Richtek RT8511AGQW, RT8511AGQWA Schematic [ru]

43V Asynchronous Boost WLED Driver

RT8511A

General Description

The RT851 1A is a n LED driver IC that ca n support up to 10 WLED in series. It is composed of a current mode boost converter integrated with a 43V/1.2A power switch running at a fixed 1MHz frequency and covering a wide VIN ra nge from 2.7V to 24V.

The white LED current is set with a n external resistor , and the feedback voltage is regulated to 200mV (typ.). During operation, the LED current ca n be controlled by the PWM input signal in which the duty cycle determines the feedback reference voltage.

For brightness dimming, the RT851 1A is able to maintain steady control of the LED current. Therefore, no audible noises are generated on the output cap acitor . The RT851 1A also has programmable over voltage pin to prevent the output from exceeding absolute maximum ratings during open LED conditions. The RT8511A is available in W DFN-8L 2x2 a nd WDF N-8L 3x3 pa ckages.

Features



Wide Input Voltage Range : 2.7V to 24V





 High Output Voltage : up to 43V



 Direct PWM Dimming Control and Frequency from



100Hz to 1kHz



 Internal Soft -Start and Compensation



 200mV Reference Voltage



 PWM Dimming with Internal Filter



 Programmable Over Voltage Protection



 Over Temperature Protection



 Current Limit Protection



 Thin 8-Lead 2x2 and 8-Lead 3x3 WDFN Packages



 RoHS Compliant and Halogen Free



Applications

 UMPC and Notebook Computer Backlight  GPS, Portable DVD Backlight

Pin Configurations

Ordering Information

RT8511A

Package Type QW : WDFN-8L 2x2 (W-Type) QWA : WDFN-8L 3x3 (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.

(TOP VIEW)

OVP

DIMC

GND

WDFN-8L 2x2 / WDFN-8L 3x3

Marking Information

RT8511AGQW

14 : Product Code

14W

RT851 1AGQWA

22=YM

DNN

W : Date Code

22= : Product Code YMDNN : Date Code

8 7 6 5

EN PWM VIN LX

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RT8511A

Typical Application Circuit

4.2V to 24V

Chip Enable

100Hz to 1kHz

2.7V to 24V

Chip Enable

100Hz to 1kHz

PWM

LED

2.7V to 4.2V

PWM

1µF x 2

DIMC

1µF

Figure 1. T ypical Application Circuit of Normal Operation

LED

1µF x 2 V

DIMC

1µF

7 3

VIN

PWM DIMC

VIN

PWM DIMC

10µH

RT8511A

10µH

RT8511A

OVP

4, 9 (Exposed Pad)

GND

OVP

4, 9 (Exposed Pad)

GND

3.3M C

OUT

1µF x 2

R1 100k

3.3M C

OUT

1µF x 2

R1 100k

OUT

: : : :

WLEDs

SET

3.3

OUT

: : : :

WLEDs

SET

3.3

Figure 2. T ypical Application Circuit of Low V oltage Operation

Functional Pin Description

Pin No. Pin Name Pin Function

1 OVP Over Voltage Protection for Boost Converter. The detecting threshold is 1.2V. 2 FB 3 DIMC PWM Filter Pin. Filter the PWM signal to a DC voltage.

4 GND Ground Pin. 5 LX Switch Node for Boost Converter. 6 VIN Power Supply Input. 7 PWM Dimming Control Input. 8 EN Chip Enable (Active High) for Boost Converter.

9 (Ex posed Pad) GND

Fe edbac k Pin. Con nect a resi stor betw een th is p in and GND to se t the LED current.

T he ex pos ed pad mus t be so lde r ed to a larg e P CB and co nne cte d to A GND for maximum power dissipation.

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Function Block Diagram

RT8511A

LXOVP

VIN

PWM

DIMC

OSC

Detection

PWM

Controller

D/A

Dimming

1.2V

0.8V

OTP

OCP

GND

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RT8511A

Absolute Maximum Ratings (Note 1)

 VIN, EN, PWM, DIMC to GND------------------------------------------------------------------------------------------ −0.3V to 26.5V  LX, FB, OVP to GND ----------------------------------------------------------------------------------------------------- −0.3V to 48V  Power Dissipation, P

WDFN-8L 2x2 -------------------------------------------------------------------------------------------------------------- 0.833W WDFN-8L 3x3 -------------------------------------------------------------------------------------------------------------- 1.429W

 Package Thermal Re sistance (Note 2)

W DF N-8L 2x2, θJA--------------------------------------------------------------------------------------------------------- 120°C/W WDFN-8L 2x2, θJC--------------------------------------------------------------------------------------------------------- 8.2°C/W W DF N-8L 3x3, θJA--------------------------------------------------------------------------------------------------------- 70°C/W WDFN-8L 3x3, θJC--------------------------------------------------------------------------------------------------------- 8.2°C/W

 Lead Temperature (Soldering, 10 sec.)------------------------------------------------------------------------------- 260°C  Junction T emperature----------------------------------------------------------------------------------------------------- 150°C  Storage T emperature Range -------------------------------------------------------------------------------------------- –65°C to 150°C  ESD Susceptibility (Note 3)

HBM (Human Body Model)---------------------------------------------------------------------------------------------- 2kV MM (Machine Model) ----------------------------------------------------------------------------------------------------- 200V

@ TA = 25°C

Recommended Operating Conditions

 Supply Input V oltage, V  Junction T emperature Range-------------------------------------------------------------------------------------------- −40°C to 125°C  Ambient T emperature Range-------------------------------------------------------------------------------------------- −40°C to 85°C

------------------------------------------------------------------------------------------------ 2.7V to 24V

(Note 4)

Electrical Characteristics

= 4.5V, T

VIN Quie scent Current

VIN Shutd ow n Cu rr ent I

Control Input

EN, PW M Threshold Voltage

EN Sink Current IIH V Shutdown Delay t PWM Dimming Frequency 0.1 -- 1 kHz

Boost Con verter

= 25°C, unless otherwise specified)

Parameter Symbol Test Conditions Min Typ Max Unit

IQ V

VFB = 0V, Switching -- -- 2.2 mA

Q_SW

SHDN

Logic-High VIH V Logic- Low V

EN high to low 26 32 40 ms

SHDN

= 1.5V, No Switching -- 725 -- A

= 4.5V, VEN = 0V -- 1 4 A

= 2.7V to 24V 1.6 -- --

= 2.7V to 24V -- -- 0.8

= 3V 1 -- 10 A

Switching Frequency f LX On Re sist ance

(N-MOSFET)

OSC

VIN > 5V -- 0.4 0.6 

DS(ON)

= 2.7V to 24V 0.8 1 1.2 MHz

Minimum ON Time -- 100 -- ns Maximum Du ty Cycl e D

MAX

= 0V, Switching -- 92 -- %

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RT8511A

Parameter Symbol Test Conditions Min Typ Max Unit

LED Current

Minimum PWM Dimming Duty Cycle

Feedback Voltage VFB -- 200 -- mV

Fault Protection

Dimming Freq. = 100Hz to 1kHz 5 -- -- %

MIN

LX Current Limit I Over Voltage Protection

Threshold Thermal Shutdown Temperature

0.85 1.2 1.55 A

LIM

-- 1.2 -- V

OVP

-- 160 -- °C

Thermal Shutdown Hysteresis TSD -- 30 -- °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

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

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RT8511A

Typical Operating Characteristics

Efficiency vs. Input Voltage

100

95 90 85 80 75

Eff iciency (%)

70 65

4 7 9 121417192224

OUT

Inp ut Voltage (V)

FB Reference Voltage vs. Temperature

200

198

196

194

192

FB Reference Volt age (mV)

190

-20 5 30 55 80 105

Temperatu re (°C)

= 29.5V

= 4.5V

FB Reference Voltage (mV)

Frequency ( kHz) 1

FB Reference Voltage vs. Input Voltage

199.5

199.2

198.9

198.6

198.3

198.0 4 8 12 16 20 24

Input Vol tage (V)

Frequency vs. Input Voltage

1100 1075 1050 1025 1000

975 950 925 900

4 6 8 1012141618202224

Input Voltage (V)

Frequency vs. Temperature

1100 1075 1050 1025 1000

975

Frequency ( kH z) 1

950 925

= 4.5V

900

-50 -25 0 25 50 75 100 125

Tempera tur e (°C)

LED Current ( m A)

LED Current vs. PWM Duty Cycle

PWM = 100Hz

PWM = 1kHz

0 102030405060708090100

PWM Dut y Cycle (%)

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Application Information

RT8511A

The RT8511A is a current mode boost converter which operates at a fixed frequency of 1MHz. It is capable of driving up to 10 white LEDs in series and integrates functions such as soft-start, compensation, and internal analog di mming control. The protection block also provides over-voltage, over-temperature, and current- limit protection features.

LED Current Setting

The loop structure of the boost converter keeps the FB pin voltage equal to the reference voltage VFB. Therefore, by connecting the resistor, R

between the FB pin and

SET

GND, the LED current will be determined by the current through R

. The LED current can be calculated by the

SET

following equation :

I =

LED

SET

Brightness Control

For the brightness dimming control of the RT8511A, the IC provides typically 200mV reference voltage when the PWM pin is constantly pulled high. However, the PWM pin allows a PWM signal to adjust the reference voltage by changing the PWM duty cycle to achieve LED brightness dimming control. The relationship between the duty cycle and the FB voltage can be calculated a ccording to the following equation :

VFB = 200mV x Duty where 200mV is the typical internal reference voltage and

Duty is the duty cycle of the PWM signal. As shown in Figure 3, the duty cycle of the PWM signal

is used to modify the internal 200mV reference voltage. With an on-chip output clamping amplifier and a serial resistor, the PWM dimming signal is easily low-pass filtered to an analog dimming signal with one external cap a citor, C

, for noise-free PWM dimming. Di mming

DIMC

frequency can be sufficiently a djusted from 100Hz to 1kHz. However, the LED current cannot be 100% proportion al to the duty cycle. Referring to T a ble 1, the minimum dimming duty can be as low as 1% for the frequency range from 100Hz to 1kHz. It should be noted that the accuracy of 1% duty is not guaranteed.

Because the voltage of DIMC a nd FB is small to 2mV a nd ea sily affected by LX switching noise.

200mV

PWM

DIMC

C 1µF

DIMC

To Controller

Figure 3. Block Di agra m of Progra mm able FB Voltage.

Table 1. Minimum Duty for Dimming Frequency

Dimming Fr equency Minimum Duty Cycle

100Hz to 1kHz 5%

It also should be noted that when the input voltage is too close to the output voltage [(V

OUT −VIN

) < 6V] , excessive audible noise may occur. Additionally, for accurate brightness dimming control, the input voltage should be kept lower than the LEDs' turn on voltage. When operating in the light load , excessive output ripple may occur..

Soft-Start

The RT851 1A provides a built-in soft-start function to limit the inrush current, while allowing for an increased PWM frequency for dimming.

Current Limiting Protection

The RT8511A can limit the peak current to achieve over current protection. The IC senses the inductor current through the LX pin in the charging period. When the value exceeds the current limiting threshold, the internal NMOSFET will be turned off. In the off period, the inductor current will descend. The internal MOSFET is turned on by the oscillator during the beginning of the next cycle.

In addition, the LX current limit threshold is about 0.8V. If the voltage of LX is over 0.8V, and the fault signal

accumulates 3 times with 32μs, the MOSFET will be latched off.

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RT8511A

Power Sequence

In order to assure that the normal soft start function is in place for suppressing the inrush current, the in put voltage and enable voltage should be ready before PWM pulls high.

PWM

soft-start

OUT

Mode1

Figure 4 and Figure 5 show the power on a nd power off sequences.

PWM

OUT

Mode1

OUT

PWM

OUT

PWM

OUT

soft-start

Mode2

soft-start

PWM

OUT

Mode3

Mode2

Shutdown

Delay

Mode3

Figure 4. Power On Sequence

Figure 5. Power Off Sequence

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RT8511A

Over Voltage Protection

The RT851 1A equips over voltage protection (OVP) function. When the voltage at the OVP pin rea ches a threshold of approximately 1.2V, the MOSFET drive output will turn off. The MOSFET drive output will turn on again once the voltage at the OVP pin drops below the threshold. Thus, the output voltage can be clamped at a certain voltage level, as shown in the f ollowing equation :

V = V1+

OUT, OVP OVP









where R1 and R2 ma ke up the voltage divider connected to the OVP pin.

Over Temperature Protection

The RT851 1A ha s an over te mperature protection (OTP) function to prevent overheating caused by excessive power dissipation from overheating the device. The OTP will shut down switching operation if the junction temperature exceeds 160°C. The boost converter will start switching again when the junction temperature is cooled down by a pproximately 30°C.

Inductor Selection

The inductance depends on the maximum in put current. As a general rule, the inductor ripple current range is 20% to 40% of the maximum input current. If 40% is selected as an example, the inductor ripple current can be calculated according to the following equation :



I =

IN(MAX)

I = 0.4I

RIPPLE IN(MAX)

where η is the ef ficiency of the boost converter , I the maximum input current, I all LED strings, and I

OUT OUT





(MIN) IN(MIN)



RIPPLE

IN(MAX)

is the total current from

OUT

is the inductor ripple current. The input peak current can be calculated by maximum input current plus half of inductor ripple current shown a s following equation :

= 1.2 x I

PEAK

IN(MAX)

Note that the saturated current of the inductor must be greater than I

. The inductance can eventually be

PEAK

determined according to the following equation :

V(VV







L =

IN OUT IN

0.4 V I f





OUT OUT OSC

)

where f

is the switching frequency . For better efficiency ,

OSC

it is suggested to choose an inductor with small series resistance.

Diode Selection

The Schottky diode is a good choice for an a synchronous boost converter due to its small forward voltage. However , when selecting a Schottky diode, important parameters such as power dissipation, reverse voltgae rating, and pulsating peak current must all be taken into consideration. A suitable Schottky diode's reverse voltage rating must be greater than the maximum output voltage, and its average current rating must exceed the average output current.

Capacitor Selection

Two 1μF ceramic input capacitors and two 1μF ceramic output cap acitors are recommended for driving 10 WLEDs in series. For better voltage filtering, ceramic capacitors with low ESR are recommended. Note that the X5R a n d X7R types are suitable because of their wide voltage and temperature ranges.

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 a mbient temperature. The maximum power dissipation can be calculated by the following formula :

where T the ambient temperature, a nd θ

D(MAX)

= (T

J(MAX)

− TA) / θ

J(MAX)

is the maximum junction temperature, T

is the junction to ambient

thermal resistance. For recommended operating condition specifications, the

maximum junction temperature is 125°C. The junction to ambient thermal re sistance, θJA, is layout dependent. For WDFN-8L 2x2 packages, the thermal resistance, θJA, is 120°C/W on a standard JEDEC 51-7 four-layer thermal test board. For WDFN-8L 3x3 packages, the thermal resistance, θJA, is 70°C/W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power

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RT8511A

dissipation at TA = 25°C can be calculated by the following formula s :

= (125°C − 25°C) / (120°C/W) = 0.833W for

D(MAX)

W DF N-8L 2X2 pa ckage P

= (125°C − 25°C) / (70°C/W) = 1.429W for

D(MAX)

W DF N-8L 3X3 pa ckage The maximum power dissipation depends on operating

ambient temperature for fixed T

and thermal

J(MAX)

resistance, θJA. The derating curves in Figure 6 allow the designer to see the effect of rising ambient temperature on the maximum power dissipation.

1.6

1.4

1.2

WDFN-8L 3x3

1.0

0.8

0.6

WDFN-8L 2x2

0.4

Four-Layer PCB

Layout Consideration

For high frequency switching power supplies, the PCB layout is important to obtain good regulation, high efficiency and sta bility . The following description s are the suggestions for better PCB layout.

 Input and output capacitors should be placed close to

the IC and connected to the ground plane to reduce noise coupling.

 The GND and Exposed Pad should be connected to a

strong ground plane for heat sinking a nd noise protection.

 The components L, D, C

and C

must be placed a s

OUT

close as possible to reduce current loop. Keep the main current traces as possible as short and wide.

 The LX node of the DC/DC converter experiences is with

high frequency voltage swings. It should be kept in a small area.

 The component R

should be placed as close as

SET

possible to the IC and kept away from noisy devices.

0.2

Maximum Power Dissipati on (W) 1

0.0 0 25 50 75 100 125

Ambient Tempera ture (°C )

Figure 6. Derating Curve of Maxi mum Power Dissi pation

WLEDs

Locate R to FB as possible

: : : :

The C

OUT

directly from the output schottky diode to ground rather than across the WLEDs.

SET

R1R2

OVP

SET

DIMC

OUT

should be connected

DIMC

GND

2 3 4

PWM

VIN

GND

OUTCIN

CIN should be placed as closed as possible to V I N pin for good filtering.

The inductor should be placed as close as possible to the switch pin to minimize the noise coupling into other circuits. LX node copper area should be minimized for reducing EMI

Figure 7. PCB Layout Guide

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Outline Dimension

RT8511A

SEE DETAIL A

DETAIL A

Pin #1 ID a nd T ie Bar Mark Option s

Note : The configuration of the Pin #1 identifier is optional, but must be located within the zone indicated.

Dimensions In Millimeters Dimen sions In Inch es

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 0.200 0.300 0.008 0.012

D 1.950 2.050 0.077 0.081

D2 1.000 1.250 0.039 0.049

E 1.950 2.050 0.077 0.081 E2 0.400 0.650 0.016 0.026

e 0.500 0.020

L 0.300 0.400

W-Type 8L DFN 2x2 Package

0.012 0.016

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RT8511A

SEE DETAIL A

DETAIL A

Pin #1 ID a nd T ie Bar Mark Option s

Note : The configuration of the Pin #1 identifier is optional, but must be located within the zone indicated.

Dimensions In Millimeters Dimen sions In Inch es

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 0.200 0.300 0.008 0.012

D 2.950 3.050 0.116 0.120

D2 2.100 2.350 0.083 0.093

E 2.950 3.050 0.116 0.120 E2 1.350 1.600 0.053 0.063

e 0.650 0.026 L 0.425 0.525

Richtek Technology Corporation

14F, No. 8, Tai Yuen 1st Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789

0.017 0.021

W-Type 8L DFN 3x3 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.

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Richtek RT8511AGQW, RT8511AGQWA Schematic [ru]

Specifications and Main Features

Frequently Asked Questions

User Manual