LM3405A [NSC]

1.6MHz, 1A Constant Current Buck LED Driver with Internal Compensation in Tiny SOT23 Package; 为1.6MHz ， 1A恒流降压LED驱动器和内部补偿，采用微型SOT23封装


型号：	LM3405A
厂家：	National Semiconductor
描述：	1.6MHz, 1A Constant Current Buck LED Driver with Internal Compensation in Tiny SOT23 Package 为1.6MHz ， 1A恒流降压LED驱动器和内部补偿，采用微型SOT23封装驱动器
文件：	总20页 (文件大小：491K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

October 2007

LM3405A

1.6MHz, 1A Constant Current Buck LED Driver with Internal

Compensation in Tiny SOT23 Package

General Description

Features

The LM3405A is a 1A constant current buck LED driver de-

signed to provide a simple, high efficiency solution for driving

high power LEDs. With a 0.205V reference voltage feedback

control to minimize power dissipation, an external resistor

sets the current as needed for driving various types of LEDs.

Switching frequency is internally set to 1.6MHz, allowing

small surface mount inductors and capacitors to be used. The

LM3405A utilizes current-mode control and internal compen-

sation offering ease of use and predictable, high performance

regulation over a wide range of operating conditions. With a

maximum input voltage of 22V, it can drive up to 5 High-

Brightness LEDs in series at 1A forward current, with the

single LED forward voltage of approximately 3.7V. Additional

features include user accessible EN/DIM pin for enabling and

PWM dimming of LEDs, thermal shutdown, cycle-by-cycle

current limit and over-current protection.

V_INoperating range of 3V to 22V

■

Drives up to 5 High-Brightness LEDs in series at 1A

Thin SOT23-6 package

1.6MHz switching frequency

EN/DIM input for enabling and PWM dimming of LEDs

■

300mΩ NMOS switch

■

40nA shutdown current at V_IN= 5V

Internally compensated current-mode control

Cycle-by-cycle current limit

Input voltage UVLO

Over-current protection

Thermal shutdown

Applications

LED Driver

■

Constant Current Source

Industrial Lighting

LED Flashlights

LED Lightbulbs

■

Typical Application Circuit

Efficiency vs LED Current (V_IN= 12V)

30015201

30015273

300152

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Connection Diagrams

30015205

6-Lead TSOT

NS Package Number MK06A

Pin 1 Identificat³io⁰⁰n¹⁵²⁶⁰

Ordering Information

Part Number

LM3405AXMKE

LM3405AXMK

LM3405AXMKX

Package Type

NS Package Drawing

Package Marking

Supplied As

SSEB

250 Units on Tape and Reel

1000 Units on Tape and Reel

3000 Units on Tape and Reel

TSOT-6

MK06A

*NOPB versions are available

Pin Descriptions

Pin(s)

Name

Application Information

Voltage at this pin drives the internal NMOS power switch. A bootstrap capacitor is

connected between the BOOST and SW pins.

BOOST

Signal and Power ground pin. Place the LED current-setting resistor as close as possible

to this pin for accurate current regulation.

GND

Feedback pin. Connect an external resistor from FB to GND to set the LED Current.

Enable control input. Logic high enables operation. Toggling this pin with a periodic logic

square wave of varying duty cycle at different frequencies controls the brightness of LEDs.

Do not allow this pin to float or be greater than V_IN+ 0.3V.

EN/DIM

V_IN

Input supply voltage. Connect a bypass capacitor locally from this pin to GND.

Switch pin. Connect this pin to the inductor, catch diode, and bootstrap capacitor.

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ESD Susceptibility (Note 2)

Storage Temperature

Soldering Information

2kV

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

-65°C to +150°C

Infrared/Convection Reflow (15sec)

220°C

V_IN

-0.5V to 24V

-0.5V to 30V

Operating Ratings (Note 1)

SW Voltage

V_IN

3V to 22V

Boost Voltage

Boost to SW Voltage

FB Voltage

-0.5V to (V_IN+ 0.3V)

2.5V to 5.5V

EN/DIM voltage

Boost to SW Voltage

-0.5V to 6.0V

-0.5V to 3.0V

-0.5V to (V_IN+ 0.3V)

150°C

Junction Temperature Range

Thermal Resistance θ_JA(Note 3)

-40°C to +125°C

EN/DIM Voltage

Junction Temperature

118°C/W

Electrical Characteristics Unless otherwise specified, V_IN= 12V. Limits in standard type are for T_J= 25°C only;

limits in boldface type apply over the junction temperature (T_J) range of -40°C to +125°C. Minimum and Maximum limits are

guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm, and are

provided for reference purposes only.

Symbol

Parameter

Feedback Voltage

Conditions

Min

Typ

Max

Units

V_FB

0.188

0.205

0.220

V_IN= 3V to 22V

ΔV_FB/(ΔV_INxV_FB) Feedback Voltage Line Regulation

0.01

%/V

I_FB

Sink/Source

V_INRising

V_INFalling

250

Feedback Input Bias Current

Under-voltage Lockout

UVLO Hysteresis

2.74

2.3

0.44

1.6

2.95

UVLO

1.9

f_SW

D_MAX

R_DS(ON)

I_CL

Switching Frequency

Maximum Duty Cycle

Switch ON Resistance

Switch Current Limit

1.2

1.9

MHz

V_FB= 0V

V_BOOST- V_SW= 3V

V_BOOST- V_SW= 3V, V_IN= 3V

Switching, V_FB= 0.195V

V_EN/DIM= 0V

300

2.0

1.8

0.3

600

2.8

mΩ

1.2

1.8

Quiescent Current

µA

I_Q

Quiescent Current (Shutdown)

Enable Threshold Voltage

Shutdown Threshold Voltage

EN/DIM Pin Current

V_EN/DIMRising

V_{EN/DIM_TH}

V_EN/DIMFalling

0.4

I_EN/DIM

I_SW

Sink/Source

0.01

0.1

µA

Switch Leakage

V_IN= 22V

Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings define the conditions under which the device

is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: Human body model, 1.5kΩ in series with 100pF.

Note 3: Thermal shutdown will occur if the junction temperature (T_J) exceeds 165°C. The maximum allowable power dissipation (P_D) at any ambient temperature

(T_A) is P_D= (T_J(MAX)– T_A)/θ_JA. This number applies to packages soldered directly onto a 3" x 3" PC board with 2oz. copper on 4 layers in still air. For a 2 layer

board using 1 oz. copper in still air, θ_JA= 204°C/W.

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Typical Performance Characteristics Unless otherwise specified, V_IN= 12V, V_BOOST- V_SW= 5V and

T_A= 25°C.

Efficiency vs LED Current (V_IN=5V)

Efficiency vs Input Voltage (I_F= 1A)

30015271

30015231

Efficiency vs Input Voltage (I_F= 0.7A)

Efficiency vs Input Voltage (I_F= 0.35A)

30015232

30015233

V_FBvs Temperature

Oscillator Frequency vs Temperature

30015236

30015227

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Current Limit vs Temperature

R_DS(ON)vs Temperature (V_BOOST- V_SW= 3V)

30015272

30015230

Quiescent Current vs Temperature

Startup Response to EN/DIM Signal

(V_IN= 15V, I_F= 0.2A)

30015234

30015268

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Block Diagram

30015252

FIGURE 1. Simplified Block Diagram

Application Information

THEORY OF OPERATION

The LM3405A is a PWM, current-mode control switching buck

regulator designed to provide a simple, high efficiency solu-

tion for driving LEDs with a preset switching frequency of

1.6MHz. This high frequency allows the LM3405A to operate

with small surface mount capacitors and inductors, resulting

in LED drivers that need only a minimum amount of board

space. The LM3405A is internally compensated, simple to

use, and requires few external components.

The following description of operation of the LM3405A will re-

fer to the Simplified Block Diagram (Figure 1) and to the

waveforms in Figure 2. The LM3405A supplies a regulated

output current by switching the internal NMOS power switch

at constant frequency and variable duty cycle. A switching

cycle begins at the falling edge of the reset pulse generated

by the internal oscillator. When this pulse goes low, the output

control logic turns on the internal NMOS power switch. During

this on-time, the SW pin voltage (V_SW) swings up to approxi-

mately V_IN, and the inductor current (I_L) increases with a linear

slope. I_Lis measured by the current sense amplifier, which

generates an output proportional to the switch current. The

sense signal is summed with the regulator’s corrective ramp

and compared to the error amplifier’s output, which is propor-

tional to the difference between the feedback voltage and

V_REF. When the PWM comparator output goes high, the in-

ternal power switch turns off until the next switching cycle

begins. During the switch off-time, inductor current dis-

charges through the catch diode D1, which forces the SW pin

to swing below ground by the forward voltage (V_D1) of the

catch diode. The regulator loop adjusts the duty cycle (D) to

maintain a constant output current (I_F) through the LED, by

forcing FB pin voltage to be equal to V_REF(0.205V).

30015207

FIGURE 2. SW Pin Voltage and Inductor Current

Waveforms of LM3405A

BOOST FUNCTION

Capacitor C3 and diode D2 in Figure 1 are used to generate

a voltage V_BOOST. The voltage across C3, V_BOOST- V_SW, is

the gate drive voltage to the internal NMOS power switch. To

properly drive the internal NMOS switch during its on-time,

V_BOOSTneeds to be at least 2.5V greater than V_SW. Large

value of V_BOOST- V_SWis recommended to achieve better ef-

ficiency by minimizing both the internal switch ON resistance

(R_DS(ON)), and the switch rise and fall times. However,

V_BOOST- V_SWshould not exceed the maximum operating limit

of 5.5V.

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When the LM3405A starts up, internal circuitry from V_INsup-

plies a 20mA current to the BOOST pin, flowing out of the

BOOST pin into C3. This current charges C3 to a voltage suf-

ficient to turn the switch on. The BOOST pin will continue to

source current to C3 until the voltage at the feedback pin is

greater than 123mV.

placing a zener diode D3 in series with D2 as shown in Figure

4. When using a series zener diode from the input, the gate

drive voltage is V_IN- V_D3- V_D2+ V_D1

There are various methods to derive V_BOOST

1. From the input voltage (V_IN)

2. From the output voltage (V_OUT

3. From a shunt or series zener diode

4. From an external distributed voltage rail (V_EXT

)

The first method is shown in the Simplified Block Diagram of

Figure 1. Capacitor C3 is charged via diode D2 by V_IN. During

a normal switching cycle, when the internal NMOS power

switch is off (T_OFF) (refer to Figure 2), V_BOOSTequals V_INmi-

nus the forward voltage of D2 (V_D2), during which the current

in the inductor (L1) forward biases the catch diode D1 (V_D1).

Therefore the gate drive voltage stored across C3 is:

30015299

FIGURE 4. V_BOOSTderived from V_INthrough a Series

Zener

V_BOOST- V_SW= V_IN- V_D2+ V_D1

When the NMOS switch turns on (T_ON), the switch pin rises

to:

An alternate method is to place the zener diode D3 in a shunt

configuration as shown in Figure 5. A small 350mW to

500mW, 5.1V zener in a SOT-23 or SOD package can be

used for this purpose. A small ceramic capacitor such as a

6.3V, 0.1µF capacitor (C5) should be placed in parallel with

the zener diode. When the internal NMOS switch turns on, a

pulse of current is drawn to charge the internal NMOS gate

capacitance. The 0.1µF parallel shunt capacitor ensures that

the V_BOOSTvoltage is maintained during this time. Resistor R2

should be chosen to provide enough RMS current to the zener

diode and to the BOOST pin. A recommended choice for the

zener current (I_ZENER) is 1mA. The current I_BOOSTinto the

BOOST pin supplies the gate current of the NMOS power

switch. It reaches a maximum of around 3.6mA at the highest

gate drive voltage of 5.5V over the LM3405A operating range.

V_SW= V_IN– (R_DS(ON)x I_L)

Since the voltage across C3 remains unchanged, V_BOOSTis

forced to rise thus reverse biasing D2. The voltage at

V_BOOSTis then:

V_BOOST= 2V_IN– (R_DS(ON)x I_L) – V_D2+ V_D1

Depending on the quality of the diodes D1 and D2, the gate

drive voltage in this method can be slightly less or larger than

the input voltage V_IN. For best performance, ensure that the

variation of the input supply does not cause the gate drive

voltage to fall outside the recommended range:

2.5V < V_IN- V_D2+ V_D1< 5.5V

The second method for deriving the boost voltage is to con-

nect D2 to the output as shown in Figure 3. The gate drive

voltage in this configuration is:

For the worst case I_BOOST, increase the current by 50%. In

that case, the maximum boost current will be:

V_BOOST- V_SW= V_OUT– V_D2+ V_D1

I_BOOST-MAX= 1.5 x 3.6mA = 5.4mA

R2 will then be given by:

Since the gate drive voltage needs to be in the range of 2.5V

to 5.5V, the output voltage V_OUTshould be limited to a certain

range. For the calculation of V_OUT, see OUTPUT VOLTAGE

section.

R2 = (V_IN- V_ZENER) / (I_{BOOST_MAX}+ I_ZENER

)

For example, let V_IN= 12V, V_ZENER= 5V, I_ZENER= 1mA, then:

R2 = (12V - 5V) / (5.4mA + 1mA) = 1.09kΩ

30015293

FIGURE 3. V_BOOSTderived from V_OUT

30015294

The third method can be used in the applications where both

V_INand V_OUTare greater than 5.5V. In these cases, C3 cannot

be charged directly from these voltages; instead C3 can be

charged from V_INor V_OUTminus a zener voltage (V_D3) by

FIGURE 5. V_BOOSTderived from V_INthrough a Shunt Zener

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The fourth method can be used in an application which has

an external low voltage rail, V_EXT. C3 can be charged through

D2 from V_EXT, independent of V_INand V_OUTvoltage levels.

Again for best performance, ensure that the gate drive volt-

age, V_EXT- V_D2+ V_D1, falls in the range of 2.5V to 5.5V.

with respect to V_INon the LED current is shown in Figure 7.

For a fast rising input voltage (200µs for example), there is no

need to delay the EN/DIM signal since soft-start can smoothly

bring up the LED current as shown in Figure 8.

SETTING THE LED CURRENT

LM3405A is a constant current buck regulator. The LEDs are

connected between V_OUTand FB pin as shown in the Typical

Application Circuit. The FB pin is at 0.205V in regulation and

therefore the LED current I_Fis set by V_FBand the resistor R1

from FB to ground by the following equation:

I_F= V_FB/ R1

I_Fshould not exceed the 1A current capability of LM3405A

and therefore R1 minimum must be approximately 0.2Ω. I_F

should also be kept above 200mA for stable operation, and

therefore R1 maximum must be approximately 1Ω. If average

LED currents less than 200mA are desired, the EN/DIM pin

can be used for PWM dimming. See LED PWM DIMMING

section.

OUTPUT VOLTAGE

30015276

The output voltage is primarily determined by the number of

LEDs (n) connected from V_OUTto FB pin and therefore V_OUT

can be written as :

FIGURE 6. Startup Response to V_INwith 5ms rise time

V_OUT= ((n x V_F) + V_FB

)

where V_Fis the forward voltage of one LED at the set LED

current level (see LED manufacturer datasheet for forward

characteristics curve).

ENABLE MODE / SHUTDOWN MODE

The LM3405A has both enable and shutdown modes that are

controlled by the EN/DIM pin. Connecting a voltage source

greater than 1.8V to the EN/DIM pin enables the operation of

LM3405A, while reducing this voltage below 0.4V places the

part in a low quiescent current (0.3µA typical) shutdown

mode. There is no internal pull-up on EN/DIM pin, therefore

an external signal is required to initiate switching. Do not allow

this pin to float or rise to 0.3V above V_IN. It should be noted

that when the EN/DIM pin voltage rises above 1.8V while the

input voltage is greater than UVLO, there is a finite delay be-

fore switching starts. During this delay the LM3405A will go

through a power on reset state after which the internal soft-

start process commences. The soft-start process limits the

inrush current and brings up the LED current (I_F) in a smooth

and controlled fashion. The total combined duration of the

power on reset delay, soft-start delay and the delay to fully

establish the LED current is in the order of 100µs (refer to

Figure 10).

30015277

FIGURE 7. Startup Response to V_INwith EN/DIM delayed

The simplest way to enable the operation of LM3405A is to

connect the EN/DIM pin to V_INwhich allows self start-up of

LM3405A whenever the input voltage is applied. However,

when an input voltage of slow rise time is used to power the

application and if both the input voltage and the output voltage

are not fully established before the soft-start time elapses, the

control circuit will command maximum duty cycle operation of

the internal power switch to bring up the output voltage rapid-

ly. When the feedback pin voltage exceeds 0.205V, the duty

cycle will have to reduce from the maximum value according-

ly, to maintain regulation. It takes a finite amount of time for

this reduction of duty cycle and this will result in a spike in LED

current for a short duration as shown in Figure 6. In applica-

tions where this LED current overshoot is undesirable, EN/

DIM pin voltage can be separately applied and delayed such

that V_INis fully established before the EN/DIM pin voltage

reaches the enable threshold. The effect of delaying EN/DIM

30015275

FIGURE 8. Startup Response to V_INwith 200µs rise time

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LED PWM DIMMING

The LED brightness can be controlled by applying a periodic

pulse signal to the EN/DIM pin and varying its frequency and/

or duty cycle. This so-called PWM dimming method controls

the average light output by pulsing the LED current between

the set value and zero. A logic high level at the EN/DIM pin

turns on the LED current whereas a logic low level turns off

the LED current. Figure 9 shows a typical LED current wave-

form in PWM dimming mode. As explained in the previous

section, there is approximately a 100µs delay from the EN/

DIM signal going high to fully establishing the LED current as

shown in Figure 10. This 100µs delay sets a maximum fre-

quency limit for the driving signal that can be applied to the

EN/DIM pin for PWM dimming. Figure 11 shows the average

LED current versus duty cycle of PWM dimming signal for

various frequencies. The applicable frequency range to drive

LM3405A for PWM dimming is from 100Hz to 5kHz. The dim-

ming ratio reduces drastically when the applied PWM dim-

ming frequency is greater than 5kHz.

30015283

FIGURE 11. Average LED Current versus Duty Cycle of

PWM Dimming Signal at EN/DIM Pin

UNDER-VOLTAGE LOCKOUT

Under-voltage lockout (UVLO) prevents the LM3405A from

operating until the input voltage exceeds 2.74V (typical). The

UVLO threshold has approximately 440mV of hysteresis, so

the part will operate until V_INdrops below 2.3V (typical). Hys-

teresis prevents the part from turning off during power up if

V_INis non-monotonic.

CURRENT LIMIT

The LM3405A uses cycle-by-cycle current limit to protect the

internal power switch. During each switching cycle, a current

limit comparator detects if the power switch current exceeds

2.0A (typical), and turns off the switch until the next switching

cycle begins.

30015266

OVER-CURRENT PROTECTION

FIGURE 9. PWM Dimming of LEDs using the EN/DIM Pin

The LM3405A has a built in over-current comparator that

compares the FB pin voltage to a threshold voltage that is

60% higher than the internal reference V_REF. Once the FB pin

voltage exceeds this threshold level (typically 328mV), the in-

ternal NMOS power switch is turned off, which allows the

feedback voltage to decrease towards regulation. This

threshold provides an upper limit for the LED current. LED

current overshoot is limited to 328mV/R1 by this comparator

during transients.

THERMAL SHUTDOWN

Thermal shutdown limits total power dissipation by turning off

the internal power switch when the IC junction temperature

exceeds 165°C. After thermal shutdown occurs, the power

switch does not turn on until the junction temperature drops

below approximately 150°C.

Design Guide

INDUCTOR (L1)

30015267

The Duty Cycle (D) can be approximated quickly using the

ratio of output voltage (V_OUT) to input voltage (V_IN):

FIGURE 10. Startup Response to EN/DIM with I_F= 1A

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The catch diode (D1) forward voltage drop and the voltage

drop across the internal NMOS must be included to calculate

a more accurate duty cycle. Calculate D by using the following

formula:

selection, refer to Circuit Examples and Recommended In-

ductance Range in Table 1.

TABLE 1. Recommended Inductance Range

I_F

Inductance Range and Inductor Current Ripple

6.8µH-15µH

Inductance

6.8µH

51%

10µH

36%

15µH

24%

1.0A

Δi_L/ I_F*

V_SWcan be approximated by:

V_SW= I_Fx R_DS(ON)

10µH-22µH

10µH

Inductance

15µH

39%

22µH

26%

0.6A

0.2A

58%

The diode forward drop (V_D1) can range from 0.3V to 0.7V

depending on the quality of the diode. The lower V_D1is, the

higher the operating efficiency of the converter.

Δi_L/ I_F*

15µH-27µH

15µH

Inductance

22µH

79%

27µH

65%

The inductor value determines the output ripple current (Δi_L,

as defined in Figure 2). Lower inductor values decrease the

size of the inductor, but increases the output ripple current.

An increase in the inductor value will decrease the output rip-

ple current. The ratio of ripple current to LED current is

optimized when it is set between 0.3 and 0.4 at 1A LED cur-

rent. This ratio r is defined as:

Δi_L/ I_F*

116%

*Maximum over full range of V_INand V_OUT

INPUT CAPACITOR (C1)

An input capacitor is necessary to ensure that V_INdoes not

drop excessively during switching transients. The primary

specifications of the input capacitor are capacitance, voltage

rating, RMS current rating, and ESL (Equivalent Series In-

ductance). The input voltage rating is specifically stated by

the capacitor manufacturer. Make sure to check any recom-

mended deratings and also verify if there is any significant

change in capacitance at the operating input voltage and the

operating temperature. The input capacitor maximum RMS

input current rating (I_RMS-IN) must be greater than:

One must also ensure that the minimum current limit (1.2A)

is not exceeded, so the peak current in the inductor must be

calculated. The peak current (I_LPK) in the inductor is calculated

as:

I_LPK= I_F+ Δi_L/2

When the designed maximum output current is reduced, the

ratio r can be increased. At a current of 0.2A, r can be made

as high as 0.7. The ripple ratio can be increased at lighter

loads because the net ripple is actually quite low, and if r re-

mains constant the inductor value can be made quite large.

An equation empirically developed for the maximum ripple

ratio at any current below 2A is:

It can be shown from the above equation that maximum RMS

capacitor current occurs when D = 0.5. Always calculate the

RMS at the point where the duty cycle D, is closest to 0.5. The

ESL of an input capacitor is usually determined by the effec-

tive cross sectional area of the current path. A large leaded

capacitor will have high ESL and a 0805 ceramic chip capac-

itor will have very low ESL. At the operating frequency of the

LM3405A, certain capacitors may have an ESL so large that

the resulting inductive impedance (2πfL) will be higher than

that required to provide stable operation. It is strongly recom-

mended to use ceramic capacitors due to their low ESR and

low ESL. A 10µF multilayer ceramic capacitor (MLCC) is a

good choice for most applications. In cases where large ca-

pacitance is required, use surface mount capacitors such as

Tantalum capacitors and place at least a 1µF ceramic capac-

itor close to the V_INpin. For MLCCs it is recommended to use

X7R or X5R dielectrics. Consult capacitor manufacturer

datasheet to see how rated capacitance varies over operating

conditions.

-0.3667

r = 0.387 x I_OUT

Note that this is just a guideline.

The LM3405A operates at a high frequency allowing the use

of ceramic output capacitors without compromising transient

response. Ceramic capacitors allow higher inductor ripple

without significantly increasing LED current ripple. See the

output capacitor and feed-forward capacitor sections for more

details on LED current ripple.

Now that the ripple current or ripple ratio is determined, the

inductance is calculated by:

OUTPUT CAPACITOR (C2)

The output capacitor is selected based upon the desired re-

duction in LED current ripple. A 1µF ceramic capacitor results

in very low LED current ripple for most applications. Due to

the high switching frequency, the 1µF capacitor alone (without

feed-forward capacitor C4) can filter more than 90% of the

inductor current ripple for most applications where the sum of

LED dynamic resistance and R1 is larger than 1Ω. Since the

internal compensation is tailored for small output capacitance

with very low ESR, it is strongly recommended to use a ce-

ramic capacitor with capacitance less than 3.3µF.

where f_SWis the switching frequency and I_Fis the LED current.

When selecting an inductor, make sure that it is capable of

supporting the peak output current without saturating. Induc-

tor saturation will result in a sudden reduction in inductance

and prevent the regulator from operating correctly. Because

of the operating frequency of LM3405A, ferrite based induc-

tors are preferred to minimize core losses. This presents little

restriction since the variety of ferrite based inductors is huge.

Lastly, inductors with lower series resistance (DCR) will pro-

vide better operating efficiency. For recommended inductor

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Given the availability and quality of MLCCs and the expected

output voltage of designs using the LM3405A, there is really

no need to review other capacitor technologies. A benefit of

ceramic capacitors is their ability to bypass high frequency

noise. A certain amount of switching edge noise will couple

through the parasitic capacitances in the inductor to the out-

put. A ceramic capacitor will bypass this noise. In cases where

large capacitance is required, use Electrolytic or Tantalum

capacitors with large ESR, and verify the loop performance

on bench. Like the input capacitor, recommended multilayer

ceramic capacitors are X7R or X5R. Again, verify actual ca-

pacitance at the desired operating voltage and temperature.

Check the RMS current rating of the capacitor. The maximum

RMS current rating of the capacitor is:

30015270

FIGURE 13. PWM Dimming with a 1µF Feed-Forward

Capacitor

One may select a 1206 size ceramic capacitor for C2, since

its current rating is typically higher than 1A, more than enough

for the requirement.

CATCH DIODE (D1)

The catch diode (D1) conducts during the switch off-time. A

Schottky diode is required for its fast switching time and low

forward voltage drop. The catch diode should be chosen such

that its current rating is greater than:

FEED-FORWARD CAPACITOR (C4)

The feed-forward capacitor (designated as C4) connected in

parallel with the LED string is required to provide multiple

benefits to the LED driver design. It greatly improves the large

signal transient response and suppresses LED current over-

shoot that may otherwise occur during PWM dimming; it also

helps to shape the rise and fall times of the LED current pulse

during PWM dimming thus reducing EMI emission; it reduces

LED current ripple by bypassing some of inductor ripple from

flowing through the LED. For most applications, a 1µF ce-

ramic capacitor is sufficient. In fact, the combination of a 1µF

feed-forward ceramic capacitor and a 1µF output ceramic ca-

pacitor leads to less than 1% current ripple flowing through

the LED. Lower and higher C4 values can be used, but bench

validation is required to ensure the performance meets the

application requirement.

I_D1= I_Fx (1-D)

The reverse breakdown rating of the diode must be at least

the maximum input voltage plus appropriate margin. To im-

prove efficiency, choose a Schottky diode with a low forward

voltage drop.

BOOST DIODE (D2)

A standard diode such as the 1N4148 type is recommended.

For V_BOOSTcircuits derived from voltages less than 3.3V, a

small-signal Schottky diode is recommended for better effi-

ciency. A good choice is the BAT54 small signal diode.

BOOST CAPACITOR (C3)

Figure 12 shows a typical LED current waveform during PWM

dimming without feed-forward capacitor. At the beginning of

each PWM cycle, overshoot can be seen in the LED current.

Adding a 1µF feed-forward capacitor can totally remove the

overshoot as shown in Figure 13.

A 0.01µF ceramic capacitor with a voltage rating of at least

6.3V is sufficient. The X7R and X5R MLCCs provide the best

performance.

POWER LOSS ESTIMATION

The main power loss in LM3405A includes three basic types

of loss in the internal power switch: conduction loss, switching

loss, and gate charge loss. In addition, there is loss associ-

ated with the power required for the internal circuitry of IC.

The conduction loss is calculated as:

If the inductor ripple current is fairly small (for example, less

than 40%) , the conduction loss can be simplified to:

P_COND= I_F²x R_DS(ON)x D

The switching loss occurs during the switch on and off tran-

sition periods, where voltage and current overlap resulting in

power loss. The simplest means to determine this loss is to

empirically measure the rise and fall times (10% to 90%) of

the voltage at the switch pin.

30015269

FIGURE 12. PWM Dimming without Feed-Forward

Capacitor

Switching power loss is calculated as follows:

P_SW= 0.5 x V_INx I_Fx f_SWx ( T_RISE+ T_FALL

)

www.national.com

The gate charge loss is associated with the gate charge Q_G

required to drive the switch:

PCB Layout Considerations

When planning layout there are a few things to consider when

trying to achieve a clean, regulated output. The most impor-

tant consideration when completing the layout is the close

coupling of the GND connections of the input capacitor C1

and the catch diode D1. These ground ends should be close

to one another and be connected to the GND plane with at

least two through-holes. Place these components as close to

the IC as possible. The next consideration is the location of

the GND connection of the output capacitor C2, which should

be near the GND connections of C1 and D1.

P_G= f_SWx V_INx Q_G

The power loss required for operation of the internal circuitry:

P_Q= I_Qx V_IN

I_Qis the quiescent operating current, and is typically around

1.8mA for the LM3405A.

The total power loss in the IC is:

P_INTERNAL= P_COND+ P_SW+ P_G+ P_Q

An example of power losses for a typical application is shown

in Table 2:

There should be a continuous ground plane on the bottom

layer of a two-layer board except under the switching node

island.

TABLE 2. Power Loss Tabulation

The FB pin is a high impedance node and care should be

taken to make the FB trace short to avoid noise pickup that

causes inaccurate regulation. The LED current setting resis-

tor R1 should be placed as close as possible to the IC, with

the GND of R1 placed as close as possible to the GND of the

IC. The V_OUTtrace to LED anode should be routed away from

the inductor and any other traces that are switching.

Conditions

Power loss

V_IN

V_OUT

I_OUT

V_D1

12V

3.9V

1.0A

0.45V

R_DS(ON)

f_SW

P_COND

108mW

288mW

High AC currents flow through the V_IN, SW and V_OUTtraces,

so they should be as short and wide as possible. Radiated

noise can be decreased by choosing a shielded inductor.

300mΩ

1.6MHz

18ns

T_RISE

T_FALL

I_Q

The remaining components should also be placed as close

as possible to the IC. Please see Application Note AN-1229

for further considerations and the LM3405A demo board as

an example of a four-layer layout.

P_SW

12ns

1.8mA

1.4nC

P_Q

P_G

22mW

27mW

Q_G

D is calculated to be 0.36

Σ ( P_COND+ P_SW+ P_Q+ P_G) = P_INTERNAL

P_INTERNAL= 445mW

www.national.com

LM3405A Circuit Examples

30015242

FIGURE 14. V_BOOSTderived from V_IN

( V_IN= 5V, I_F= 1A )

Bill of Materials for Figure 14

Part ID

Part Value

Part Number

Manufacturer

National Semiconductor

TDK

1A LED Driver

10µF, 6.3V, X5R

1µF, 10V, X7R

0.01µF, 16V, X7R

1µF, 10V, X7R

LM3405A

C1, Input Cap

C2, Output Cap

C3, Boost Cap

C4, Feedforward Cap

D1, Catch Diode

D2, Boost Diode

C3216X5R0J106M

GRM319R71A105KC01D

0805YC103KAT2A

GRM319R71A105KC01D

Murata

AVX

Murata

Schottky, 0.37V at 1A, V_R= 10V MBRM110LT1G

ON Semiconductor

Central Semiconductor

TDK

Schottky, 0.36V at 15mA

4.7µH, 1.6A

CMDSH-3

SLF6028T-4R7M1R6

WSL2010R2000FEA

LXK2-PW14

Vishay

0.2Ω, 0.5W, 1%

1.5A, White LED

LED1

Lumileds

www.national.com

30015243

FIGURE 15. V_BOOSTderived from V_OUT

( V_IN= 12V, I_F= 1A )

Bill of Materials for Figure 15

Part ID

Part Value

Part Number

Manufacturer

National Semiconductor

Panasonic

Murata

1A LED Driver

10µF, 25V, X5R

1µF, 10V, X7R

0.01µF, 16V, X7R

1µF, 10V, X7R

LM3405A

C1, Input Cap

C2, Output Cap

C3, Boost Cap

C4, Feedforward Cap

D1, Catch Diode

D2, Boost Diode

ECJ-3YB1E106K

GRM319R71A105KC01D

0805YC103KAT2A

GRM319R71A105KC01D

AVX

Murata

Schottky, 0.5V at 1A, V_R= 30V SS13

Vishay

Schottky, 0.36V at 15mA

4.7µH, 1.6A

CMDSH-3

Central Semiconductor

TDK

SLF6028T-4R7M1R6

WSL2010R2000FEA

LXK2-PW14

Vishay

0.2Ω, 0.5W, 1%

1.5A, White LED

LED1

Lumileds

www.national.com

30015244

FIGURE 16. V_BOOSTderived from V_INthrough a Shunt Zener Diode (D3)

( V_IN= 18V, I_F= 1A )

Bill of Materials for Figure 16

Part ID

Part Value

Part Number

Manufacturer

National Semiconductor

Panasonic

Murata

1A LED Driver

10µF, 25V, X5R

1µF, 10V, X7R

0.01µF, 16V, X7R

1µF, 10V, X7R

0.1µF, 16V, X7R

LM3405A

C1, Input Cap

C2, Output Cap

C3, Boost Cap

C4, Feedforward Cap

C5, Shunt Cap

D1, Catch Diode

D2, Boost Diode

D3, Zener Diode

ECJ-3YB1E106K

GRM319R71A105KC01D

0805YC103KAT2A

GRM319R71A105KC01D

GRM219R71C104KA01D

AVX

Murata

Schottky, 0.5V at 1A, V_R= 30V SS13

Vishay

Schottky, 0.36V at 15mA

4.7V, 350mW, SOT-23

6.8µH, 1.5A

CMDSH-3

Central Semiconductor

Fairchild

BZX84C4V7

SLF6028T-6R8M1R5

WSL2010R2000FEA

CRCW08051K91FKEA

LXK2-PW14

TDK

Vishay

0.2Ω, 0.5W, 1%

1.91kΩ, 1%

Vishay

LED1

1.5A, White LED

Lumileds

www.national.com

30015249

FIGURE 17. V_BOOSTderived from V_INthrough a Series Zener Diode (D3)

( V_IN= 15V, I_F= 1A )

Bill of Materials for Figure 17

Part ID

Part Value

Part Number

Manufacturer

National Semiconductor

Panasonic

Murata

1A LED Driver

10µF, 25V, X5R

1µF, 10V, X7R

0.01µF, 16V, X7R

1µF, 10V, X7R

LM3405A

C1, Input Cap

C2, Output Cap

C3, Boost Cap

C4, Feedforward Cap

D1, Catch Diode

D2, Boost Diode

D3, Zener Diode

ECJ-3YB1E106K

GRM319R71A105KC01D

0805YC103KAT2A

GRM319R71A105KC01D

AVX

Murata

Schottky, 0.5V at 1A, V_R= 30V SS13

Vishay

Schottky, 0.36V at 15mA

11V, 350mW, SOT-23

6.8µH, 1.5A

CMDSH-3

Central Semiconductor

Fairchild

BZX84C11

SLF6028T-6R8M1R5

WSL2010R2000FEA

LXK2-PW14

TDK

Vishay

0.2Ω, 0.5W, 1%

1.5A, White LED

LED1

Lumileds

www.national.com

30015250

FIGURE 18. V_BOOSTderived from V_OUTthrough a Series Zener Diode (D3)

( V_IN= 18V, I_F= 1A )

Bill of Materials for Figure 18

Part ID

Part Value

Part Number

Manufacturer

National Semiconductor

Panasonic

Murata

1A LED Driver

10µF, 25V, X5R

1µF, 16V, X7R

0.01µF, 16V, X7R

1µF, 16V, X7R

LM3405A

C1, Input Cap

C2, Output Cap

C3, Boost Cap

C4, Feedforward Cap

D1, Catch Diode

D2, Boost Diode

D3, Zener Diode

ECJ-3YB1E106K

GRM319R71A105KC01D

0805YC103KAT2A

GRM319R71A105KC01D

AVX

Murata

Schottky, 0.5V at 1A, V_R= 30V SS13

Vishay

Schottky, 0.36V at 15mA

3.6V, 350mW, SOT-23

6.8µH, 1.5A

CMDSH-3

Central Semiconductor

Fairchild

BZX84C3V6

SLF6028T-6R8M1R5

WSL2010R2000FEA

LXK2-PW14

TDK

Vishay

0.2Ω, 0.5W, 1%

1.5A, White LED

LED1

Lumileds

LED2

LXK2-PW14

Lumileds

www.national.com

30015251

FIGURE 19. LED MR16 Lamp Application

( V_IN= 12V AC, I_F= 0.75A )

Bill of Materials for Figure 19

Part ID

Part Value

Part Number

Manufacturer

1A LED Driver

LM3405A

National Semiconductor

Panasonic

C1, Input Cap

C2, Output Cap

C3, Boost Cap

C5, Input Cap

D1, Catch Diode

D2, Boost Diode

D3, Rectifier Diode

D4, Rectifier Diode

D5, Rectifier Diode

D6, Rectifier Diode

10µF, 25V, X5R

1µF, 10V, X7R

ECJ-3YB1E106K

GRM319R71A105KC01D

0805YC103KAT2A

ECE-A1EN221U

Murata

0.01µF, 16V, X7R

220µF, 25V, electrolytic

AVX

Panasonic

Schottky, 0.5V at 1A, V_R= 30V SS13

Vishay

Schottky, 0.36V at 15mA

Schottky, 0.385V at 500mA

6.8µH, 1.5A

CMDSH-3

Central Semiconductor

TDK

CMHSH5-2L

SLF6028T-6R8M1R5

ERJ8BQFR27

LXHL-PW09

Panasonic

0.27Ω, 0.33W, 1%

1A, White LED

LED1

Lumileds

www.national.com

Physical Dimensions inches (millimeters) unless otherwise noted

6-Lead TSOT Package

NS Package Number MK06A

www.national.com

Notes

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