OB3350

Cost Effective LED Controller

GENERAL DESCRIPTION

OB3350 is a highly integrated and cost effective

Light Emitting Diode (LED) driver optimized for

LCD monitor and LCD TV backlighting application.

It provides a high performance LED backlight solution with minimized BOM count.

The OB3350 contains a PWM boost driver which uses current mode control and fixed frequency operation to regulate the LED current. The LED current is sensed through an external current sense resistor. The voltage across the sensing resistor is compared with reference level of 300mV, the error is amplified to control the pulse width of the power switch thus to regulate the current flowing the LED.

The OB3350 offers external low frequency PWM

(LPWM) dimming method for a wide range of dimming control.

The OB3350 offers comprehensive protection such as power MOSFET over current protection

(OCP), output over voltage protection (OVP), IC power supply under voltage lockout (UVLO), and

LED open protection.

FEATURES

Current Mode PWM Controller With Good

Dynamic Response

8V to 35V Input Voltage Range

External PWM Dimming Mode

Over Voltage Protection

Over Current Protection

Under Voltage Lockout (UVLO)

Thermal Shutdown

APPLICATIONS

LCD Monitor

LCD TV

Flat panel display is offered in SOP

TYPICAL APPLICATION

Figure1. OB3350 Typical Application Schematic

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OB3350

Cost Effective LED Controller

Absolute Maximum Ratings

Parameter

VIN Input Voltage to GND

I/O to GND

Operating Ambient Temp. T

A

Operating Junction Temp. T

J

Min/Max Storage Temp. T stg

Lead Temp. (10 Sec)

Value

-0.3V to 40V

-0.3V to 7V

-40 ~ 85

-40 ~ 150

-55 ~150

260

Note:

Stresses beyond those listed under “absolute maximum ratings” may cause permanent damage to the device. These are stress ratings only, functional operation of the device at these or any other conditions beyond those indicated under

“recommended operating conditions” is not implied. Exposure to absolute maximum-rated conditions for extended periods may affect device reliability.

Recommended Operating Range

Parameter Value

Vin Voltage 8V to 35V

Burst Dimming Frequency 100Hz to 1KHz

Package Thermal Characteristics

Parameter

Thermal Resistance JA

(SOP)

Value

Ordering Information

Part Number

OB3350CP

Cost Effective LED Controller

Package Pb- free

P:Pb-free

Packing

Blank:Tube

A: Taping

Y:Year Code

WW:Week Code(01-52)

ZZZ:Lot Code

C:SOP8 Package

P:Pb-free Package

S:Internal Code(Optional)

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6

7

4

5

8

No.

1

2

3

OB3350

Cost Effective LED Controller

Pin Configuration

VIN

GATE

GND

CS

1 8

2

3

OB3350

7

6

4 5

PWM

OVP

COMP

FB

Name

VIN

GATE

GND

CS

FB

COMP

OVP

PWM

I/O

Power

I/O

GND

I/O

I/O

I/O

I/O

Ground

Over Voltage Protection Sense Input

External PWM Dimming Control

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OB3350

Cost Effective LED Controller

Functional Block Diagram

COMP

FB

DIM

OSC

Ramp

Adjust

Soft-start

Thermal

Shutdown

5V

UVLO

Reference

EA

Sense

Amplifer

S PW

M

PWM

Comparator

Vth_ovp

Control

Logic

DIM

Control

CS

VIN

GATE

PWM

GND

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OB3350

Cost Effective LED Controller

Electrical Characteristics

VDDH=12V,PWM=5V, T

A

=25 , if not otherwise noted.

Symbol Conditions Parameter

Current Consumption

Input voltage range

Operating supply current

Stand by current

I

I

VDD

VDD

_

STB

Vin=12V,PWM=5V no load

PWM low >50ms sleep mode

VDD UVLO

UVLO

UVLO Hysteresis

Soft start

UVLO(ON)

UVLO(Hys)

Soft start slope

I sst

_

slop

PWM Control

Operating frequency

Max duty cycle

PWM Input Threshold

F

OSC

D max

PWM(high) High Level

PWM(low) Low Level

Error Amplifier

Reference voltage

Open loop voltage gain

Transconductance of EA

Output source current

V ref

_

ea

A m

Gm

I source

Reference voltage at non-inverting input

Output sink current

I

sin

k

Protection Threshold

OTP ON

OTP OFF

T

OTP

_

ON

T

OTP

_

OFF

Threshold of output over voltage

Threshold of over current protection

(duty=90%)

T dio

V

OVP UV

Normal operation

System startup

Gate Driver Output

T rise

T fall

500pF load

500pF load

Min

8

-

-

V mA

175 200 225 KHz

93 95

2

97 %

V

V

1.8

-

6

Typ Max Units

- 35

2.5 3

310 400 uA

7

400

2.0 2.2 V

330

8

40 80

20 40 mV/mS

0.289 0.3 0.311 V

80

24

70

30

100 120

36 dB uA/V uA

48 60 72 uA

140 160 180

120 140 160

1

200

100 mV mS mV mV ns ns

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OB3350

Cost Effective LED Controller

Function Description

General Operation

The OB3350 is a LED driver designed for boost

DC-DC converters in a constant frequency mode.

The controller implements a peak current mode control scheme and an internal transconductance amplifier to accurately control the output current over a wide input and load conditions.

Low frequency PWM dimming input that can accept an external control signal with a duty ratio of 1%-100%. OB3350 offers comprehensive protection features to protect the system in various fault conditions such as output over voltage protection, boost diode disconnection protection and output short circuit protection. The cycle-by-cycle current limit function limits the maximum current flowing through the external

MOSFET. The over temperature protection ensures that the system will not run into condition of thermal runaway and blow up.

Startup

OB3350 is enabled by applying a voltage of greater than approximately 2.5V to PWM pin. An on-chip internal 150k ohm pull down resistor is

When the

, also the pin of PWM is logic high, the OB3350 will power up and remains power up until the PWM input has been hold low for at least 50ms(typ), this time the chip will enter sleep mode. At sleep mode, the power of theOB3350 will be lower than

400uA (typ). will be lower than is enabled the internal will be activated to source a current less than 4mA to the whole chip.

When PWM is active high, the OB topology connection first. The chip monitors the

OVP pin to see if the Schottky diode (boost diode) is connected or the boost output is short to GND, if the voltage at OVP pin is lower than 100mV, the output will be disabled. The OB3350 will also check other faults (UVLO, OCP and OTP), if there is no fault, then the boost converter will boost up the output with the internal soft start. switching operation when the voltage at pin VIN increases to a voltage above 7.5V.

LED Current Regulation

The LED current is sensed by current sense resistors connected between pin FB and GND.

The sensed error signal is amplified and compared to the CS pin sensing current signal plus the slope compensation to determine the on-time of the switching MOSFET. The error amplifier sources or sinks the current to the COMP pin to adjust the required inductor current as the load changes. The slope-compensation signal is added to the current-sense signal to improve the stability at high duty cycles.

The average LED current is approximated by the following equation

300 [

mV

]

I

[

mA

]

R

FB

[ ]

At light loading, the OB3350 automatically skips some pulses to improve efficiency and prevent overcharging the output capacitor. In this pulse-skipping mode, the inductor current ramps up for a minimum on deliver the stored energy to the output. The switch remains off until another pulse is needed to boost

Slope Compensation

The OB3350 uses a current mode control scheme.

The main advantages of current mode control are inherent cycle-by-cycle current limit for the switch and simpler control loop characteristics. However, current mode control has an inherent instability for duty cycles greater than 50%, which is also called sub-harmonic oscillation. The OB3350 has a build-in slope compensation to avoid sub-harmonic oscillation.

An under-voltage lockout protection feature with a hysteresis of about 300mv is provided for VDDH.

When the voltage at this pin exceeds a threshold of approximately 7.5V (typ), the IC starts the normal operation. If the voltage at this pin drops below a threshold of approximately 7(typ) V, the IC stops switching operation. The IC resumes

Figure4 Sub-harmonic oscillation at duty>50% and compensation slope to avoid sub-harmonic oscillation

If the control signal (which applied to the negative input of the PWM comparator) has no slope and

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OB3350

Cost Effective LED Controller

the slope which representing inductor current ramp up until the comp signal is reached, then ramp down. If the duty > 50%, any distribution won’t be converged from duty to duty and thus sub-harmonic oscillation happens. For the equation below:

I

(

n

1 )

S

S n f

S e

S e

I

(

n

)

To avoid sub-harmonic oscillation happening, just make sure (S f

-S e

) / (S n

+S e

) smaller than 1.

For boost applications, the slopes Se, Sf and Sn can be calculated with the formulas below and a typical value of VSL is 50mV.

S e

V

SL f s

S f

R sen

V out

V in

/

L

S n

R sen

V in

/

L

When Se increases then the factor which can determine whether the sub-harmonic oscillation will occur decreases. When the duty cycle is greater than 50% and the inductance becomes less, the factor increases. And for more flexibility slope compensation amplitude can be increased by adding one external resistor, RSL, in the CS path. Figure 4 shows the setup. The externally generated slope compensation is then added to

3350.

When using external slope compensation, the formula for Se becomes:

S e

(

V

SL

I s

R

SL

)

f s

A typical value of I s

is 40uA.

Dimming Control

The LED brightness is controlled by the PWM signal at PWM pin which has different duty cycle.

OB3350 can accept an external PWM signal to

PWM pin in the range of 100Hz to 1 KHz with a swing voltage of 0V to a level greater than 2.5V.

OVP Protection

OB3350 shuts down the output if the output voltage is higher than the predetermined threshold the current of LED and the resulted voltage drop on sensing resistor the converter will run at maximum duty cycle that boosts the output voltage to a very high level. To prevent this happen, the output voltage is monitored through Pin OVP and converter is shut down if the output voltage exceeds the predetermined level is latched and OB3350

PWM. n if the output voltage exceeds the predetermined level. This protection can be reset from VIN or

VOUT short condition is monitored by the voltage at pins OVP. During normal operation, when the voltage at OVP pin drops below a threshold of approximately 0.2V, the VOUT-short protection function is activated and the GATE is turned off.

This state is latched and can be reset from VIN or

PWM.

Thermal Shutdown

The OB3350 includes a thermal protection circuit.

When the junction temperature exceeds 160°C

(typical), the controller and current sources shut down and do not restart until the junction temperature drops below 140°C (typical).

It’s good design practice to only add as much slope compensation as needed to avoid sub-harmonic oscillation. Additional slope compensation minimized the influence of the sensed current in the control loop. With very large slope compensation the control loop characteristics are similar to a voltage mode regulator which compares the error voltage to a saw tooth waveform rather than the inductor current.

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Reference Application

OB3350

Cost Effective LED Controller

Figure6. OB3350 for external LPWM dimming Reference Application Schematic

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PACKAGE MECHANICAL DATA

OB3350

Cost Effective LED Controller

Symbol

D

E

L

Max

0.310

0.100

4.700

3.800

0.400

0º

1.270 (BSC)

1.650

0.510

0.250

5.150

4.000

6.200

1.270

8º

Dimensions In Inches

Min

0.053

0.002

0.049

0.012

0.004

0.185

0.150

0.228

Max

0.069

0.010

0.065

0.020

0.010

0.203

0.157

0.244

0.050 (BSC)

0.016

0º

0.050

8º

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OB3350

Cost Effective LED Controller

Important Notice

Right to make changes

On-Bright Electronics Corp. reserves the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete.

Warranty information

Customers are responsible for their products and applications using On-Bright’s components, data sheet ciated with customer products and applications customers should provide adequate design and operating safeguards.

Life support

resulting from the use of its products in medical applications.

Military

military applications.

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