LM3414HV

Application Note 2076 LM3414HV 1A 65V LED Driver Evaluation Board

Literature Number: SNVA451B

LM3414HV 1A 65V LED

Driver Evaluation Board

National Semiconductor

Application Note 2076

SH Wong

August 10, 2010

Introduction

The LM3414HV is a 65V floating buck LED driver that designed to drive up to 18 pieces of serial High Brightness LEDs

(HBLEDs) with up to 1000mA LED forward current. With the incorporation of the proprietary Pulse-Level-Modulation

(PLM) technology, the LM3414HV requires no external current sensing resistor to facilitate LED current regulation. The

LM3414HV features a dimming control input (DIM pin) that allows PWM dimming control. The LM3414HV is available in

LLP-8 (3mm x 3mm outline) and ePSOP8 to fulfil the requirements of small solution size and high thermal performance respectively. In order to demonstrate the performance of the

LM3414 family, the LM3414HV is selected for the evaluation boards because of the wide input voltage range (4.5V to 65V) providing the best flexibility to fit the requirements of different applications. Two versions of evaluation board with identical schematic are available with the LM3414HV in either LLP-8 or PSOP-8 package. The board with LLP-8 package demonstrates the high power density of the device. The board with

PSOP-8 package demonstrates the functionality of the

LM3414HV with enhanced thermal performance. The schematic, bill of materials and PCB layout for the evaluation boards are provided in this document. The evaluation boards can be adapted to different application requirements by changing the values of a few components only. This evaluation board is also suitable for the LM3414 with maximum acceptable input voltage reduced to 42VDC.

Standard Settings of the LM3414HV

Evaluation Board

Vin range: 4.5V to 65V

No. of LEDs: 1 - 18

LED current: 1A

Switching frequency: 500 kHz

30128701

FIGURE 1. Standard Schematic for the LM3414HV Evaluation Board

© 2010 National Semiconductor Corporation 301287 www.national.com

Board Connectors and Test Pins

Terminal Designation

VIN

GND

LED+

LED-

DIM

THM+

THM-

30128702

FIGURE 2. Connection Diagram

Description

Power supply positive (+ve) connection

Power supply negative (-ve) connection

Connect to cathode of the serial LED string

Connect to anode of the serial LED string

PWM dimming signal input (TTL signal compatible)

Connect to PTC thermal sensor for thermal foldback control

Connect to PTC thermal sensor for thermal foldback control www.national.com

2

Connecting to LEDs and Power

Supply

The LM3414HV evaluation board can be powered by a DC voltage source in the range of 4.5V to 65V through the banana-plug type connectors (VIN and GND) on the board as shown in figure 2. This evaluation board is designed to provide 1A (I

LED

)output current to a LED string containing up to

18 pieces of serial HBLEDs. The anode and cathode of the

LED string should connect to the LED+ and LED- bananaplug type connectors on the board respectively. By default, the LM3414HV on the evaluation board is enabled. The LEDs will light up as long as appropriate input voltage is applied to the evaluation board.

Adjusting the Output Current

The resistor RIADJ defines the output current of the

LM3414HV evaluation board. The default value of RIADJ is

3.09k

Ω, which sets the LED driving current to 1A. The LED current can be changed by adjusting the value of RIADJ with equation (1):

(1)

Table 1 shows the suggested value of RIADJ for common output current settings:

I

LED

(mA)

350

400

500

600

700

800

900

1000

TABLE 1. Examples for RIADJ Setting

R

IADJ

(k

Ω)

8.93

7.81

6.25

5.21

4.46

3.91

3.47

3.13

Adjusting the Switching Frequency

The resistor RFS defines the switching frequency of the

LM3414HV evaluation board. The default value of the RIADJ is 40k

Ω that sets the switching frequency to 500kHz. The LED current is adjustable by altering the resistance of RFS according to the equation (2):

(2)

Table 2 shows the suggested value of RFS for different switching frequencies: f

SW

(kHz)

250

500

1000

R

FS

(k

Ω)

8.93

7.81

6.25

TABLE 2. Examples for RFS Setting

When setting the switching frequency, it is necessary to ensure the on time of the internal switch is no shorter than

400ns; otherwise the driving current to the LEDs will increase and may eventually damage the LEDs.

Design Example

Assuming a LED string containing six serial HBLEDs is being driven by the board with 700mA (I

LED

). The forward voltages of one HBLED with 700mA driving current under different operation temperatures are:

V f(60C)

@700mA = 3.0V

V f(25C)

@700mA = 3.2V

V f(-10C)

@700mA = 3.5V

Step 1. Defining input voltage range

Because the LM3414HV is a floating buck LED driver, the input voltage to the LED driver must be higher than the total forward voltage of the LEDs under all conditions. As the forward voltage of a common HBLED could increase as the driving current increases or the operation temperature decreases, it is essential to ensure the minimum supply voltage is at least 10% higher than the possible highest forward voltage of the LED string. For example, assuming the forward voltage of a HBLED is 3.2V at T

A

= 25°C and 3.5V at T

A

=

-10°C at 700mA driving current. When 6 pieces of LED are connected in series, the total forward voltage of the LED string at 25°C and -10°C are 19.2V and 21V respectively. In order to secure current regulation under -10°C, the input voltage should not be lower than 23.1V. In this example, a standard

24V DC power supply with no more than +/– 3% output voltage variation can be used.

Step 2. Defining switching frequency fSW

When the maximum LED forward voltage and minimum input voltage are identified, the switching frequency of the

LM3414HV can be defined. The switching frequency of the

LM3414HV must be set in the range of 250kHz to 1MHz. Because the LM3414HV is designed to operate in continuous conduction mode (CCM) with 400ns minimum switch ON time limit, the maximum allowable switching frequency is restricted by the minimum input voltage, V

IN(MIN) forward voltage, V f(MAX)

and maximum LED

according to equation (3):

(3)

In this example, because a 24V DC power supply with +/- 3% output voltage variation is used, V

IN(MAX) imum forward voltage of the LED string V

is 24.72V. The minf(MIN)

is 18V because the forward voltage of the LED string will be at the lowest level when the operation temperature rises to 60°C. According to equation (3), with V

IN(MAX) switching frequency, f

SW

=24.72V and V f(MIN)

=18V, the

should not set higher than 1.82MHz.

However, because the switching frequency of the LM3414HV must set in the range of 250kHz to 1MHz, 1MHz switching frequency is selected.

Step 3. Inductor Selection

The inductance of the inductor, L1 can be decided according to the switching frequency and output current settings determined in step 1 and step 2. The inductance must be adequate to maintain the LM3414HV to operate in CCM. The minimum inductance can be calculated by following equation (4):

3 www.national.com

(4)

In equation (4), I

LED

is the average output current of the

LM3414HV circuit to drive the LED string. I

RIP(P-P)

is the peakto-peak value of the inductor current ripple. Assuming that the required LED current is 700mA, 50% inductor current ripple and 1MHz switching frequency, the inductance should be no less than 14uH. Because common power inductor carries +/-

20% inductance tolerance, a standard 18uH inductor with

+/-20% tolerance can be used.

Other than deciding a suitable inductance value, it is essential to ensure the peak inductor current is not exceeding the rated saturation current of the inductor. The peak inductor current is governed by the following equation:

(5)

In equation (5), I

L(PEAK)

is the peak inductor current. As a 18uH with +/- 20% variation is used, the minimum inductance L

(MIN)

is 14.4uH. With 700mA LED current, the peak inductor current is 836mA, thus a standard 18uH power inductor with

1A saturation current (I

SAT

) can be used.

PWM Dimming Control

The average LED current can be controlled by applying PWM dimming signal across the DIM and GND terminals of the

LM3414HV evaluation board. The board accepts standard

TTL level dimming signal. The output of the board is enabled when the DIM terminal is pulled high. The average LED current is adjustable according to the ON duty ratio of the PWM dimming signal by equation (6):

(6)

In equation (6), I

LED(AVG) the LED string and D

DIM

is the average current flows through

is the ON duty ratio of the PWM dimming signal being applied to the DIM pin of the LM3414HV.

Analog Dimming Control

As the output current of the LM3414HV is defined by the current being drawn to GND through RIADJ proportionally, analog dimming control (true output current control) can be accommodated by applying external current to RIADJ of the

LM3414HV evaluation board. Figure 3 shows an example circuit for analog dimming control. With analog dimming control.

Injecting additional current through the RIADJ to GND can effectively reduce the LED current (I

LED

I

LED

and I

EXT

). The relationship of

is governed by equation (7).

30128720

FIGURE 3. Reducing LED current with external current to the IADJ pin

(7)

In equation (7), I

EXT

RIADJ. As I

EXT

is the external current being injected into

increases, ILED decreases.

Figure 4 shows a practical thermal foldback control circuit which reduces the LED current when the temperature of the

LED sting is exceeding certain preset threshold. Because the temperature threshold for thermal foldback control depends on end application, the components required in this thermal foldback control circuitry are not included in the LM3414HV evaluation board. Physical pads and connections for R1, R2 and Q1 have been reserved on the board for component mounting. In order to detect the temperature of the LED string, a Positive Temperature Coefficient (PTC) thermistor, RPTC should be connected across the THM+ and THM- terminals of the LM3414HV evaluation board. In figure 4, the bipolar transistor, Q1 is biased by a potential divider composes of R1 and RPTC. When the temperature of the LEDs rises, the voltwww.national.com

4

age drop across RPTC increases as the resistance of RPTC increases. As the emitter voltage of Q1 reaches 1.255V, thermal foldback control is activated and the LED current reduces according to I

EXT

.

30128721

Design Example

FIGURE 4. Thermal Foldback Control with PTC thermistor

The LM3414HV evaluation board is used to drive a LED string at 700mA and thermal foldback control is needed to take place when the temperature of the LED strings exceeds 80°

C as presented in Figure 5.

RPTC

(25C)

RPTC

(80C)

= 330Ω

= 1.2k

Ω

RPTC

(100C)

= 10k

Ω

In Figure 5, the LED current with the LED temperature below

80°C (I

LED(normal)

) is 700mA. As the temperature of the LED goes up to 80°C, thermal foldback begins and reduces the

LED driving current with respect to the increase of resistance of RPTC. As the temperature of the LEDs reaches 100°C, the

LED current reduces to zero. Provided that the resistance of the thermistor RPTC under 80C and 100°C are 1.2k

Ω and

10k

Ω respectively, the values of R1 and R2 can be calculated following the steps listed below.

At 80°C:

30128722

FIGURE 5. Reduction of LED current with thermal foldback control

Assume the resistance of the PTC thermistor under 25°C, 80°

C and 100°C are:

5

(8) www.national.com

At 100°C:

(9)

Tiny Board Outline

The tiny packages of the LM3414 family are exceptionally suitable for the applications that require high output power in limited space. In order to demonstrate the high power density of the LM3414HV, the core circuitry of this evaluation boards are completed in compact form factors: 22mm x 19mm for

LLP-8 package, 26mm x 19mm for PSOP-8 package. The schematic of the core circuitry is as shown in Figure 6. The core circuitry can be extracted by cutting out from the PCB frame of the board as shown in Figure 7.

FIGURE 6. Core Circuitry of the LM3414HV Evaluation Boards

30128725 www.national.com

30128726

FIGURE 7. Extracting the core circuitry from the LM3414HV evaluation boards

6

30128727

FIGURE 8. Connecting to the core circuitry

The board of the core circuitry features four connection pads for connections to DC power supply and LED string, as shown in Figure 8. To ensure thermal performance of the board, a heatsink attaches to the bottom layer of the board may be required depending on actual operation environment.

Bill of Materials

Designation

U1

D1

L1

CIN

CVCC

RIADJ

RFS

VIN, GND,

LED+, LED-

VIN, GND,

LED+,LED-,

THM+, THM-,

DIM,

PCB

Q1

R1,R2,RFS_1,

RFS_2,RIADJ_1

JP1,JP2,JP3

Description

LED Driver IC, LM3414HV

Schottky Diode 100V 2A

Power Inductor 47 µH

Cap MLCC 100V 2.2 µF X7R 1210

Cap MLCC 10V 1 µF X5R 0603

Chip Resistor 3.09 k

Ω 1% 0603

Chip Resistor 40.2 k

Ω 1% 0603

Banana Jack 5.3(mm) Dia

Turret 2.35(mm) Dia

LM3414EVAL PCB 85 X 54 (mm)

NPN Bipolar Transistor

NA

NA

Package

LLP8 / PSOP8

1210

603

603

603

5.3 (mm) Dia.

2.35 (mm) Dia.

85 X 54 (mm)

SOT23

603

603

Manufacturer Part #

LM3414MH

SS2PH10-M3/84A

MMD-08EZ-470M-S1

GRM32ER72A225KA35L

GRM185R61A105KE36D

CRCW06033K09FKEA

CRCW060340K2FKEA

575-8

1502-2

Vendor

NSC

Vishay

MAG.Layers

Murata

Murata

Vishay

Vishay

KEYSTONE

KEYSTONE

NSC

7 www.national.com

Typical Performance Characteristics

All curves taken at V

IN

= 48V with configuration in typical application for driving twelve power LEDs with four output channels active and output current per channel = 350 mA. T

A

= 25°C, unless otherwise specified.

Output Current (A) Efficiency (%)

I

LED

(A) vs R

IADJ

(k

Ω)

30128730

f

SW

(kHz) vs R

FS

(k

Ω)

30128731

I

LED

with V

DIM

rising

30128732

I

LED

with V

DIM

falling

30128733 www.national.com

30128708

8

30128709

Evaluation Board Layout (LLP-8 Package)

Top Layer and Top Overlay

30128712

Bottom Layer and Bottom Overlay

9

30128713 www.national.com

Evaluation Board Layout (PSOP-8 Package)

Top Layer and Top Overlay

30128714 www.national.com

Bottom Layer and Bottom Overlay

10

30128715

Notes

11 www.national.com

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