LM3435SQ/NOPB - NATIONAL SEMICONDUCTOR

LM3435

LM3435 Compact Sequential Mode RGB LED Driver with I 2 C Control Interface

Literature Number: SNVS724A

LM3435

October 4, 2011

Compact Sequential Mode RGB LED Driver with I2C Control

Interface

General Description

The LM3435, a Synchronously Rectified non-isolated Flyback

Converter, features all required functions to implement a high-

ly efficient and cost effective RGB LED driver. Different from

conventional Flyback converter, LEDs connect across the

VOUT pin and the VIN pin through internal passing elements

at corresponding LED pins. Thus, voltage across LEDs can

be higher than, equal to or lower than the input supply voltage.

Load current to LEDs is up to 2A with voltage across LEDs

ranging from 2.0V to 4.5V. Integrated N-Channel main MOS-

FET, P-Channel synchronous MOSFET and three N-Channel

current regulating pass switches allow low component count,

thus reducing complexity and minimize board size. The

LM3435 is designed to work exceptionally well with ceramic

output capacitors with low output ripple voltage. Loop com-

pensation is not required resulting in a fast load transient

response. Non-overlapping RGB LEDs are driven sequen-

tially through individual control. Output voltage hence can be

optimized for different forward voltage of LEDs during the

non-overlapping period. I2C interface eases the programming

of the individual RGB LED current up to 1,024 levels per

channel.

The LM3435 is available in the thermally enhanced LLP-40

package.

Key Specifications

■Support up to 2A LED current

■Typical ±3% LED current accuracy

■Integrated N-Channel main and P-Channel synchronous

MOSFETs

■3 Integrated N-Channel current regulating pass switches

■LED Currents programmable via I2C bus independently

■Input voltage range 2.7V - 5.5V

■Thermal shutdown

■Thermally enhanced LLP-40 package

Features

■Sequential RGB driving mode

■Low component count and small solution size

■Stable with ceramic and other low ESR capacitors, no loop

compensation required

■Fast transient response

■Programmable converter switching frequency up to 1 MHz

■MCU interface ready with I2C bus

■Peak current limit protection for the switcher

■LED fault detection and reporting via I2C bus

Applications

■Li-ion batteries / USB Powered RGB LED driver

■Pico / Pocket RGB LED Projector

LM3435 Compact Sequential Mode RGB LED Driver with I2C Control Interface

Typical Application Circuit

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LM3435

Connection Diagram

Top View

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40-pin Leadless Leadframe Package (LLP)

5.0 x 5.0 x 0.8mm, 0.4mm pitch

NS Package Number SQF40A

Ordering Information

Order Number Spec. Package Type NSC Package Drawing Supplied As

LM3435SQ NOPB LLP-40 SQF40A 1000 Units, Tape and Reel

LM3435SQX NOPB LLP-40 SQF40A 4500 Units, Tape and Reel

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LM3435

Pin Descriptions

Pin Name Type Description Application Information

1, 2, 38, 39,

PGND Ground Power Ground Ground for power devices, connect to GND.

3 CG Output GREEN LED capacitor Connect a capacitor to Ground for GREEN LED.

Minimum 1nF.

4 CB Output BLUE LED capacitor Connect a capacitor to Ground for BLUE LED. Minimum

1nF.

5 CR Output RED LED capacitor Connect a capacitor to Ground for RED LED. Minimum

1nF.

6 IREFG Output Current Reference for GREEN LED Connect a resistor to Ground for GREEN LED current

reference generation.

7 IREFB Output Current Reference for BLUE LED Connect a resistor to Ground for BLUE LED current

reference generation.

8 IREFR Output Current Reference for RED LED Connect a resistor to Ground for RED LED current

reference generation.

9 GND Ground Ground

10, 29 SGND Ground I2C Ground Ground for I2C control, connect to GND.

11 SVDD Power I2C VDD VDD for I2C control.

12 SDATA Input /

Output

DATA bus Data bus for I2C control.

13 SCLK Input CLOCK bus Clock bus for I2C control.

14, 15, 16,

17, 37

VIN Power Input supply voltage Supply pin to the device. Nominal input range is 2.7V to

5.5V.

18 GCTRL Input GREEN LED control On/Off control signal for GREEN LED. Internally pull-low.

19 BCTRL Input BLUE LED control On/Off control signal for BLUE LED. Internally pull-low.

20 RCTRL Input RED LED control On/Off control signal for RED LED. Internally pull-low.

21, 22 RLED Output RED LED cathode Connect RED LED cathode to this pin.

23, 24 BLED Output BLUE LED cathode Connect BLUE LED cathode to this pin.

25, 26 GLED Output GREEN LED cathode Connect GREED LED cathode to this pin.

27 FAULT Output Fault indicator Pull-up when LED open or short is being detected.

28 EN Input Enable pin Internally pull-up. Connect to a voltage lower than 0.2 x

VIN to disable the device.

30, 31, 32 VOUT Input /

Output

Output voltage Connect anodes of LEDs to this pin.

33 RT Input ON-time control An external resistor connected from VOUT to this pin sets

the main MOSFET on-time, hence determine the

switching frequency.

34, 35, 36 SW Output Switch node Internally connected to the drain of the main N-channel

MOSFET and the P-channel synchronous MOSFET.

Connect to the output inductor.

EP EP Ground Exposed Pad Thermal connection pad, connect to the GND pin.

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LM3435

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

VIN to GND -0.3V to 6.0V

VOUT, RT to VIN -0.3V to 5.5V

RLED, GLED, BLED to VIN -0.3V to 5.5V

SW to GND -0.3V to 11.5V

SW to GND (Transient) -2V to 13V

(<100 ns)

All other inputs to GND -0.3V to 6.0V

ESD Rating (Note 2)

Human Body Model ±1.5 kV

Storage Temperature -65°C to +150°C

Junction Temperature (TJ) -40°C to +125°C

Operating Ratings (Note 1)

Supply Voltage Range (VIN) 2.7V to 5.5V

Junction Temp. Range (TJ) -40°C to +125°C

Thermal Resistance (θJB)

(Note 3)

28°C/W

Electrical Characteristics Specification with standard type are for TA = TJ = +25°C only; limits in boldface type

apply over the full Operating Junction Temperature (TJ) range. Minimum and Maximum are guaranteed through test, design or

statistical correlation. Typical values represent the most likely parametric norm at TJ = +25°C, and are provided for reference

purposes only. Unless otherwise stated the following conditions apply: VIN = 5V and VOUT – VIN = 3V.

Symbol Parameter Conditions Min Typ Max Units

Supply Characteristics

IIN IIN operating current No switching 5 10 mA

IIN-SD IIN Shutdown current VEN = 0V 8 30 µA

ISVDD SVDD standby supply current VSVDD = 5V, I2C Bus idle 1µA

VINUVLO VIN under-voltage lock-out VIN decreasing 2.5 V

VINUVLO_hys VIN under-voltage lock-out hysteresis VIN increasing 0.2 V

Enable Input

VEN EN Pin input threshold VEN rising 0.8 x

VIN

VEN falling 0.2 x

VIN

IEN Enable Pull-up Current VEN = 0V 5 µA

Logic Inputs (RCTRL, GCTRL and BCTRL)

VCTRL CTRL pins input threshold VCTRL rising

(VIN = 2.7V to 5.5V)

1.35 V

VCTRL falling

(VIN = 2.7V to 5.5V)

0.63

Switching Characteristics

RDS-M-ON Main MOSFET RDS(ON) VGS(MAIN) =VIN = 5.0V

ISW(sink) = 100mA

0.04 0.1 Ω

RDS-S-ON Syn. MOSFET RDS(ON) VGS(SYN) = VOUT - 5.0V

ISW(source) = 100mA

0.06 0.2 Ω

Current Limit

ICL Peak current limit through main MOSFET

threshold

6 8.5 A

ON/OFF Timer

tON ON timer pulse width RRT = 499 kΩ 750 ns

tON-MIN ON timer minimum pulse width 80 ns

tOFF OFF timer minimum pulse width 155 ns

RGB Driver Characteristics (RLED, BLED and GLED)

RDS(RED) Red LED Switch RDS VOUT - VIN = 3.3V

ILED = 1.5A

I2C code = 3FFh

0.1 0.2 Ω

RDS(BLU) Blue LED Switch RDS VOUT - VIN = 3.3V

ILED = 1.5A

I2C code = 3FFh

0.1 0.2 Ω

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LM3435

Symbol Parameter Conditions Min Typ Max Units

RDS(GRN) Green LED Switch RDS VOUT - VIN = 3.3V

ILED = 1.5A

I2C code = 3FFh

0.1 0.2 Ω

ILEDMAX Max. LED current (Note 4) VIN = 4.5V to 5.5V,

0°C ≤ TA ≤ 50°C

2 A

I1.5A,3FFh Current accuracy (3FFh) VIN = 2.7V to 5.5V

RIREF = 16.5 kΩ,

VOUT – VIN = 2.4V (RLED),

3.3V (GLED/BLED)

1.455 1.5 1.545 A

1.425 1.575 A

I1.5A,1FFh Current (1FFh) 0.8 A

I1.5A,001h Current (001h) 1.2 mA

FAULT Output Characteristics

VOH Output high voltage VIN = 2.7V to 5.5V,

IOH = -100µA

VIN –

0.1

VIN = 2.7V to 5.5V,

IOH = -5mA

VIN –

0.5

VOL Output low voltage VIN = 2.7V to 5.5V,

IOL = 100µA

0.1 V

VIN = 2.7V to 5.5V,

IOL = 5mA

0.5 V

Thermal Shutdown

TSD Thermal shutdown temperature TJ rising 163 °C

TSD-HYS Thermal shutdown temperature hysteresis TJ falling 20 °C

I2C Logic Interface Electrical Characteristics (1.7 V < SVDD < VIN )

Logic Inputs SCL, SDA

VIL Input Low Level 0.2 x

SVD

VIH Input High Level 0.8 x

SVDD

ILLogic Input Current -1 1 µA

fSCL Clock Frequency 400 kHz

Logic Output SDA

VOL Output Low Level ISDA = 3mA 0.3 0.5 V

ILOutput Leakage Current VSDA = 2.8V 2 µA

Note 1: Absolute Maximum Ratings are limits which damage to the device may occur. Operating ratings are conditions under which operation of the device is

intended to be functional. For guaranteed specifications and test conditions, see the electrical characteristics.

Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.

Note 3: θJB is junction-to-board thermal characteristic parameter. For packages with exposed pad, θJB is significantly dependent on PC boards. So, only when

the PC board under end-user environments is similar to the 2L JEDEC board, the corresponding θJB can be used to predict the junction temperature. θJB value

is obtained by NS Thermal Calculator© for reference only.

Note 4: Maximum LED current measured at VIN = 4.5V to 5.5V with heat sink on top of LM3435 with no air flow at 0°C ≤ TA ≤ 50°C. Operating conditions differ

from the above is not guaranteed.

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LM3435

Typical Performance Characteristics All curves taken at VIN = 5V with configuration in typical application

for driving one red (OSRAM LRW5AP-KZMX), one green (OSRAM LTW5AP-LZMY) and one blue (OSRAM LBW5AP-JYKX) LEDs

with IOUT per channel = 1.5A under TA = 25°C, unless otherwise specified.

IIN-SD vs VIN

23456

IIN-SD (μA)

VIN (V)

-40°C

25°C

125°C

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IIN (no switching) vs VIN

23456

3.0

3.5

4.0

4.5

5.0

5.5

6.0

IIN (mA)

VIN (V)

125°C

25°C

-40°C

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ISVDD vs VIN

23456

ISVDD (nA)

VSVDD (V)

125°C

25°C

-40°C

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RDS-M-ON vs VIN

23456

RDS-M-ON (mΩ)

VIN (V)

125°C

25°C

-40°C

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RDS-S-ON vs VIN

23456

RDS-S-ON (mΩ)

VIN (V)

125°C

25°C

-40°C

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RIREFx vs ILEDx

5 15 25 35 45 55

0.0

0.5

1.0

1.5

2.0

2.5

ILEDx(A)

RIREFx (kΩ)

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LM3435

ILED(RED) vs VIN

23456

1.46

1.48

1.50

1.52

1.54

ILED(RED) (A)

VIN (V)

125°C

25°C

-40°C

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RDS(RED) vs VIN

23456

100

120

140

160

RDS(RED) (mΩ)

VIN (V)

125°C

25°C

-40°C

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ILED(GRN) vs VIN

23456

1.46

1.48

1.50

1.52

1.54

ILED(GRN) (A)

VIN (V)

125°C

-40°C

25°C

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RDS(GRN) vs VIN

23456

100

120

140

160

RDS(GRN) (mΩ)

VIN (V)

125°C

25°C

-40°C

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ILED(BLU) vs VIN

23456

1.46

1.48

1.50

1.52

1.54

ILED(BLU) (A)

VIN (V)

125°C

25°C

-40°C

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RDS(BLU) vs VIN

23456

100

120

140

160

RDS(BLU) (mΩ)

VIN (V)

125°C

25°C

-40°C

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LM3435

RED Efficiency vs VIN @ TA = 25°C

23456

100

RED EFFICIENCY, ηRED(%)

VIN (V)

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GREEN Efficiency vs VIN @ TA = 25°C

23456

100

GREEN EFFICIENCY, ηGRN(%)

VIN (V)

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BLUE Efficiency vs VIN @ TA = 25°C

23456

100

BLUE EFFICIENCY, ηBLU(%)

VIN (V)

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Power Up Transient

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10ms/DIV

RGB Sequential Mode Operation

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1ms/DIV

Color Transition Delay

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100µs/DIV

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LM3435

Simplified Functional Block Diagram

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Operation Description

INTRODUCTION

The LM3435 is a sequential LED driver for portable and pico

projectors. The device is integrated with three high current

regulators, low side MOSFETs and a synchronous flyback

DC-DC converter. Only single LED can be enabled at any

given time. The DC-DC converter quickly adjusts the output

voltage to an optimal level based on each LED’s forward volt-

age. This minimizes the power dissipation at the current

regulators and maximizes the system efficiency. The I2C com-

patible synchronous serial interface provides access to the

programmable functions and registers of the device. I2C pro-

tocol uses a two-wire interface for bi-directional communica-

tions between the devices connected to the bus. The two

interface lines are the Serial Data Line (SDA), and the Serial

Clock Line (SCL). These lines should be connected to a pos-

itive supply, via a pull-up resistor and remain HIGH even when

the bus is idle. Every device on the bus is assigned an unique

address and acts as either a Master or a Slave depending on

whether it generates or receives the serial clock (SCL).

SYNCHRONOUS FLYBACK CONVERTER

The LM3435 integrates a synchronous flyback DC-DC con-

verter to power the three-channel current regulator. The LEDs

are connected across VOUT of the flyback converter and VIN

through an internal power MOSFET connecting to corre-

sponding LED channel. The maximum current to LED is 2A

and the maximum voltage across VOUT and VIN is limited at

around 4.7V. The LM3435 integrates the main N-channel

MOSFET, the synchronous P-channel MOSFET of the fly-

back converter and three N-channel MOSFETs as internal

passing elements connecting to LED channels in order to

minimize the solution components count and PCB space.

The flyback converter of LM3435 employs a proprietary Pro-

jected On-Time (POT) control scheme to determine the on-

time of the main N-channel MOSFET with respect to the input

and output voltages together with an external switching fre-

quency setting resistor connected to RT pin, RRT. POT control

use information of the current passing through RRT from

VOUT, voltage information of VOUT and VIN to find an ap-

propriate on-time for the circuit operations. During the on-time

period, the inductor connecting to the flyback converter is

charged up and the output capacitor is discharged to supply

power to the LED. A cycle-by-cycle current limit of typical 6A

is imposed on the main N-channel MOSFET for protection.

After the on-time period, the main N-channel MOSFET is

turned off and the synchronous P-channel MOSFET is turned

on in order to discharge the inductor. The off state will last

until VOUT is dropped below a reference voltage. Such ref-

erence voltage is derived from the required LED current to be

regulated at a particular LED channel. The flyback converter

under POT control can maintain a fairly constant switching

frequency that depends mainly on value of the resistor con-

nected across VOUT and RT pins, RRT. The relationship

between the flyback converter switching frequency, FSW and

RRT is approximated by the following relationship:

RRT in Ω and FSW in kHz

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LM3435

In addition, POT control requires no external compensation

and achieves fast transient response of the output voltage

changes that perfectly matches the requirements of a se-

quential RGB LED driver. The POT flyback converter only

operates at Continuous Conduction Mode. Dead-time be-

tween main MOSFET and synchronous MOSFET switching

is adaptively controlled by a minimum non-overlap timer to

prevent current shoot through. Initial VOUT will be regulated

at around 3.2V to 3.5V above VIN before any control signals

being turned on. Three small capacitors connected to CR, CG

and CB pins are charged by an internal current source and

act as soft-start capacitors of the flyback converter during

start-up. Once initial voltage of VOUT is settled, the capaci-

tors will be used as a memory element to store the VOUT

information for each channel respectively. This information

will be used for VOUT regulation of respective LED channel

during channel switching. In between the channel switching,

a small I2C programmable blank out time of 5 µs to 35 µs is

inserted so that the LED current is available after the correct

VOUT for the color is stabilized. This control scheme ensures

the minimal voltage headroom for different color LED and

hence best conversion efficiency can be achieved.

HIGH CURRENT REGULATORS

The LM3435 contains three internal current regulators pow-

ered by the output of the synchronous Flyback Converter,

VOUT. Three low side power MOSFETs are included. These

current regulators control the current supplied to the LED

channels individually and maintain accurate current regula-

tion by internal feedback and control mechanism. The regu-

lation is achieved by a Gm-C circuit comparing the sensing

voltage of the internal passing N-channel MOSFET and an

internal LED current reference voltage generated from the

external reference current setting resistor, RIREFx connect to

IREFG, IREFB or IREFR pin, of the corresponding LED chan-

nel. The nominal maximum LED current is governed by the

equation in below:

RIREFx in Ω and ILEDx in Ampere

The LED current setting can be in the range of 0.5A up to 2A

maximum. The nominal maximum of the device is 1.5A and

for applications need higher than 1.5A LED current, VIN and

thermal constrains must be complied. The actual LED current

can be adjusted on-the-fly by the internal ten bits register for

individual channel. The content of these registers are user

programmable via I2C bus connection. The user can control

the LED output current on-the-fly during normal operation.

The resolution is 1 out of 1024 part of the LED current setting.

The user can program the registers in the range of 1(001H)

to 1023(3FFH) for each channel independently, provided the

converter is not entered the Discontinuous Conduction Mode.

Whenever the converter operation entered the Discontinuous

Conduction Mode, the regulation will be deteriorated. A value

of “0” may cause false fault detection, so it must be avoided.

SEQUENTIAL MODE RGB TIMING

LM3435 is a sequential mode RGB driver dedicatedly de-

signed for pico and portable projector applications. By using

this device, the system only require one power driver stage

for three color LEDs. With LM3435, only single LED can be

enabled at any given time period and the DC-DC converter

can quickly adjusts the output voltage to an optimized level

by controlling the current flowing into the respective LED

channel. This approach minimizes the power dissipation of

the internal current regulator and effectively maximizes the

system efficiency. Timing of the RGB LEDs depends solely

on the RCTRL, GCTRL and BCTRL inputs. The Timing Chart

in below shows a typical timing of two cycles of even RGB

scan. In real applications, the RGB sequence is totally con-

trolled by the system or the video processor. It’s not manda-

tory to follow the simple RGB sequence, but for any change

instructed by the I2C control will only take place at the falling

edge of the corresponding CTRL signal.

30162520

RGB Control Signals Timing Chart

PRIORITIES OF LED CONTROL SIGNALS

The LM3435 does not support color overlapping mode oper-

ation. At any instant, only one LED will be enabled even

overlapping control signals applied to the control inputs. The

decision logics of the device determine which LED channel

should be enabled in case overlapping control signals are

detected at the control inputs. The GREEN channel has the

higher priority over BLUE channel and the RED channel has

the lowest priority. However, if a low priority channel is already

turned on before the high priority channel control signal

comes in, the low priority channel will continue to take the

control until the control signal ceased. The timing diagram in

below illustrates some typical cases during operation.

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LM3435

30162521

Priorities of LED Control Signals

LED OPEN FAULT REPORTING

The fly-back converter tries to keep VOUT to the forward volt-

age required by the LED with the desired LED current output.

However, if the LED channel is being opened no matter it is

due to LED failure or no connection, the fly-back converter will

limit the VOUT voltage at around 4.7V above VIN. Once such

voltage is achieved, an open-fault-suspect signal will go high.

If this open-fault-suspect signal is being detected at 3 con-

secutive falling edges of the opened channel control signal,

“Fault” pin will be latched high and the corresponding channel

open fault will be reported through I2C. The open fault report

can be removed either by pulling EN pin low for less than

100ns (a true shutdown will be triggered if the negative pulse

on EN is more than 100ns) or by writing a “0” to “bit 0” of the

I2C register ”05h”. The “Fault” pin will be cleared and the I2C

fault register will be reset. In order to reinstate the fault re-

porting feature, the system need to write a “1” to “bit 0” of the

I2C register “05h”.

LED SHORT FAULT REPORTING

If the VOUT is prohibited to regulate at a potential higher than

1.5V above VIN at a LED channel, such LED is considered

being shorted and a short-fault-suspect signal will go high. If

this short-fault-suspect signal is being detected at 3 consec-

utive falling edges of the shorted channel control signal,

“Fault” pin will be latched high and the corresponding channel

short fault will be reported through I2C. The short fault report

can be removed either by pulling EN pin low for less than

100ns (a true shutdown will be triggered if the negative pulse

on EN is more than 100ns) or by writing a “0” to “bit 0” of the

I2C register ”05h”. The “Fault” pin will be cleared and the I2C

fault register will be reset. In order to reinstate the fault re-

porting feature, the system need to write a “1” to “bit 0” of the

I2C register “05h”. Persistently short of LED can cause per-

manent damage to the device. Whenever the short fault is

detected, the system should turn off the faulty channel imme-

diately by pulling the corresponding PWM control pin to GND.

THERMAL SHUTDOWN

Internal thermal shutdown circuitry is included to protect the

device in the event that the maximum junction temperature is

exceeded. The threshold for thermal shutdown in LM3435 is

around 160°C and it will be resumed to normal operation

again once the temperature cools down to below around 140°

I2C Compatible Interface

INTERFACE BUS OVERVIEW

The I2C compatible synchronous serial interface provides ac-

cess to the programmable functions and registers on the

device. This protocol uses a two-wire interface for bi-direc-

tional communications between the devices connected to the

bus. The two interface lines are the Serial Data Line (SDA),

and the Serial Clock Line (SCL). These lines should be con-

nected to a positive supply, via a pull-up resistor and remain

HIGH even when the bus is idle. Every device on the bus is

assigned a unique address and acts as either a Master or a

Slave depending on whether it generates or receives the se-

rial clock (SCL).

DATA TRANSACTIONS

One data bit is transferred during each clock pulse. Data is

sampled during the high state of the serial clock (SCL). Con-

sequently, throughout the clock’s high period, the data should

remain stable. Any changes on the SDA line during the high

state of the SCL and in the middle of a transaction, aborts the

current transaction. New data should be sent during the low

SCL state. This protocol permits a single data line to transfer

both command/control information and data using the syn-

chronous serial clock.

I2C DATA VALIDITY

The data on SDA line must be stable during the HIGH period

of the clock signal (SCL). In other words, state of the data line

can only be changed when CLK is LOW.

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I2C Signals : Data Validity

I2C START and STOP CONDITIONS

START and STOP bits classify the beginning and the end of

the I2C session. START condition is defined as SDA signal

transitioning from HIGH to LOW while SCL line is HIGH.

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LM3435

STOP condition is defined as the SDA transitioning from LOW

to HIGH while SCL is HIGH. The I2C master always generates

START and STOP bits. The I2C bus is considered to be busy

after START condition and free after STOP condition. During

data transmission, I2C master can generate repeated START

conditions. First START and repeated START conditions are

equivalent, function-wise.

30162516

I2C Start and Stop Conditions

I2C ADDRESSES AND TRANSFERRING DATA

Every byte put on the SDA line must be eight bits long, with

the most significant bit (MSB) being transferred first. Each

byte of data has to be followed by an acknowledge bit. The

acknowledge bit related clock pulse is generated by the mas-

ter. The transmitter releases the SDA line (HIGH) during the

acknowledge clock pulse. The receiver must pull down the

SDA line during the 9th clock pulse, signifying an acknowl-

edgement. A receiver which has been addressed must gen-

erate an acknowledge bit after each byte has been received.

After the START condition, the I2C master sends a chip ad-

dress. This address is seven bits long followed by an eighth

bit which is a data direction bit (R/W). The LM3435 address

is 50h or 51H which is determined by the R/W bit. I2C address

(7 bits) for LM3435 is 28H. For the eighth bit, a “0” indicates

a WRITE and a “1” indicates a READ. The second byte se-

lects the register to which the data will be written. The third

byte contains data to write to the selected register.

30162517

I2C Chip Address

the last ACK from slave.

30162532

w = write (SDA = “0”)

r = read (SDA = “1”)

ack = acknowledge (SDA pulled down by either master or slave)

rs = repeated start

id = 7-bit chip address, 50H (ADDR_SEL=0) or 51H (ADDR_SEL=1) for LM3435.

I2C Write Cycle

When a READ function is to be accomplished, a WRITE func-

tion must precede the READ function, as shown in the Read

Cycle waveform.

30162533

I2C Read Cycle

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LM3435

I2C TIMING PARAMETERS (VIN = 2.7V to 5.5V, SVDD = 1.7V

to VIN)

30162534

I2C Timing Diagram

Symbol Parameter Limit Units

Min Max

1 Hold Time (repeated) START Condition 0.6 µs

2 Clock Low Time 1.3 µs

3 Clock High Time 600 ns

4 Setup Time for a Repeated START Condition 600 ns

5 Data Hold Time (Output direction) 300 ns

5 Data Hold Time (Input direction) 0 ns

6 Data Setup Time 100 ns

7 Rise Time of SDA and SCL 20+0.1Cb 300 ns

8 Fall Time of SDA and SCL 15+0.1Cb 300 ns

9 Set-up Time for STOP condition 600 ns

10 Bus Free Time between a STOP and a START

Condition

1.3 µs

Cb Capacitive Load for Each Bus Line 10 200 pF

Note: Data guaranteed by design.

www.national.com 14

LM3435

I2C REGISTER DETAILS

The I2C bus interacts with the LM3435 to realize the features

of LED current program inter-color delay time program and

Fault reporting function. The operation of these functions re-

quires the writing and reading of the internal registers of the

LM3435. In below is the master register map of the device.

Master Register Map

ADDR REGISTER D7 D6 D5 D4 D3 D2 D1 D0 DEFAULT

00h LEDLO 0 0 RLED[1:0] BLED[1:0] GLED[1:0] 0011 1111

01h GLEDH GLED[9:2] 1111 1111

02h BLEDH BLED[9:2] 1111 1111

03h RLEDH RLED[9:2] 1111 1111

05h FLT_RPT 0 0 0 0 0 0 0 FLT_RPT 0000 0001

06h DELAY RDLY[1:0] 1 BDLY[1:0] 1 GDLY[1:0] 1111 1111

07h FAULT GO GS 0 BO BS 0 RO RS 0000 0000

LED Current Register Definitions

The LED currents for each color can be accurately adjusted

by 10 bits resolution (1024 steps) independently. By writing

control bytes into the LM3435 LED current Registers, the LED

currents can be precisely set to any value in the range of

IMIN to IREF.

In below is the LED Current Low register bit definition:

ADDR REGISTER D7 D6 D5 D4 D3 D2 D1 D0 DEFAULT

00h LEDLO 0 0 RLED[1:0] BLED[1:0] GLED[1:0] 0011 1111

Bits Description

7:6 Reserved. These bits always read zeros.

5:4 The least significant bits of the 10-bit RLED register. This register is for programming the level of current for the

Red LED.

3:2 The least significant bits of the 10-bit BLED register. This register is for programming the level of current for the

Blue LED.

1:0 The least significant bits of the 10-bit GLED register. This register is for programming the level of current for the

Green LED.

In below is the LED Current High register bit definition:

ADDR REGISTER D7 D6 D5 D4 D3 D2 D1 D0 DEFAULT

01h GLEDH GLED[9:2] 1111 1111

02h BLEDH BLED[9:2] 1111 1111

03h RLEDH RLED[9:2] 1111 1111

Bits Description

7:0 The most 8 significant bits of the 10-bit GLED, BLED and RLED registers respectively. These registers are for

programming the level of current of the Green LED, Blue LED and Red LED independently.

Fault Reporting Register Definition

The fault reporting feature of the LM3435 can be selected by

the system designer according to their application needs. To

select or de-select this feature is realized by writing one bit

into the FLT_RPT register.

ADDR REGISTER D7 D6 D5 D4 D3 D2 D1 D0 DEFAULT

05h FLT_RPT 0 0 0 0 0 0 0 FLT_RPT 0000 0001

Bits Description

7:1 Reserved. These bits always read zeros.

0This bit defines the mode of fault report feature. Writing a “ 1 “ into this bit enables the fault reporting feature,

otherwise no Fault signal output at Pin 27.

Color Transition Delay Register Definition

The transition of one color into next color is not executed im-

mediately. Certain delay is inserted in between to guarantee

the LED rail voltage stabilized before turning the next LED on.

This delay is user programmable by writing control bits into

the DELAY register for each color individually. The power up

default delay time is 35µs and this delay can be programmed

from 5 µs to 35 µs maximum in step of 10 µs.

15 www.national.com

LM3435

ADDR REGISTER D7 D6 D5 D4 D3 D2 D1 D0 DEFAULT

06h DELAY RDLY[1:0] 1 BDLY[1:0] 1 GDLY[1:0] 1111 1111

Bits Description

7:6 These two bits are for programming the Red transition delay.

5 Reserved. This bit always read “ 1“.

4:3 These two bits are for programming the Blue transition delay.

2 Reserved. This bit always read “ 1“.

1:0 These two bits are for programming the Green transition delay.

Fault Register Definition

The LM3435 features LED fault detection capability. When-

ever a LED fault is detected (open or short), the FAULT output

(pin 27) will go high to indicate a LED fault is detected. The

details of the fault can be investigated by reading the FAULT

be cleared by clearing and then re-enabling the FLT_RPT

ADDR REGISTER D7 D6 D5 D4 D3 D2 D1 D0 DEFAULT

07h FAULT GO GS 0 BO BS 0 RO RS 0000 0000

Bits Description

7 GO – Green Open. This read only register bit indicates the presence of an OPEN fault of the GREEN LED.

6 GS – Green Short. This read only register bit indicates the presence of an SHORT fault of the GREEN LED.

5 Reserved. This bit always read “ 0 “.

4 BO – Blue Open. This read only register bit indicates the presence of an OPEN fault of the BLUE LED.

3 BS – Blue Short. This read only register bit indicates the presence of an SHORT fault of the BLUE LED.

2 Reserved. This bit always read “ 0 “.

1 RO – Red Open. This read only register bit indicates the presence of an OPEN fault of the RED LED.

0 RS – Red Short. This read only register bit indicates the presence of an SHORT fault of the RED LED.

www.national.com 16

LM3435

Design Procedures

This section presents a design example of a typical pico pro-

jector application. By using LM3435, the system requires only

single DC-DC converter to drive three color LEDs instead of

using three DC-DC converters with conventional design. The

suggested approach not only saves components cost, but al-

so releases invaluable PCB space to the system and en-

hances system reliability. The handy projector is powered by

a single lithium battery cell or a 5Vdc wall mount adaptor. The

key specifications of the design are as in below:

Supply voltage range, VIN = 2.7V to 5.5V

Preset LED current per channel, ILED = 1.5A

Minimum LED current per channel, ILED(MIN) = 600mA

Maximum LED forward voltage drop, VLED = 3.5V @ 1.5A

Flyback converter switching Frequency, FSW = ~500kHz

SETTING THE FLYBACK CONVERTER SWITCHING

FREQUENCY, FSW

The LM3435 employs a proprietary Projected On-Time (POT)

control scheme, the switching frequency, FSW of the converter

is simply set by an external resistor, RRT across RT pin of

LM3435 and VOUT of the converter. The flyback converter

under POT control can maintain a fairly constant switching

frequency that depends mainly on the value of RRT. The re-

lationship between the flyback converter switching frequency,

FSW and RRT is approximated by the following relationship:

RRT in Ω and FSW in kHz

In order to set the flyback converter switching frequency,

FSW to 500kHz, the value of RRT can be calculated as in be-

low:

A standard resistor value of 499kΩ can be used in place and

the period of switching, TSW is about 2µs.

SETTING THE NOMINAL LED CURRENT

The nominal LED current of the LEDs are set by resistors

connected to IREFR, IREFG and IREFB pins. The current for

each channel can be set individually and it is not mandatory

that all channel currents are the same. Just for simplicity, we

assume all channels are set to 1.5A in this example. The LED

current and the value of RIREFR, RIREFG and RIREFB is gov-

erned by the following equation.

RIREFx in Ω and ILEDx in Ampere

The resistance value for the current setting resistors is cal-

culated as in below:

In order to achieve the required LED current accuracy, high

quality resistors with tolerance not higher than +/-1% are rec-

ommended.

INDUCTOR SELECTION

Selecting the correct inductor is one of the major task in ap-

plication design of a LED driver system. The most critical

inductor parameters are inductance, current rating, DC resis-

tance and size. As an rule of thumb, for same physical size

inductor, higher the inductance means higher the DC resis-

tance, consequently more power loss with the inductor and

lower the DC-DC conversion efficiency. With LM3435, the in-

ductor governs the inductor ripple current and limits the min-

imum LED current that can be supported. However for the

POT control in LM3435, a minimum inductor ripple current of

about 300mApk-pk is required for proper operation. The rela-

tionship of the ON-Duty, D and the input/output voltages can

be derived by applying the Volt-Second Balance equation

across the inductor. The waveforms of the inductor current

and voltage are shown in below.

30162547

Inductor Switching Waveforms

Applying the Volt-Second Balance equation with the inductor

voltage waveform,

Referring to the inductor current waveform, the average in-

ductor current, IL(AVG) can be derived as in below:

17 www.national.com

LM3435

The minimum LED current, ILED(MIN) happens when the in-

ductor current just entered the Critical Conduction Mode op-

eration, i.e. ILripple(MIN)=0.

Applying this condition to the last equation:

The relationship of the LED current, ILED and the average in-

ductor current, IL(AVG) is shown in below:

By combining last two equations, the minimum LED current,

ILED(MIN) can be calculated as in below:

By rearranging the terms, the inductance, L required for any

specific minimum LED current, ILED(MIN) can be found with the

equation in below:

From the equation, it can be noted that for lower minimum

LED current, the inductance required will be higher. As men-

tioned in before, higher the inductance means higher DC

resistance in same size inductor. Additionally, the POT con-

trol in LM3435, a minimum inductor ripple current is required

to maintain proper operation. The restrictions limit the lowest

current can be programed by I2C control.

In this example, the ILED(MIN)=600mA and the highest ripple

will happen when the input voltage is maximum, i.e. VIN=5.5V.

The ON Duty, D with average LED forward voltage of 3.5V is

calculated in below:

The required inductance for this case is:

A standard inductance value of 2.2µH is suggested and the

minimum LED current, ILED(MIN) is about 595mA @ VIN=5.5V.

Other than the inductance, the worst case inductor current,

IL(MAX) must be calculated so that an inductor with appropriate

saturation current level can be specified. The maximum in-

ductor current, IL(MAX) can be calculated with the equation in

below:

The highest inductor current occurs when the input voltage is

minimum, i.e. VIN=2.7V. The ON Duty, D for this condition can

be calculated as in below:

The maximum inductor current, IL(MAX) is calculated in below:

The calculated maximum inductor current is 4.1A, however

the inductance can drop as temperature rise. In order to ac-

commodate all possible variations, an inductor with saturation

current specification not less than 5A is suggested.

INPUT CAPACITORS SELECTION

Input capacitors are required for all supply input pins to ensure

that VIN does not drop excessively during high current switch-

ing transients. LM3435 have supply input pins located in

different sides of the device. Individual capacitors are needed

for the supply input pins locally. All capacitors must be placed

as close as possible to the supply input pins and have low

impedance return ground path to the device grounds and

back to supply ground. Capacitors CIN1 and CIN2 are the main

input capacitors and additionally, CIN3 is added to de-couple

high frequency interference. The capacitance for CIN1 and

CIN2 is recommended in the range of 22μF to 47µF and CIN3

is 0.1µF. Compact applications normally have stringent space

limitations, small size surface mount capacitors are usually

preferred. Low ESR Multi-Layer Ceramic Capacitors (MLCC)

are the best choices. MLCC capacitors with X5R and X7R

dielectrics are recommended for its low leakage and low ca-

pacitance variation against temperature and frequency.

OUTPUT CAPACITORS SELECTION

Two output capacitors are required with LM3435 configura-

tion, one for VOUT to Ground, COUT2 and one for de-coupling

the LED current ripple, COUT1. The LM3435 operates at fre-

quencies high enough to allow the use of MLCC capacitors

without compromising transient response. Low ESR charac-

teristic of the MLCC allow higher inductor ripple without sig-

nificant increase of the output ripple. The capacitance

recommended for COUT1 is 10µF and COUT2 is 22µF. Again,

high quality MLCC capacitors with X5R and X7R dielectrics

are recommended. For certain conditions, acoustic problem

may be encountered with using MLCC, Low Acoustic Noise

Type capacitors are strongly recommended for all output ca-

pacitors. Alternatively, the acoustic noise can also be lowered

by using smaller size capacitors in parallel to achieve the re-

quired capacitance.

OTHER CAPACITORS SELECTION

Three small startup capacitors connected to CG, CB and CR

pins are needed for proper operation. The suggested capac-

itance for CCR, CCG and CCB is 1nF. Also three capacitors

connected to GLED, BLED and RLED pins to protect the de-

vice from high transient stress due to the inductance of the

connecting wires for the LEDs. The suggested capacitance

www.national.com 18

LM3435

for CG, CB and CR is 0.47µH. MLCC capacitors with X5R and

X7R dielectrics are recommended. All capacitors must be

placed as close as possible to the device pins.

DIODE SELECTION

A schottky barrier diode is added across the SW and VOUT

pins, equivalently, its across the internal P-channel MOSFET

of the synchronous converter, that can help to improve the

conversion efficiency by few percents. A very low forward

voltage and 1A rated forward current part is suggested in the

schematic diagram. The key selection criteria are the forward

voltage and the rated forward current.

PCB LAYOUT CONSIDERATIONS

The performance of any switching converters depends as

much upon the layout of the PCB as the component selection.

PCB layout considerations are therefore critical for optimum

performance. The layout must be as neat and compact as

possible, and all external components must be as close as

possible to their associated pins. The PGND connection to

CIN and VOUT connection to COUT should be as short and

direct as possible with thick traces. The inductor should con-

nect close to the SW pin with short and thick trace to reduce

the potential electro-magnetic interference.

It is expected that the internal power elements of the LM3435

will produce certain amount of heat during normal operation,

good use of the PC board's ground plane can help consider-

ably to dissipate heat. The exposed pad on the bottom of the

IC package can be soldered to a copper pad with thermal vias

that can help to conduct the heat to the bottom side ground

plane. The bottom side ground plane should be as large as

possible.

Schematic of the Example

Application for Pico Projector

30162535

19 www.national.com

LM3435

Physical Dimensions inches (millimeters) unless otherwise noted

LLP-40 Pin Package (SQF)

For Ordering, Refer to Ordering Information Table

NS Package Number SQF40A

www.national.com 20

LM3435

Notes

21 www.national.com

LM3435

Notes

LM3435 Compact Sequential Mode RGB LED Driver with I2C Control Interface

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