Richtek RT8511BGQW Schematic [ru]

43V Asynchronous Boost WLED Driver

RT8511B

General Description

The RT851 1B is an LED driver IC that can support up to 10 WLED in series. It is composed of a current mode boost converter integrated with a 43V/2.2A power switch running at a fixed 500kHz 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 1B is able to maintain steady control of the LED current. Therefore, no audible noises are generated on the output cap acitor . The RT851 1B also has programmable over voltage pin to prevent the output from exceeding absolute maximum ratings during open LED conditions. The RT8511B is available in W DF N-8L 2x2 package.

Features



Wide Input Voltage Range : 2.7V to 24V





 High Output Voltage : up to 43V



 Direct PWM Dimming Control and Frequency from



100Hz to 8kHz



 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 WDFN Package



 RoHS Compliant and Halogen Free



Applications

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

Ordering Information

RT8511B

Package Type QW : WDFN-8L 2x2 (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.

Pin Configurations

(TOP VIEW)

OVP

DIMC

GND

WDFN-8L 2x2

GND

Marking Information

0F : Product Code

0FW

W : Date Code

8 7 6 5

EN PWM VIN LX

DS8511B-05 February 2015 www.richtek.com

RT8511B

Typical Application Circuit

4.2V to 24V

Chip Enable

100Hz to 8kHz

2.7V to 24V

Chip Enable

100Hz to 8kHz

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

RT8511B

10µH

RT8511B

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 Feedback. Connect a resistor between this pin and GND to set the LED current. 3 DIMC PW M Fil t er. Filter the PW M sig nal to a DC vol t ag e. 4 GND Ground. 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 (Exp ose d Pa d) GN D

The exposed pad must be soldered to a large PCB and connected to AGND for ma x imum power dissipation.

DS8511B-05 February 2015www.richtek.com

Function Block Diagram

RT8511B

LXOVP

VIN

PWM

DIMC

OSC

PWM

Controller

D/A

Dimming

1.2V

OTP

OCP

GND

DS8511B-05 February 2015 www.richtek.com

RT8511B

Absolute Maximum Ratings (Note 1)

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

< 500ns ---------------------------------------------------------------------------------------------------------------------- −1V to 48V

 Power Dissipation, P

WDFN-8L 2x2 -------------------------------------------------------------------------------------------------------------- 0.833W

 Package Thermal Re sistance (Note 2)

W DF N-8L 2x2, θJA--------------------------------------------------------------------------------------------------------- 120°C/W WDFN-8L 2x2, 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 Quiescent Current

VIN Shutdown Current I

Control In put

EN, PWM Threshold Voltage

EN Sink Current IIH V Shutdown Delay t PWM Dimming Frequenc y 0.1 - - 8 kHz

Boost Converter

= 25°C, unless otherwise specified)

Parameter Symbol Test Conditions Min Typ Max Unit

Logic-High V

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

Q_SW

SHDN

= 1.5V, No Switching -- 725 -- A

= 4.5V, VEN = 0V -- 1 4 A

= 2.7V to 24V 1.6 -- --

Logic-Low VIL V

EN high to low 52 64 80 ms

SHDN

= 2.7V to 24V -- -- 0.8

= 3V 1 -- 10 A

Switching Frequency f LX On Resistance

(N-MOSFET)

OSC

VIN > 5V -- 0.4 0.6 

DS(ON)

= 2.7V to 24V 0.4 0.5 0.6 MHz

Minimum ON T i me -- 6 0 -- ns Maximum Duty Cycle D

MAX

= 0V, Switching -- 92 -- %

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RT8511B

Parameter Symbol Test Conditions Min Typ Max Unit

LED Current

Minimum PWM Dimming Duty Cycle

Feedback Voltage VFB 195 200 205 mV

Fault Protection

Dimming Freq. = 100Hz to 8kHz 1 -- -- %

MIN

LX Current Limit I Over Voltage Protection

Threshold Thermal Shutdown Temperature

1.66 2.2 2.74 A

LIM

1.14 1.2 1.26 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

DS8511B-05 February 2015 www.richtek.com

RT8511B

Typical Operating Characteristics

Efficiency vs. Input Voltage

100

95 90 85 80 75

Efficiency (%)

70 65

4 7 9 121417192224

OUT

Inp ut Vol tage (V)

FB Reference Voltage vs. Te m perature

200

198

196

= 29.5V

FB Reference Voltage vs. Input Voltage

199.5

199.2

198.9

198.6

198.3

FB Refer ence V oltage ( m V)

198.0 4 8 12 16 20 24

Input Vol tage (V)

Frequency vs. Input Voltage

600

550

500

194

192

FB Reference Voltage (mV)

190

-20 5 30 55 80 105

Temperature (°C)

Current Lim it vs. Input Voltage

3.0

2.6

2.2

1.8

Current Lim it ( A)

1.4

1.0

2.5 5.25 8 10.75 13.5 16.25 19 21.75 24.5

Inpu t Voltage (V)

= 4.5V

450

Frequency ( kH z) 1

400

350

4 8 12 16 20 24

Input Vol tage (V)

LED Current vs. PWM Duty Cycle

LED Current (mA)

0 102030405060708090100

PWM Duty Cycle (%)

PWM = 100Hz

= 2kHz

PWM

PWM = 8kHz

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Application Information

RT8511B

The RT8511B is a current mode boost converter which operates at a fixed frequency of 500kHz. It is ca pa ble 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 RT8511B, 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 8kHz. 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 8kHz. 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

DIMC

PWM

DIMC

1µF

To Controller

Figure 3. Block Di agra m of Progra mma ble FB Voltage

Table 1. Minimum Duty for Dimming Frequency

Dimming Frequency Minimum Duty Cycle

100Hz to 8kHz 1%

The FB pin voltage will be decrea sed by lower PWM duty ratio . That will achieve LED current diming function for different brightness. But LED current is more accurate when higher PWM duty. The Table 2. shows typical variation value comparison between different PWM duty and condition is VIN = 3.7V, LED array = 6S2P, R

SET

5Ω.

Table 2. LED Current Variation vs PWM Duty

PWM Duty

(%)

Variation

(%)

PWM Duty

(%)

Variation

(%)

1 ±60 8 ±7 2 ±25 9 ±6 3 ±17 10 ±5 4 ±13 20 ±4 5 ±10 50 ±3 6 ±9 100 ±2.5 7 ±8

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

OUT −VIN

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.

) < 6V] , excessive

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RT8511B

Soft-Start

The RT851 1B 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 RT8511B 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 N-

PWM

soft-start

OUT

Mode1

MOSFET 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.

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

PWM

OUT

Mode1

OUT

soft-start

Mode2

PWM

soft-start

OUT

Mode3

Figure 4. Power On Sequence

PWM

OUT

Mode2

PWM

OUT

Shutdown

Delay

Mode3

Figure 5. Power Off Sequence

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RT8511B

Over Voltage Protection

The RT8511B equips Over Voltage Protection (OVP) function. When the voltage at the OVP pin reaches a threshold of approxi mately 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 1B has a n Over T e 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

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 voltage 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.

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 =

0.4 V I f

IN OUT IN





OUT OUT OSC

)

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 :

P 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 package, the thermal resistance, θJA, is 120°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 f ormulas :

DS8511B-05 February 2015 www.richtek.com

RT8511B

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

D(MAX)

W DF N-8L 2X2 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.

1.0

0.8

0.6

0.4

0.2

Maximum Power Dissipation (W) 1

0.0 0 25 50 75 100 125

Ambient Temperatu re (°C)

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.

Figure 6. Derating Curve of Maxi mum Power Dissi pation

SET

R1R2

SET

DIMC

GND

OUT

OVP

2 3 4

should be connected

WLEDs

Locate R to FB as possible

: : : :

The C

OUT

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

Figure 7. PCB Layout Guide

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

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Outline Dimension

RT8511B

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 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 0.200 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

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.

DS8511B-05 February 2015 www.richtek.com

Richtek RT8511BGQW Schematic [ru]

Specifications and Main Features

Frequently Asked Questions

User Manual