AH102A-TRG - TriQuint Semiconductor

LM3508

LM3508 Synchronous Magnetic Constant Current White LED Driver

Literature Number: SNVS494B

March 6, 2008

LM3508

Synchronous Magnetic Constant Current White LED Driver

General Description

The LM3508 is a synchronous boost converter (no external

Schottky diode required) that provides a constant current out-

put. It is designed to drive up to 4 series white LEDs at 30mA

from a single-cell Li-Ion battery. A single low power external

resistor is used to set the maximum LED current. The LED

current can be adjusted by applying a PWM signal of up to

100kHz to the DIM pin. Internal soft-start circuitry is designed

to eliminate high in-rush current at start-up. For maximum

safety, the device features an advanced short-circuit protec-

tion when the output is shorted to ground. Additionally, over-

voltage protection and an 850kHz switching frequency allow

for the use of small, low-cost output capacitors with lower

voltage ratings. During shutdown, the output is disconnected

from the input preventing a leakage current path through the

LEDs to ground. The LM3508 is available in a tiny 9-bump

chip-scale micro-SMD package.

Features

■Drives 4 Series White LEDs with up to 30mA

■>80% Peak Efficiency

■Up to 100kHz PWM Brightness Control

■Accurate ±5% LED Current Regulation across VIN range

■Internal Synchronous PFET (No Schottky Diode

Required)

■True Shutdown Isolation

■Output Short-Circuit Protection

■17.5V Over-Voltage Protection

■Internal Soft-Start Eliminates Inrush Current

■Wide Input Voltage Range: 2.7V to 5.5V

■850kHz Fixed Frequency Operation

■Low Profile 9-Bump Micro-SMD Package (1.514mm x

1.514mm x 0.6mm)

Applications

■White LED Backlighting

■Handheld Devices

■Digital Cameras

■Portable Applications

Typical Application Circuit

30004201

LM3508 Synchronous Magnetic Constant Current White LED Driver

Connection Diagram

Top View

30004202

9-Bump (Large) µ-SMD (1.514mm x 1.514mm x 0.6mm) NS Package Number TLA09

Ordering Information

Part Number Package Type NSC Package Drawing Top Mark Supplied As

LM3508TL 9-Bump micro SMD TL09SDA D31 250 Units, Tape and Reel

LM3508TLX 9-Bump micro SMD TL09SDA D31 3000 Units, Tape and Reel

Pin Descriptions/Functions

Pin Name Function

A1 PGND Power Ground Connection.

A2 SW Inductor connection and drain connection for both NMOS and PMOS power devices.

A3 OUT Output capacitor connection, PMOS source connection for synchronous rectifier, and OVP

sensing node.

B1 ILED Regulated current source input.

B2 DIM Current source modulation input. A logic low at DIM turns off the internal current source. A logic

high turns the LEDs fully on (VSET=200mV). Apply a PWM signal at DIM for LED brightness control.

B3 IN Input voltage connection.

C1 SET Current sense connection and current source output. Connect a 1% resistor (RSET) from SET to

PGND to set the maximum LED current (ILED = 200mV/RSET) .

C2 EN Enable input. A logic low at EN turns off the LM3508. A logic high turns the device on.

C3 AGND Analog ground. Connect AGND to PGND through a low impedance connection.

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LM3508

Absolute Maximum Ratings (Notes 1, 2)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

VIN −0.3V to 6V

VOUT −0.3V to 22V

VSW −0.3V to 22V

VILED, VSET, VDIM, VEN −0.3V to 6V

Continuous Power Dissipation

(Note )

Internally Limited

Junction Temperature +150°C

Lead Temperature

(Note 4) +300°C

Storage Temperature Range -65°C to +150°C

ESD Rating(Note 9)

Human Body Model 2kV

Operating Conditions (Notes 1, 2)

Input Voltage Range 2.7V to 5.5V

Ambient Temperature Range

(Note 5) −30°C to +85°C

Junction Temperature Range −30°C to +105°C

Thermal Properties

Junction to Ambient Thermal

Resistance (θJA)(Note 6)

64.7°C/W

ESD Caution Notice

National Semiconductor recommends that all integrated cir-

cuits be handled with appropriate ESD precautions. Failure to

observe proper ESD handling techniques can result in dam-

age to the device.

Electrical Characteristics

Specifications in standard type face are for TA = 25°C and those in boldface type apply over the Operating Temperature Range

of TA = −30°C to +85°C. Unless otherwise specified VIN =3.6V.(Note 7)

Symbol Parameter Conditions Min Typ Max Units

IDLED Current

Regulation

RSET = 10Ω 20 mA

RSET = 6.67Ω 30

VSET Voltage at SET Pin 3.0V < VIN < 5.5V 190 200 210 mV

VILED Voltage at ILED Pin 500 mV

VHR Current Sink

Headroom Voltage

Where ILED = 95% of

nominal, RSET = 20Ω 400 mV

RDSON NMOS Switch On

Resistance

ISW = 100mA 0.5

Ω

PMOS Switch On

Resistance

VOUT = 10V, ISW = 65mA 2.2

ICL NMOS Switch

Current Limit

370 500 620 mA

ILSW SW Leakage Current VSW = VIN = 5.5V, OUT

Floating, VEN = PGND 0.01 µA

IOUT_SHUTDOWN Outout Pull-Down

Resistance in

Shutdown

VEN = 0V

630 Ω

VOVP Output Over-Voltage

Protection

ON Threshold (VOUT rising) 17.5 19.8 21.8

OFF Threshold (VOUT

falling)

18.6

fSW Switching Frequency 3.0V < VIN < 5.5V 715 850 1150 kHz

DMAX Maximum Duty Cycle 91 %

VSC Output Voltage

Threshold for Short

Circuit Detection

VOUT Falling 0.93×VIN

VOUT Rising 0.95×VIN

VEN_TH EN Threshold

Voltage

On Threshold 1.1

Off Threshold 0.5 V

VDIM_TH DIM Threshold

Voltage

On Threshold 1.1 V

Off Threshold 0.5

IDIM DIM Bias Current

(Note 8)

VDIM = 1.8V 4.7 µA

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LM3508

Symbol Parameter Conditions Min Typ Max Units

IEN EN Bias Current

(Note 8)

VEN = 1.8V 4.7 µA

IOUT OUT Bias Current VOUT = 16V, device not

switching

420 µA

ROUT_SHUTDOWN Output Pull-Down

Resistance in

Shutdown

VEN = 0V, VOUT < VIN

630

Ω

IQQuiescent Current

Device Not Switching

VILED > 0.5V, 3.0V < VIN <

5.5V, SW Floating

0.18 0.3 mA

VEN = 0V, 3.0V < VIN < 5.5V 0.01 0.5

IQ_SW Switching Supply

Current

825 µA

tSTART_UP From EN Low to High

to Inductor Current

Steady State

VOUT = 17V, ILED = 20mA

470

µs

Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended

to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: All voltages are with respect to PGND.

Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown enagaes at TJ = +150°C (typ.) and disengages at

TJ = +140°C (typ.).

Note 4: For more detailed soldering information and specifications, please refer to National Semiconductor Application Note 1112: Micro SMD Wafer Level Chip

Scale Package (AN-1112), available at www.national.com.

Note 5: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be

derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = +125ºC), the maximum power

dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the

following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX).

Note 6: Junction-to-ambient thermal resistance (θJA) is taken from thermal modeling performed under the conditions and guidelines set forth in the JEDEC

standard JESD51-7. The test board is a 4-layer FR-4 board mesuring (102mm × 76mm × 1.6mm) with a 2 × 1 array of thermal vias. The ground plane on the

board is (50mm × 50mm). Thickness of copper layers are (36µm/18µm/18µm/36µm) (1.5oz/1oz/1oz/1.5oz copper). Ambient temperature in simulation is +22°C,

still air. Power dissipation is 1W.

Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm.

Unless otherwise specified, conditions for typical specifications are VIN = 3.6V, TA = +25°C.

Note 8: There is a typical 383kΩ pull-down on this pin.

Note 9: The human body model is a 100pF capacitor discharged through 1.5kΩ resistor into each pin. (MIL-STD-883 3015.7).

Typical Performance Characteristics VIN = 3.6V, RSET = 10Ω, L = TDK VLF3012AT-220MR33 (22µH),

LEDs are OSRAM (LW M67C), COUT = CIN = 1µF, TA = +25°C, unless otherwise noted.

4 LED Efficiency vs ILED

(L = TDK VLF3012AT-220MR33, RL = 0.66Ω)

30004242

3 LED Efficiency vs ILED

(L = TDK VLF3012AT-220MR33, RL = 0.66Ω)

30004241

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LM3508

2 LED Efficiency vs ILED

(L = TDK VLF3012AT-220MR33, RL = 0.66Ω)

30004258

Converter Output Voltage vs LED Current

30004243

Efficiency vs VIN (ILED = 20mA)

30004260

Efficiency vs VIN (ILED = 30mA)

30004261

Peak Current Limit vs VIN

30004213

Switching Frequency vs VIN

30004214

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LM3508

Maximum Duty Cycle vs VIN

30004215

Quiescent Current vs VIN

(EN = GND)

30004217

Quiescent Current vs VIN

(Device Not Switching, VIN = VSW)

30004216

Quiescent Current vs VIN

(Device Switching)

30004218

SET Voltage vs DIM Frequency

(50% Duty Cycle at DIM)

30004219

SET Voltage vs VIN

30004259

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LM3508

SET Voltage vs DIM Duty Cycle

30004262

NFET On-Resistance vs VIN

(ISW = 250mA)

30004224

PFET On-Resistance vs Temperature

(VSW = 10.4V, VOUT = 10V)

30004225

Over Voltage Limit vs VIN

(VOUT Rising)

30004227

Over Voltage Limit vs VIN (VOUT Falling)

30004228

Start-Up Waveform

30004257

4 LEDs, ILED = 30mA, VIN = 3.6V

Channel 1: VOUT (10V/div)

Channel 2: EN (2V/div)

Channel 4: IIN (200mA/div)

Time Base: 100µs/div

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LM3508

Over-Voltage Protection Function

30004237

VIN = 3.6V, VOUT = 18.86V

Channel 1: VOUT (1V/div)

Channel 4: IIN (500mA/div)

Time Base: 400µs/div

Line-Step Response

30004255

VIN = 3.6V, 4 LEDs

Channel 1: VOUT (AC Copupled, 1V/div)

Channel 3: VIN (AC Coupled, 500mV/div)

Channel 4: ILED (DC Coupled, 5mA/div)

Time Base: 200µs/div

Output Short-Circuit Response

30004256

VIN = 3.6V, ILED = 30mA

Channel 1: VOUT (10V/div)

Channel 2: IIN (100mA/div)

Time Base: 200µs/div

Typical Operating Waveforms (DIM High)

30004229

VIN = 3.6V, 4 LEDs, ILED = 30mA, VOUT = 15.8V

Channel 1: VOUT (AC Coupled, 100mV/div)

Channel 2: VSW (DC Coupled, 10V/div)

Channel 4: IL (DC Coupled, 100mA/div)

Time Base: 400ns/div

Typical Operating Waveforms (DIM With 20kHz Square

Wave)

30004230

VIN = 3.6V, 4 LEDs, ILED = 15mA

Channel 1: VOUT (AC Coupled, 200mV/div)

Channel 3: VIN (AC Coupled, 100mV/div)

Channel 2: IL (DC Coupled, 100mA/div)

Channel 4: DIM (DC Coupled, 2V/div)

Time Base: 10µs/div

DIM Operation (ILED changing from 30mA to 15mA)

30004238

VIN = 3.6V

Channel 4: ILED (DC Coupled, 10mA/div)

Channel 2: VOUT (AC Coupled, 2V/div)

Channel 1: DIM (DC Coupled, 2V/div, 20kHz, 50% duty cycle)

Channel 3: IIN (DC Coupled, 200mA/div)

Time Base: 400µs/div

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LM3508

Operation

30004203

FIGURE 1. LM3508 Block Diagram

The LM3508 utilizes a synchronous step-up current mode

PWM controller and a regulated current sink to provide a

highly efficient and accurate LED current for white LED bias.

The internal synchronous rectifier increases efficiency and

eliminates the need for an external diode. Additionally, inter-

nal compensation eliminates the need for external compen-

sation components resulting in a compact overall solution.

Figure 1 shows the detailed block diagram of the LM3508.

The output of the boost converter (OUT) provides power to

the series string of white LED’s connected between OUT and

ILED. The boost converter regulates the voltage at ILED to

500mV. This voltage is then used to power the internal current

source whose output is at SET.

The first stage of the LM3508 consists of the synchronous

boost converter. Operation is as follows: At the start of each

switching cycle the oscillator sets the PWM controller. The

controller turns the low side (NMOS) switch on and the syn-

chronous rectifier (PMOS) switch off. During this time current

ramps up in the inductor while the output capacitor supplies

the current to the LED’s. The error signal at the output of the

error amplifier is compared against the sensed inductor cur-

rent. When the sensed inductor current equals the error sig-

nal, or when the maximum duty cycle is reached, the NMOS

switch turns off and the PMOS switch turns on. When the

PMOS turns on, the inductor current ramps down, restoring

energy to the output capacitor and supplying current to the

LED’s. At the end of the clock period the PWM controller is

again set and the process repeats itself. This action regulates

ILED to 500mV.

The second stage of the LM3508 consists of an internal cur-

rent source powered by the ILED voltage and providing a

regulated current at SET. The regulated LED current is set by

connecting an external resistor from SET to PGND. VSET is

adjusted from 0 to 200mV by applying a PWM signal of up to

typically 100kHz at DIM (see Typical Performance Charac-

teristic of SET voltage vs DIM frequency). The PWM signal at

DIM modulates the internal 200mV reference and applies it to

an internal RC filter resulting in an adjustable SET voltage and

thus an adjustable LED current.

Start-Up

The LM3508 features a soft-start to prevent large inrush cur-

rents during start-up that can cause excessive voltage ripple

on VIN. During start-up the average input current is ramped

up at a controlled rate. For the typical application circuit, driv-

ing 4LED’s from a 3.6V lithium battery at 30mA, when EN is

driven high the average input current ramps from zero to 160-

mA in 470µs. See plot of Soft Start functionality in the Typical

Performance Characteristics.

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LM3508

DIM Operation

DIM is the input to the gate of an internal switch that accepts

a logic level PWM waveform and modulates the internal

200mV reference through an internal RC filter. This forces the

current source regulation point (VSET) to vary by the duty cycle

(D) of the DIM waveform making ILED = D × 200mV / RSET.

The cutoff frequency for the filter is approximately 500Hz. DIM

frequencies higher than 100kHz cause the LED current to

drastically deviate from their nominal set points. The graphs

of SET voltage vs DIM frequency, SET voltage vs VIN and SET

voltage vs DIM duty cycle (see Typical Performance Charac-

teristics) show the typical variation of the current source set

point voltage.

Enable Input and Output Isolation

Driving EN high turns the device on while driving EN low

places the LM3508 in shutdown. In shutdown the supply cur-

rent reduces to less than 1µA, the internal synchronous PFET

turns off as well as the current source (N2 in figure 1). This

completely isolates the output from the input and prevents

leakage current from flowing through the LED’s. In shutdown

the leakage current into SW and IN is typically 400nA. EN has

an internal 383kΩ pull-down to PGND.

Peak Current Limit/Maximum

Output Current

The LM3508 boost converter provides a peak current limit.

When the peak inductor current reaches the peak current limit

the duty cycle is terminated. This results in a limit on the max-

imum output power and thus the maximum output current the

LM3508 can deliver. Calculate the maximum LED current as

a function of VIN, VOUT, L and IPEAK as:

and fSW = 850kHz. Efficiency and IPEAK can be found in the

efficiency and IPEAK curves in the Typical Performance Char-

acteristics.

Output Current Accuracy

The LM3508 provides highly accurate output current regula-

tion of ±5% over the 3V to 5.5V input voltage range. Accuracy

depends on various key factors. Among these are; the toler-

ance of RSET, the frequency at DIM (ƒDIM), and the errors

internal to the LM3508 controller and current sink. For best

accuracy, use a 1% resistor for RSET and keep ƒDIM between

1kHz and 100kHz. Refer to the Typical Performance Char-

acteristics for VSET vs VIN, VSET vs ƒDIM, and VSET vs DIM duty

cycle.

Voltage Head Room at ILED

If the LED current is increased to a point where the peak in-

ductor current is reached, the boost converter's on-time is

terminated until the next switching cycle. If the LED current is

further increased the 500mV regulated voltage at ILED begins

to drop. When VILED drops below the current sink headroom

voltage (VHR = 400mV typ.) the current sink FET (see N2 in

figure 1) will be fully on, appearing as a 5Ω resistor between

ILED and SET.

Output Short Circuit Protection

The LM3508 provides a short circuit protection that limits the

output current if OUT is shorted to PGND. During a short at

OUT when VOUT falls to below VIN × 0.93, switching will stop.

The PMOS will turn into a current source and limit the output

current to 35mA. The LM3508 can survive with a continuous

short at the output. The threshold for OUT recovering from a

short circuit condition is typically VIN × 0.95.

Output Over-Voltage Protection

When the load at the output of the LM3508 goes high

impedance the boost converter will raise VOUT to try and

maintain the programmed LED current. To prevent over-volt-

age conditions that can damage output capacitors and/or the

device, the LM3508 will clamp the output at a maximum of

21.8V. This allows for the use of 25V output capacitors avail-

able in a tiny 1.6mm × 0.8mm case size.

During output open circuit conditions when the output voltage

rises to the over voltage protection threshold (VOVP = 19.8V

typical) the OVP circuitry will shut off both the NMOS and

PMOS switches. When the output voltage drops below 18.6V

(typically) the converter will begin switching again. If the de-

vice remains in an over voltage condition the cycle will be

repeated resulting in a pulsed condition at the output. See

waveform for OVP condition in the Typical Performance Char-

acteristics.

Light Load Operation

During light load conditions when the inductor current reaches

zero before the end of the switching period, the PFET will turn

off, disconnecting OUT from SW and forcing the converter

into discontinuous conduction. At the beginning of the next

switching cycle, switching will resume. (see plot of discontin-

uous conduction mode in the Typical Performance Charac-

teristics graphs).

Boost converters that operate in the discontinuous conduc-

tion mode with fixed input to output conversion ratios (VOUT/

VIN) have load dependent duty cycles, resulting in shorter

switch on-times as the load decreases. As the load is de-

creased the duty cycle will fall until the converter hits its

minimum duty cycle (typically 15%). To prevent further de-

creases in the load current altering the VOUT/VIN ratio, the

LM3508 will enter a pulsed skip mode. In pulse skip mode the

device will only switch as necessary to keep the LED current

in regulation.

Thermal Shutdown

The LM3508 provides a thermal shutdown feature. When the

die temperature exceeds +150°C the part will shutdown, turn-

ing off both the NMOS and PMOS FET’s. The part will start-

up again with a soft-start sequence when the die temperature

falls below +115°C.

Applications Information

Brightness Adjustment

A logic high at DIM forces SET to regulate to 200mV. Adjust

the maximum LED current by picking RSET (the resistor from

SET to GND) such that:

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LM3508

Once ILED_MAX is set, the LED current can be adjusted from

ILED_MAX down to ILED_MIN by applying a logic level PWM signal

to DIM. This results in:

where D is the duty cycle of the PWM pulse applied to DIM.

The LM3508 can be brought out of shutdown while a signal

is applied to DIM, allowing the device to turn on into a low LED

current mode. A logic low at DIM will shut off the current

source making ILED high impedance however, the boost con-

verter continues to operate. Due to an offset voltage at SET

(approximately +/-2mV) the LED’s can faintly illuminate even

with DIM pulled to GND. If zero LED current is required then

pulling EN low will shutdown the current source causing the

LED current to drop to zero. DIM has an internal 383kΩ pull

down to PGND.

Input Capacitor Selection

Choosing the correct size and type of input capacitor helps

minimize the input voltage ripple caused by the switching ac-

tion of the LM3508’s boost converter. For continuous inductor

current operation the input voltage ripple is composed of 2

primary components, the capacitor discharge (delta VQ) and

the capacitor’s equivalent series resistance (delta VESR). The

ripple due to strictly to the capacitor discharge is:

The ripple due to strictly to the capacitors ESR is:

In the typical application circuit, a 1µF ceramic input capacitor

works well. Since the ESR in ceramic capacitors is typically

less than 5mΩ and the capacitance value is usually small, the

input voltage ripple is primarily due to the capacitive dis-

charge. With larger value capacitors such as tantalum or

aluminum electrolytic the ESR can be greater than 0.5Ω. In

this case the input ripple will primarily be due to the ESR.

Output Capacitor Selection

In a boost converter such as the LM3508, during the on time,

the inductor is disconnected from OUT forcing the output ca-

pacitor to supply the LED current. When the PMOS switch

(synchronous rectifier) turns on the inductor energy supplies

the LED current and restores charge to the output capacitor.

This action causes a sag in the output voltage during the on

time and a rise in the output voltage during the off time.

The LM3508’s output capacitor is chosen to limit the output

ripple to an acceptable level and to ensure the boost converter

is stable. For proper operation use a 1µF ceramic output ca-

pacitor. Values of 2.2µF or 4.7µF can be used although start-

up current and start-up time will be increased. As with the

input capacitor, the output voltage ripple is composed of two

parts, the ripple due to capacitor discharge (delta VQ) and the

ripple due to the capacitors ESR (delta VESR). Most of the time

the LM3508 will operate in continuous conduction mode. In

this mode the ripple due to capacitor discharge is given by:

The output voltage ripple component due to the output ca-

pacitors ESR is found by:

Table 1. Recommended Output Capacitor Manufacturers

Manufact

urer

Part

Number

Value Case Size Voltage

Rating

Murata

GRM39X5

R105K25

D539

1µF 0603 25V

TDK C1608X5

R1E105M

1µF 0603 25V

Inductor Selection

The LM3508 is designed to operate with 10µH to 22µH

inductor’s. When choosing the inductor ensure that the in-

ductors saturation current rating is greater than

Additionally, the inductor’s value should be large enough such

that at the maximum LED current, the peak inductor current

is less than the LM3508’s peak switch current limit. This is

done by choosing L such that

Values for IPEAK and efficiency can be found in the plot of peak

current limit vs. VIN in the Typical Performance Characteris-

tics graphs.

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LM3508

Table 2. Recommended Inductor Manufacturers

Manufacture

L Part

Number

Size Saturation

Current

TDK 22µ

VLF3010

AT-220M

R33

2.6mm×2.

8mm×1m

330mA

TDK 22µ

VLF3012

AT-220M

R33

2.6mm×2.

8mm×1.2

330mA

Toko 22u

D3313FB

(1036FB-

220M)

3.3mm×3.

3mm×1.3

350mA

Layout Considerations

Proper layout is essential for stable, jitter free operation, and

good efficiency. Follow these steps to ensure a good layout.

1, Use a separate ground plane for power ground (PGND)

and analog ground (AGND).

2, Keep high current paths such as SW and PGND connec-

tions short.

3, Connect the return terminals for the input capacitor and the

output capacitor together at a single point as close as possible

to PGND.

4, Connect PGND and AGND together as close as possible

to the IC. Do not connect them together anywhere else.

5, Connect the input capacitor (CIN) as close as possible to

IN.

6, Connect the output capacitor (COUT) as close as possible

to OUT.

7, Connect the positive terminal of RSET as close as possible

to ILED and the negative terminal as close as possible to

PGND. This ensures accurate current programming.

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LM3508

Physical Dimensions inches (millimeters) unless otherwise noted

9-Bump Micro SMD Package (TL09AAA)

For Ordering, Refer to Ordering Information Table

NS Package Number TLA09AAA

X1 = 1.514mm (±0.03mm), X2 = 1.514mm (±0.03mm), X3 = 0.6mm (±0.075mm)

13 www.national.com

LM3508

Notes

LM3508 Synchronous Magnetic Constant Current White LED Driver

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