Texas Instruments LM3405A Manual de usuario

User's Guide

SNVA271B–October 2007–Revised April 2013

AN-1685 LM3405A Demo Board

1 Introduction

The LM3405A demo board is configured to drive a series string of high power, high brightness LEDs at a

forward current of 1A using the LM3405A constant current buck regulator. The board can accept a full

input operating range of 3V to 22V. The converter output voltage adjusts as needed to maintain a constant

current through the LED array. The LM3405A is a step-down regulator with an output voltage range

extending from a VO(MIN) of 205 mV (the reference voltage) to a VO(MAX) determined by the maximum duty

cycle (typically 94%). It can drive up to 5 LEDs in series at 1A forward current, with the single LED forward

voltage of approximately 3.7 V (typical of white, blue, and green LEDs using InGaN technology).

As shown in the demo board schematic circuit in Figure 1, the board is configured with the boost voltage

derived from VIN through a shunt zener (D3). This will ensure that the gate drive voltage VBOOST - VSW falls

in the recommended range of 2.5 V to 5.5 V when VIN varies from 5 V to 22 V. When input voltage is in

the range of 3 V to 5 V, the anode of boost diode (D2) should be directly connected to VIN by replacing R3

with a jumper and removing C4 and D3, to obtain sufficient gate drive voltage for best performance.

Table 1 lists the bill of materials (BOM) of this demo board. The measured performance characteristics

and layout of this board are also included below. Additionally, the Circuit Configuration Schematics section

illustrates other possible circuit configurations of this board to accommodate various input and output

requirements as discussed in the LM3405A 1.6MHz, 1A Constant Current Buck LED Driver with Internal

Compensation in Tiny SOT and MSOP PowerPAD Packages Data Sheet (SNVS508).

2 Connecting to LED Array

The LM3405A demo board includes a female 6-position SIP connector P1 as well as two standard 72mil

turret connectors for the cathode and anode connections of the LED array. Solid 18 or 20 gauge wire with

about 1cm of insulation stripped away makes a convenient, solderless connection to P1.

3 Setting the LED Current

The default forward current IFdelivered to the LED array is 1.0A. To adjust this value the current setting

resistor R1 can be changed according to Equation 1:

IF= VFB / R1 (1)

The feedback voltage VFB is typically regulated at 0.205 V. The resistor R1 should be rated to handle the

power dissipation of the LED current. R1 should be less than approximately 1 Ω, to ensure that the LED

current is kept above 200 mA. If average LED currents of less than 200 mA are desired, the EN/DIM pin

should be used for pulse width modulation (PWM) dimming.

4 PWM Dimming

The default connection of the PWM terminal is tied to VIN through a 100 kΩresistor (R2) to enable the

chip, which allows the set current to flow through the LEDs continuously. This PWM terminal can also be

connected to a periodic pulse signal at different frequencies and/or duty cycle for PWM dimming. A typical

LED current waveform in PWM dimming mode is shown in Figure 2.Figure 3 shows the average LED

current versus duty cycle of various dimming signal frequencies. Due to an approximately 100 µs delay

between the dimming signal and LED current, the dimming ratio reduces dramatically if the applied PWM

dimming frequency is greater than 5 kHz.

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SNVA271B–October 2007–Revised April 2013 AN-1685 LM3405A Demo Board

LM3405A

VIN VIN

EN/DIM

BOOST

GND

PWM

VOUT

6P1

D4 *

* Not installed

GND

ANODE of

First LED

CATHODE of

Last LED

PWM Dimming

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Figure 1. LM3405A Demo Board Schematic

Figure 2. PWM Dimming of LEDs Figure .

2AN-1685 LM3405A Demo Board SNVA271B–October 2007–Revised April 2013

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PWM Dimming

Figure 3. Average LED Current versus Duty Cycle of PWM Dimming Signal at PWM Terminal

SNVA271B–October 2007–Revised April 2013 AN-1685 LM3405A Demo Board

Bill of Materials (BOM)

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5 Bill of Materials (BOM)

Table 1. Bill of Materials (BOM)

Part ID Part Value Part Number Manufacturer

U1 1A constant current buck regulator, SOT-6 LM3405A Texas Instruments

L1 10 µH, 1.3A, 53 mΩ, 6.0 x 6.0 x 2.8 mm SLF6028T-100M1R3-PF TDK

C1 10 µF, 25 V, X5R, 1206 GRM31CR61E106KA12L Murata

C2 1 µF, 35 V, X7R, 1206 GMK316BJ105KL-T Taiyo Yuden

C3 0.01 µF, 16 V, X7R, 0805 0805YC103KAT2A AVX

C4 0.1 µF, 16 V, X7R, 0805 GRM219R71C104KA01D Murata

C5 1 µF, 35 V, X7R, 0805 GMK212BJ105KG-T Taiyo Yuden

D1 Schottky, 40 V, 1A, SMA SS14-E3/61T Vishay

D2 Schottky, 30 V, 200 mA, SOD-323 BAT54WS-TP Micro Commercial Co.

D3 5.1 V, 0.35W, SOT23 MMBZ5231B-7-F Diodes

D4 Not installed

R1 0.5W, 0.2 Ω, 1%, 2010 WSL2010R2000FEA Vishay

R2 100 kΩ, 1/8W, 1%, 0805 CRCW0805100KFKEA Vishay

R3 1.0 kΩ, 1%, 1/8W, 0805 CRCW08051K00FKEA Vishay

P1 6-position connector 5535676-5 Tyco/AMP

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Typical Performance Characteristics

6 Typical Performance Characteristics

Figure 4. Efficiency vs LED Current Figure 5. Efficiency vs LED Current

(VIN = 5 V, VBOOST Derived from VIN) (VIN = 12 V)

Figure 6. Efficiency vs Input Voltage Figure 7. Switching Waveforms

(IF= 1A)

SNVA271B–October 2007–Revised April 2013 AN-1685 LM3405A Demo Board

Typical Performance Characteristics

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Figure 8. Startup During PWM Dimming Figure 9. Shutdown During PWM Dimming

(VIN = 12 V, IF= 1A) (VIN = 12 V, IF= 1A)

6AN-1685 LM3405A Demo Board SNVA271B–October 2007–Revised April 2013

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Layout

7 Layout

Figure 10. Top Layer and Top Overlay

Figure 11. Bottom Layer and Bottom Overlay

SNVA271B–October 2007–Revised April 2013 AN-1685 LM3405A Demo Board

Layout

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Figure 12. Internal Plane 1 (GND)

Figure 13. Internal Plane 2 (VIN)

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LM3405A

VIN VIN

EN/DIM

BOOST

GND

PWM

VOUT

6P1

GND

LED

ANODE

LED

CATHODE

LM3405A

VIN VIN

EN/DIM

BOOST

GND

PWM

VOUT

6P1

GND

LED

ANODE

LED

CATHODE

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Additional Circuit Configuration Schematics

8 Additional Circuit Configuration Schematics

Figure 14. VBOOST Derived from VIN

Figure 15. VBOOST Derived from VOUT

SNVA271B–October 2007–Revised April 2013 AN-1685 LM3405A Demo Board

LM3405A

VIN VIN

EN/DIM

BOOST

GND

PWM

VOUT

6P1

GND

LED

ANODE

LED

CATHODE

LM3405A

VIN VIN

EN/DIM

BOOST

GND

R3*

PWM

VOUT

6P1

GND

LED ANODE

LED

CATHODE

* a zener diode is connected in R3 spot

Additional Circuit Configuration Schematics

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Figure 16. VBOOST Derived from VIN through a Series Zener Diode

Figure 17. VBOOST Derived from VOUT through a Series Zener Diode

10 AN-1685 LM3405A Demo Board SNVA271B–October 2007–Revised April 2013

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