RT8472GJ5 - Richtek - Product.Network

DS8472-00 December 2011 www.richtek.com

RT8472

1A, Hysteretic, High Brightness LED Driver with Internal

Switch

General Description

The RT8472 is a high efficiency, continuous mode inductive

step-down converter, designed for driving single or multiple

series connected LEDs from a voltage source higher than

the LED voltage. It operates from an input voltage of 7V to

30V and employs hysteretic control with a high side current

sense resistor to set the constant output current.

The RT8472 includes an output switch and a high side

output current sensing circuit, which uses an external

resistor to set the nominal average output current. LED

brightness control is achieved with PWM dimming from

an analog or PWM input signal.

The RT8472 is available in a small TSOT-23-5 package.

Features

z7V to 30V Input Voltage Range

zHysteretic Control with High Side Current Sensing

zInternal N-MOSFET with 350mΩΩ

ΩΩ

Ω Low RDS(ON)

z1A Output Current

zUp to 97% Efficiency

zTypical ±±

±±

±5% LED Current Accuracy

zAnalog or PWM Control Signal for LED Dimming

z300Hz On-Board Ramp Generator

z Input Under Voltage Lockout

zThermal Shutdown Protection

zRoHS Compliant and Halogen Free

Applications

zAutomotive LED Lighting

zHigh Power LED Lighting

zIndicator and Emergency Lighting

zArchitectural Lighting

zLow Voltage Industrial Lighting

zSignage and Decorative LED Lighting

Ordering Information

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)

TSOT-23-5

Marking Information

LX GND ADJ

VIN SENSE

Package Type

J5 : TSOT-23-5

RT8472

Lead Plating System

G : Green (Halogen Free and Pb Free)

07=DNN

07= : Product Code

YMDNN : Date Code

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RT8472

Function Block Diagram

Typical Application Circuit

Functional Pin Description

Pi n No. Pi n Name Pi n Funct io n

1 LX Switching Node. Open drain output of internal N-MOSFET.

2 GND Ground.

3 ADJ

Dimming Control Input :

--- Analog signal input for analog PWM dimming.

--- PWM signal input for digital PWM dimming.

4 SENSE Output Current Sense Terminal. Sense LED string current.

5 VIN Supply Input Voltage.

RT8472

VIN

GND

SENSE

ADJ

VIN

7V to 30V

optional

CIN

10µF/50V

Regulator Bandgap

UVLO

Ramp Gen.

VIN

ADJ

GND

SENSE

Dimming

UVLO

1.25V

VCC

UVLO

Dimming

MOSFET

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RT8472

Absolute Maximum Ratings (Note 1)

zSupply Input Voltage, VIN ------------------------------------------------------------------------------------- −0.3V to 33V

zSwitch Voltage, LX --------------------------------------------------------------------------------------------- −0.3V to (VIN + 0.7V)

zSense Voltage, SENSE -------------------------------------------------------------------------------------- (VIN − 5V) to (VIN + 0.3V)

zAll Other Pins ----------------------------------------------------------------------------------------------------- −0.3V to 6V

zPower Dissipation, PD @ TA = 25°C

TSOT-23-5 (Single-layer PCB) ------------------------------------------------------------------------------- 0.400W

TSOT-23-5 (Four-layer PCB) --------------------------------------------------------------------------------- 0.625W

zPackage Thermal Resistance (Note 2)

TSOT-23-5, θJA (Single-layer PCB) ------------------------------------------------------------------------- 250°C/W

TSOT-23-5, θJC (Single-layer PCB) ------------------------------------------------------------------------- 130°C/W

TSOT-23-5, θJA (Four-layer PCB) --------------------------------------------------------------------------- 160°C/W

zJunction Temperature ------------------------------------------------------------------------------------------ 150°C

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

zStorage Temperature Range --------------------------------------------------------------------------------- −65°C to 150°C

zESD Susceptibility (Note 3)

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

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

Recommended Operating Conditions (Note 4)

zSupply Input Voltage, VIN ------------------------------------------------------------------------------------- 7V to 30V

zJunction Temperature Range --------------------------------------------------------------------------------- −40°C to 125°C

Electrical Characteristics

(VIN = 12V, TA = 25°C, unless otherwise specified)

Parameter Symbol Test Conditions Min Typ Max Unit

Mean Current Sense Threshold

Voltage VSENSE

Measure on SENSE Pin with

Respect to VIN. ADJ pin is

Floating.

95 100 105 mV

Sense Threshold Hysteresis VSENSEHYS -- ±15 -- %

MOSFET On-Resistance RDS(ON) -- 350 -- mΩ

MOSFET Leakage Current VLX = 5V -- 0.01 10 μA

Under Voltage Lockout Threshold VUVLO V

IN Rising -- 4 -- V

Under Voltage Lockout Threshold

Hysteresis ΔVUVLO -- 200 -- mV

Ramp Frequency fRAMP -- 300 -- Hz

Logic-High VADJ_H 1.4 -- 5.5

ADJ Input Threshold

Voltage Logic-Low VADJ_L -- -- 0.2

Analog Dimming Range 0.4 -- 1.2 V

Minimum Switch On Time tON(MIN) LX Switch On -- 210 -- ns

Minimum Switch Off Time tOFF(MIN) LX Switch Off -- 170 -- ns

To be continued

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RT8472

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 single-layer and four-layer test board of JEDEC 51. The measurement case position

of θJC is on the lead 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.

Parameter Symbol Test Conditions Min Typ Max Unit

Quiescent Supply Current with

Output Off IVIN_Off V

ADJ = 0V -- 450 -- μA

Quiescent Supply Current with

Output Switching IVIN_On ADJ Pin is Floating, fSW = 250kHz,

VIN = 8V -- 1000 -- μA

Internal Propagation Delay tPD -- 25 -- ns

Sense Pin Input Current ISENSE V

SENSE = VIN – 0.1V -- 300 -- nA

Thermal Shutdown TSD -- 150 -- °C

Thermal Shutdown Hysteresis ΔTSD -- 30 -- °C

DS8472-00 December 2011 www.richtek.com

RT8472

Typical Operating Characteristics

Duty Cyc le vs. Input Voltage

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 7 14 21 28 35

Input Voltage (V)

Duty cycle (%)

100

1 LED

2 LED

3 LED

4 LED

5 LED

6 LED

7 LED

8 LED

VIN = 7V to 30V, ILED = 1A, L = 33μH

Output Current vs. Input Voltage

0.96

0.97

0.98

0.99

1.00

1.01

1.02

1.03

1.04

0 7 14 21 28 35

Input Voltage (V)

Output Current (A

)

1 LED

2 LED

3 LED

4 LED

5 LED

6 LED

7 LED

8 LED

VIN = 7V to 30V, ILED = 1A, L = 33μH

Output Current De viation vs. Input Voltage

-4%

-3%

-2%

-1%

0 7 14 21 28 35

Input Voltage (V)

Output Current Deviation (%) 1

1 LED

2 LED

3 LED

4 LED

5 LED

6 LED

7 LED

8 LED

VIN = 7V to 30V, ILED = 1A, L = 33μH

-1

-2

-3

-4

Output Current Devia tion vs. Input Voltage

-6%

-4%

-2%

0 7 14 21 28 35

Input Voltage (V)

Output Current Deviation (%) 1

-2

-4

-6

1 LED

7 LED

3 LED

VIN = 7V to 30V, ILED = 1A, L = 33μH

Efficiency vs. Input Voltage

70%

75%

80%

85%

90%

95%

100%

0 7 14 21 28 35

Input Voltage (V)

Efficiency (%)

1 LED

100

7 LED

3 LED

VIN = 7V to 30V, ILED = 1A, L = 33μH

Switching Frequency vs. Input Voltage

150

250

350

450

550

650

750

850

0 7 14 21 28 35

Input Voltage (V)

Switching Frequency (kHz) 1

1 LED

2 LED

3 LED

4 LED

5 LED

6 LED

7 LED

8 LED

VIN = 7V to 30V, ILED = 1A, L = 33μH

DS8472-00 December 2011www.richtek.com

RT8472

RS = 0.1Ω, fDIMMING = 500Hz

Output Curren t vs. P WM Duty Cycle

200

400

600

800

1000

1200

0 20406080100

PWM Duty Cycle (%)

Output Current (mA

)

RS = 0.1Ω, fDIMMING = 10kHz

Output Current vs. ADJ Voltage

200

400

600

800

1000

1200

0.2 0.5 0.8 1.1 1.4 1.7

ADJ Voltage (V)

Output Current (mA

)

VIN = 12V, 1LED

RS = 350mΩ

RS = 150mΩ

RS = 100mΩ

Outp ut Curren t vs. PWM Dut y Cycle

200

400

600

800

1000

1200

020406080100

PWM Duty Cycle (%)

Output Current (mA

)

Quiesce nt Input Current vs. Input Voltage

200

400

600

800

1000

1200

0 7 14 21 28 35

Input Voltage (V)

Quiescent Input Current (μA

)

Output Switching

Quiesc e nt Input Current vs. Input Voltage

460

465

470

475

480

0 7 14 21 28 35

Input Voltage (V)

Quiescent Input Current (μA) 1

Output Off

MOSFET On-Resistance vs. Temperature

250

300

350

400

450

500

550

600

-50 -25 0 25 50 75 100 125 150

Temperature (°C)

On-Resistance (m )

(mΩ)

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RT8472

Ramp Frequency vs. Te m pe rature

290

295

300

305

310

315

320

325

330

-50 -25 0 25 50 75 100 125 150

Temperature (°C)

Ramp Frequency (Hz) 1

Time (5μs/Div)

Digital Dimming from ADJ On

VIN = 12V, RS = 0.1Ω, fDIMMING = 500Hz, 1 LED

VADJ

(2V/Div)

IOUT

(500mA/Div)

Time (5μs/Div)

Digital Dimming from ADJ Off

VIN = 12V, RS = 0.1Ω, fDIMMING = 500Hz, 1 LED

VADJ

(2V/Div)

IOUT

(500mA/Div)

Ramp Frequency vs. Input Voltage

290

295

300

305

310

315

320

325

330

0 7 14 21 28 35

Input Voltage (V)

Ramp Frequency (Hz

)

Power Off from VIN

Time (500μs/Div)

RS = 0.1Ω, 1 LED

VIN

(5V/Div)

IOUT

(500mA/Div)

Time (500μs/Div)

Power On from VIN

RS = 0.1Ω, 1 LED

VIN

(5V/Div)

IOUT

(500mA/Div)

DS8472-00 December 2011www.richtek.com

RT8472

Application Information

The RT8472 is a simple high efficiency, continuous mode

inductive step-down converter. The device operates with

an input voltage range from 7V to 30V and delivers up to

1A of output current. A high side current sense resistor

sets the output current and a dedicated PWM dimming

input enables pulsed LED dimming over a wide range of

brightness levels. A high side current sensing scheme

and an onboard current setting circuitry minimize the

number of external components. A 1% sense resistor

performs a ±3% LED current accuracy for the best

performance.

Under Voltage Lockout (UVLO)

The RT8472 includes a UVLO feature with 200mV

hysteresis. The internal MOSFET turns off when VIN falls

below 3.8V (typ.).

Setting Average Output Current

The RT8472 output current which flows through the LEDs

is set by an external resistor (RS) connected between the

VIN and SENSE terminal. The relationship between output

current (IOUT) and RS is shown as below :

ADJ

OUTavg

V 0.4

0.1V

I = R0.8

⎛⎞ −

⎜⎟

⎝⎠

OUTavg

0.1V

I = R

Analog Dimming Control

The ADJ terminal can be driven by an external voltage

(VADJ) to adjust the output current to an average value set

by RS. The average output current is given by :

where VADJ is ranged from 0.4V to 1.2V. When VADJ is

larger than 1.2V, the output current value will just be set

by the external resistor (RS).

Digital Dimming Control

A Pulse Width Modulated (PWM) signal can drive the ADJ

terminal directly. Notice that the PWM signal logic high

level must be above 1.4V and the logic low level must be

below 0.2V at the ADJ terminal. It's recommended to

maintain the PWM dimming at low frequency (ex. 500Hz

) in order to obtain a linear dimming curve.

PWM Soft-Start Behavior

The RT8472 features an optional PWM soft-start behavior

that allows for gradual brightness transition. This is

achieved by simply connecting an external capacitor

between the ADJ pin and GND. An internal current source

will then charge this capacitor for soft-start behavior,

resulting in steady LED current increase and decrease

during power on and power off, as shown in Figure 1.

Figure 1. PWM Soft-Start Behavior Mechanism

( A )

1.2V

0.4V

Internal

VRAMP

VADJ 0V

ILED

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RT8472

The capacitor can be selected according to below

equation :

C = 1.5 x 10-6 x tSS

where tSS is the soft-start period.

LED Current Ripple Reduction

Higher LED current ripple will shorten the LED life time

and increase heat accumulation of LED. By adding an

output capacitor in parallel with the LED. This will then

allow the use of a smaller inductor.

Inductor Selection

The inductance is determined by inductor current ripple,

switching frequency, duty ratio, circuit specifications and

component parameters, as expressed in the following

equation :

where

VD is the rectifier diode forward voltage (V)

VSEN is the voltage cross current sense resistor (V)

RL is the inductor DC resistance (Ω)

L is the inductance (H)

The saturation current of the selected inductor must be

higher than the peak output LED current, and the

continuous current rating must be above the average output

LED current. In general, the inductor saturation current

should be 1.5 times the LED current. In order to reduce

the output current ripple, a higher inductance is

recommended at higher supply voltages. However, it could

also cause a higher line resistance and result in a lower

efficiency.

Diode Selection

To obtain better efficiency, the Schottky diode is

recommended for its low reverse leakage current, low

recovery time and low forward voltage. With its low power

dissipation, the Schottky diode outperforms other silicon

diodes and increase overall efficiency.

Input Capacitor selection

Input capacitor has to supply peak current to the inductor

and flatten the current ripple on the input. The low ESR

condition is required to avoid increasing power loss. The

ceramic capacitor is recommended due to its excellent

high frequency characteristic and low ESR, which are

suitable for the RT8472. For maximum stability over the

entire operating temperature range, capacitors with better

dielectric are suggested.

Thermal Protection

A thermal protection feature is included to protect the

RT8472 from excessive heat damage. When the junction

temperature exceeds a threshold of 150°C, the thermal

protection will turn off the LX terminal. When the junction

temperature drops below 125°C, the RT8472 will turn back

on the LX terminal and return to normal operations.

where

fSW is the switching frequency (Hz)

RDS(ON) is the on-resistance of internal MOSFET ( = 0.35Ω

typical)

D is the duty cycle determined by VOUT/VIN

IOUT is the required LED current (A)

ΔIL is the inductor peak-peak ripple current (internally set

to 0.3 x IOUT)

VIN is the input supply voltage (V)

VOUT is the total LED forward voltage (V)

Besides, the selected inductance has also to satisfy the

limit of the minimum switch on/off time. The calculated

on time must be greater than 210ns of the minimum on

time, and the off time must be greater than 170ns of the

minimum off time. The following equation can be used to

verify the suitability of the inductor value.

()

IN OUT SEN DS(ON) OUT

SW L

L > V V V R I fI

⎡⎤

−−− × ×

⎣⎦

×Δ

()

×Δ

=−− ++

IN OUT OUT SEN L DS(ON)

ON(MIN)

tVV I R RR

t (210ns typ.)

()

OFF

LOFF(MIN)

OUT D SEN OUT L

LI t (170ns typ.)

VVV IR

×Δ >

++ + ×

DS8472-00 December 2011www.richtek.com

RT8472

Figure 3. PCB Layout Guide

Figure 2. Derating Curves for the RT8472 Packages

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX) − TA) / θJA

where TJ(MAX) is the maximum junction temperature, TA is

the ambient temperature, and θJA is the junction to ambient

thermal resistance.

For recommended operating condition specifications of

the RT8472, the maximum junction temperature is 125°C

and TA is the ambient temperature. The junction to ambient

thermal resistance, θJA, is layout dependent. For

TSOT-23-5 package, the thermal resistance, θJA, is 250°C/

W on a standard JEDEC 51-3 single-layer thermal test

board and 160°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 formulas :

PD(MAX) = (125°C − 25°C) / (250°C/W) = 0.4W for

TSOT-23-5 package (single-layer PCB)

PD(MAX) = (125°C − 25°C) / (160°C/W) = 0.625W for

TSOT-23-5 package (four-layer PCB)

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance, θJA. For the RT8472 package, the derating

curves in Figure 2 allow the designer to see the effect of

rising ambient temperature on the maximum power

dissipation.

Layout Considerations

For best performance of the RT8472, please abide the

following layout guide.

`The capacitor CIN, CADJ and external resistor, RS, must

be placed as close as possible to the VIN and SENSE

pins of the device respectively.

`The GND should be connected to a strong ground plane.

`Keep the main current traces as short and wide as

possible.

`The inductor (L) should be mounted as close to the

device with low resistance connections.

`The ADJ pin trace need to be kept far away from LX

terminal.

LX GND ADJ

VIN SENSE

CIN

LLED-

VIN

GND

LED+

Place the capacitor

CIN as close as

possible to VIN pin.

Place the resistor RS as close as

possible to VIN and SENSE pins.

Place the capacitor

CADJ as close as

possible to the ADJ

pin.

CADJ

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 25 50 75 100 125

Ambient Temperature (°C)

Maximum Power Dissipation (W)1

TSOT23-5 (Four-Layer PCB)

TSOT23-5 (Single-Layer PCB)

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RT8472

Richtek Technology Corporation

Headquarter

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

Hsinchu, Taiwan, R.O.C.

Tel: (8863)5526789 Fax: (8863)5526611

Information that is provided by Richtek Technology Corporation is believed to be accurate and reliable. Richtek reserves the right to make any change in circuit

design, specification or other related things if necessary without notice at any time. No third party intellectual property infringement of the applications should be

guaranteed by users when integrating Richtek products into any application. No legal responsibility for any said applications is assumed by Richtek.

Richtek Technology Corporation

Taipei Office (Marketing)

5F, No. 95, Minchiuan Road, Hsintien City

Taipei County, Taiwan, R.O.C.

Tel: (8862)86672399 Fax: (8862)86672377

Email: marketing@richtek.com

TSOT-23-5 Surface Mount Package

Dimension s In Millimeters Dimensions In Inches

Symbol Min Max Min Max

A 0.700 1.000 0.028 0.039

A1 0.000 0.100 0.000 0.004

B 1.397 1.803 0.055 0.071

b 0.300 0.559 0.012 0.022

C 2.591 3.000 0.102 0.118

D 2.692 3.099 0.106 0.122

e 0.838 1.041 0.033 0.041

H 0.080 0.254 0.003 0.010

L 0.300 0.610 0.012 0.024

AA1

Outline Dimension