------

z Ambient Temperature Range --------------------------------------------------------------------------------------------

−40°C to 125°C

−40°C to 85°C

Electrical Characteristics

(VCC = 17V, T

Supply Current I

VIN Under Voltage Lockout Threshold

Shutdown Current I

EN Threshold Voltage

PWM Threshold Voltage

EN Pin Input Current IEN VEN ≤ 5V -- -- 0.1 μA

LED Current Pro grammin g

LED Curr ent I

LEDs Curr ent Matching

RISET Pin Voltage V

Input Current of ACTL I

6

©

= 25°C, unless otherwise specified)

A

Parameter Symbol Test Conditions Min Typ Max Unit

VC ≤ 0.2V (Switching off) -- 3 5 mA

VCC

VIN Rising -- 5.6 5.95

V

Falling -- 4.9 --

IN

VEN ≤ 0.7V -- -- 10 μA

1.6 -- 5

-- -- 1

1.3 -- 5

-- -- 0.15

2V > V V

ACTL

2V > V V

ACTL

(I

(MAX)

3.6kΩ ≤ R

0.3V ≤ V

> 0.6V, R

LED

= 4.75kΩ,

ISET

> 1.2V

> 0.6V, R

LED

= 4.75kΩ,

ISET

> 1.2V, Calculating

− I

) / I

(MIN)

≤ 9.6k Ω, V

ISET

≤ 1.2V -- 1 2 μA

ACTL

Average

x 100%

ACTL

19 20 21 mA

-- -- 1.5 %

> 1.2V 1.17 1.2 1.23 V

DS8561A-02 April 2012www.richtek.com

Logic-High V

Logic-Low

Logic-High V

Logic-Low

V

UVLO

SHD N

EN_ H

V

EN_ L

PWM_H

V

PWM_L

LED

RISET

ACTL

V

RT8561A

Parameter Symbol Test Conditions Min Typ Max Unit

Threshold of ACTL V

Input Current of DCTL I

V

Threshold Un-connection -- 0.1 -- V

LED

LED Current Off -- 0.2 -- V

ACTL

0.3V ≤ V

DCTL

≤ 5.5V -- 1 2 μA

DCTL

PWM Boost Con verter

Switching Frequency 0.8 1 1.2 MHz

Minimum On Time -- 100 -- ns

Regulated V

Highest Voltage LED String 0.5 0.6 0.7 V

LED

Amplifier (gm) Output Current 2.4V > VC > 0.2V -- ±15 -- μA

VC Threshold PWM Switch Off 0.1 0.2 -- V

SW R

SW Current Limit I

-- 0.3 0.5 Ω

DS(ON)

2.5 -- -- A

LIM

SW Maximum Duty 80 88 -- %

OVP & Soft-Start

OVP Threshold V

OVP Input Current I

1.1 1.2 1.3 V

OVP

V

OVP

≤ 3V -- -- 50 nA

OVP

Soft-Start Current ISS VSS ≤ 2.5V 3 5 8 μA

Thermal Shutdown Temperature

-- 150 -- °C

T

SD

Thermal Shutdown Hysteresis -- 20 -- °C

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. θ

Note 3. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions.

is measured at T

JA

measured at the exposed pad of the package.

= 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is

A

DS8561A-02 April 2012 www.richtek.com

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7

RT8561A

Typical Operating Characteristics

Efficiency vs. Input Voltage

100

90

80

70

60

50

40

Efficiency (%)

30

20

10

0

8 1012141618202224

Input Voltage (V)

LED Curre nt vs. Temperature

24

23

22

21

20

19

LED Current (mA)

18

17

16

-50 -25 0 25 50 75 100 125

Temperature (°C)

V

80LEDs

= 12V

IN

LED Current vs. Inpu t Voltage

26

24

22

20

18

16

LED Current (mA)

14

12

10

8 1012141618202224

Input Voltage (V)

V

1.24

1.22

1.20

1.18

(V)

1.16

RISET

V

1.14

1.12

1.10

-50 -25 0 25 50 75 100 125

vs. Temperature

RISET

Temperature (°C)

V

LED1

LED2

LED3

LED4

LED5

LED6

LED7

LED8

IN

= 12V

V

vs. Inp ut Voltage

1.25

1.24

1.23

1.22

RISET

(V)

1.21

RISET

V

1.20

1.19

1.18

1.17 8 1012141618202224

Input Voltage (V)

©

LED Current vs. ACT L PWM Duty Cycle

25

20

15

10

LED Current (mA)

5

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 10 20 30 40 50 60 70 80 90 100

ACTL = 0V to 3V, V

Duty Cycle (%)

ACTL = 200Hz

ACTL = 1kHz

ACTL = 10kHz

PWM = 30kHz

DS8561A-02 April 2012www.richtek.com

8

IN

= 12V

RT8561A

)

LED Current vs. DCT L PW M Dut y Cycle

25

20

15

10

DCTL = 200Hz

DCTL = 1kHz

DCTL = 10kHz

LED Current (mA)

5

0

0 102030405060708090100

DCTL = 0V to 3V, V

Duty Cycle (%)

Frequency vs. Input Voltage

1100

1060

1020

IN

= 12V

LED Curre nt vs. ACTL Analog Voltage

28

24

20

16

12

8

LED Current (mA)

4

0

0 0.25 0.5 0.75 1 1.25 1.5

ACTL Analog Voltage (V)

Shutdown Current vs . Input Voltage

10

8

6

V

IN

= 12V

980

4

Frequency (kHz)

940

900

8 101214 1618 2022 24

Input Voltage (V)

Supply Current vs. Input Voltage

4.0

3.9

3.8

3.7

3.6

3.5

3.4

3.3

3.2

Switch Off Current (mA)

3.1

3.0 8 1012141618202224

Input Voltage (V)

VC = 0V

Shutdown Current (µA

2

0

8 1012141618202224

Input Voltage (V)

SS Current vs. Temperature

8.0

7.6

7.2

6.8

6.4

6.0

5.6

SS Current (µA)

5.2

4.8

4.4

4.0

-50 -25 0 25 50 75 100 125

Temperature (°C)

V

IN

= 12V, C

V

EN

= 0.1μF

SS

= 0V

©

DS8561A-02 April 2012 www.richtek.com

9

RT8561A

SS Current vs. Input Voltage

9

8

7

6

5

SS Current (µA)

4

3

C

2

8 1012141618202224

SS

Input Voltage (V)

OVP Voltage vs. Input Voltage

1.3

1.2

1.1

1.0

OVP Voltage (V)

0.9

0.8 8 101214161820 2224

Input Voltage (V)

= 0.1μF

OVP Voltage v s. Temperature

1.30

1.28

1.26

1.24

1.22

1.20

1.18

1.16

OVP Voltage (V)

1.14

1.12

1.10

-50 -25 0 25 50 75 100 125

V

IN

= 12V

Temperature (°C)

ACTL Off Threshold Voltage vs. Tem perature

0.22

0.21

0.20

0.19

0.18

0.17

0.16

0.15

ACTL Off Threshold Voltage (V)

0.14

-50 -25 0 25 50 75 100 125

Temperature (°C)

V

IN

= 12V

Power On from EN

V

EN

Line Transient Response

V

= 10.8V to 13.2V

IN

(2V/Div)

V

IN

V

OUT

(5V/Div)

(20V/Div)

I

IN

(500mADiv)

V

= 12V, C

IN

SS

= 0.1μF

Time (10ms/Div)

©

I

OUT

(100mA/Div)

Time (50ms/Div)

DS8561A-02 April 2012www.richtek.com

10

V

IN

(10V/Div)

V

OUT

(20V/Div)

SW

(20V/Div)

R1 = 3.3MΩ, R2 = 100kΩ

OVP

V

= 12V, All LED Pin Open

IN

Time (2.5ms/Div)

RT8561A

©

DS8561A-02 April 2012 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.

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.

Power on sequence

VIN

PWM

EN

VOUT

VIN must be turned off early than EN and PWM signal

Soft-Start

Power off sequence

VIN must be turned on late than EN and PWM signal

Figure 6. Power On Sequence Control by VIN

EN/VIN

EN and/or VIN should be pulled low once PWM pull low for over 10 ms

Abnormal Power on sequence

UVLO

No Soft-Start If PWM turns

on late

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.

Abnormal Power on sequence

No Soft-Start If PWM turns on late

VIN

PWM

EN

VOUT

Power on sequence

EN must be turned on late than VIN and PWM signal

Soft-Start

Power off sequence

EN must be turned off early than VIN and PWM signal

Figure 5. Power On Sequence Control by EN

UVLO

PWM

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.

LED Connection

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 :

I(mA) =

LED

12

©

20 x 4.75

R(kΩ)

ISET

DS8561A-02 April 2012www.richtek.com

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 < V

< 1.2V). With an on-chip

ACTL

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. ACT L PWM Duty Cycle

25

20

15

Table 1.

Dimming Freque nc y (Hz) D uty (Min.) Duty (Max.)

100 < f 200 < f 500 < f 1k < f 2k < f 5k < f 10k < f

Note : The minimum duty in Table 1 is based on the application

circuit and does not consider the deviation of current linearity.

≤ 200 0.16% 100%

PWM

≤ 500 0.40% 100%

PWM

≤ 1k 0.80% 100%

PWM

≤ 2k 1.6 0% 1 00%

PWM

≤ 5k 4.0 0% 1 00%

PWM

≤ 10k 8.00% 100%

PWM

≤ 20k 16.00% 1 00%

PW M

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 :

R1

V = V1+

OUT, OVP OVP

⎛⎞

×

⎜⎟

R2

⎝⎠

Where

R1 and R2 are the voltage divider connected to OVP pin.

V

is typically 1.2V.

OVP

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.

10

LED Current (mA)

5

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 10 20 30 40 50 60 70 80 90 100

ACTL = 0V to 3V, V

Duty Cycle (%)

ACTL = 200Hz

ACTL = 1kHz

ACTL = 10kHz

ACTL = 30kHz

= 12V

IN

Figure 8. LED Current vs. ACTL PWM Dimming Duty Cycle

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.

DS8561A-02 April 2012 www.richtek.com

©

13

RT8561A

Over Temperature Protection

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.

Inductor Selection

The value of the output inductor (L), where the transition

from discontinuous to continuous mode occurs is

approximated by the following equation :

2

(V V V

OUT IN IN

L =

2I f V

×××

OUT OUT

)

−×

2

Where,

V

= maximum output voltage.

OUT

VIN = minimum input voltage.

f = operating frequency.

I

= sum of current from all LED strings.

OUT

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 :

VI VTV V

I =

PEAK

××−

OUT OUT IN OUT IN

η

××

+

V2LV

IN OUT

⎛⎞ ⎜⎟ ⎝⎠

Capacitor Selection

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.

It is recommended to choose a ceramic capacitor bases

on the output voltage ripple requirements. The minimum

value of the output capacitor C

is approximately given

OUT

by the following equation :

(V V ) I

−×

C =

OUT

OUT IN OUT

VVf

η

×××

RIPPLE OUT

Layout Guideline

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.

` The power components L, D, C

VIN1

, C

OUT1

and C

OUT2

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 C

` Pin7 is the compensation point to adjust system

close to VIN pin.

VIN2

stability. Place the compensation components to pin7

as close as possible, no matter the compensation is

RC or capacitance.

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.

14

©

DS8561A-02 April 2012www.richtek.com

RT8561A

LED1

LED2

LED3

LED4

LED5

OVP

C

VC2

Place these components as close as possible.

GND

24 2223

1

2

3

4

5

6

7

VC

21 20 19

GND

8 9 10 12

SS

NC

RISET GND

R

Locate the compensation

VC

components to VC pin as close as possible.

C

VC1

GND

C

OUT1

D

C

L

SW

18

VCC

17

CREG

16

LED8

15

LED7

14

LED6

13

EN

25

11

ACTL

DCTL

V

OUT

C

OUT2

GND

VIN1

V

IN

GND

R

C

VIN2

Locate the C

VIN

VIN2

as close to VCC pin as possible.

Figure 9. PCB Layout Guide

©

DS8561A-02 April 2012 www.richtek.com

15

RT8561A

Outline Dimension

D

E

A

A3

A1

D2

SEE DETAIL A

1

be

E2

L

1 2

DETAIL A

Pin #1 ID and Tie Bar Mark Options

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 0.500 0.020

L 0.350 0.450

Richtek Technology Corporation

5F, No. 20, Taiyuen Street, Chupei City

Hsinchu, Taiwan, R.O.C.

Tel: (8863)5526789

0.014 0.018

W-Type 24L QFN 4x4 Package

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.

DS8561A-02 April 2012www.richtek.com

16

Richtek RT8561AGQW Schematic [ru]

Specifications and Main Features

Frequently Asked Questions

User Manual