National Semiconductor LM3410 Technical data

April 8, 2008

LM3410 PowerWise® 525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with Internal Compensation

LM3410 PowerWise

Internal Compensation

General Description

The LM3410 constant current LED driver is a monolithic, high frequency, PWM DC/DC converter in 5-pin SOT23, 6-pin LLP, & 8-pin eMSOP packages. With a minimum of external components the LM3410 is easy to use. It can drive 2.8A typical peak currents with an internal 170 mΩ NMOS switch. Switching frequency is internally set to either 525 kHz or 1.60 MHz, allowing the use of extremely small surface mount inductors and chip capacitors. Even though the operating frequency is high, efficiencies up to 88% are easy to achieve. External shutdown is included, featuring an ultra-low standby current of 80 nA. The LM3410 utilizes current-mode control and internal compensation to provide high-performance over a wide range of operating conditions. Additional features include dimming, cycle-by-cycle current limit, and thermal shutdown.

Typical Boost Application Circuit

Features

Space Saving SOT23-5 & 6-LLP Package

■

Input voltage range of 2.7V to 5.5V

■

Output voltage range of 3V to 24V

■

2.8A Typical Switch Current

■

High Switching Frequency

■

525 KHz (LM3410-Y)

—

1.6 MHz (LM3410-X)

— 170 mΩ NMOS Switch

■

190 mV Internal Voltage Reference

■

Internal Soft-Start

■

Current-Mode, PWM Operation

■

Thermal Shutdown

■

Applications

LED Backlight Current Source

■

LiIon Backlight OLED & HB LED Driver

■

Handheld Devices

■

LED Flash Driver

■

525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with

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Connection Diagrams

LM3410

Top View

5–Pin SOT23

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Top View

6-Pin LLP

Top View

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8-Pin eMSOP

Ordering Information

Order Number Frequency Package Type Package Drawing Supplied As

LM3410YMF

LM3410YMFX 3000 units Tape & Reel

LM3410YMFE 250 units Tape & Reel

LM3410YSD

LM3410YSDX 4500 units Tape & Reel

LM3410YSDE 250 units Tape & Reel

LM3410YMY

LM3410YMYX 3500 units Tape & Reel

LM3410YMYE 250 units Tape & Reel

LM3410XMF

LM3410XMFX 3000 units Tape & Reel

LM3410XMFE 250 units Tape & Reel

LM3410XSD

LM3410XSDX 4500 units Tape & Reel

LM3410XSDE 250 units Tape & Reel

LM3410XMY

LM3410XMYX 3500 units Tape & Reel

LM3410XMYE 250 units Tape & Reel

525 kHz

1.6 MHz

SOT23-5 MF05A

LLP-6 SDE06A

eMSOP-8 MUY08A

SOT23-5 MF05A

LLP-6 SDE06A

eMSOP-8 MUY08A

1000 units Tape & Reel

1000 units tape & reel

1000 units Tape & Reel

30038505

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Pin Descriptions - 5-Pin SOT23

Pin Name Function

1 SW Output switch. Connect to the inductor, output diode.

2 GND

3 FB Feedback pin. Connect FB to external resistor divider to set output voltage.

4 DIM

5 VIN Supply voltage pin for power stage, and input supply voltage.

Signal and power ground pin. Place the bottom resistor of the feedback network as close as possible to this pin.

Dimming & shutdown control input. Logic high enables operation. Duty Cycle from 0 to 100%. Do not allow this pin to float or be greater than VIN + 0.3V.

Pin Descriptions - 6-Pin LLP

Pin Name Function

1 PGND Power ground pin. Place PGND and output capacitor GND close together.

2 VIN Supply voltage for power stage, and input supply voltage.

3 DIM

4 FB Feedback pin. Connect FB to external resistor divider to set output voltage.

5 AGND

6 SW Output switch. Connect to the inductor, output diode.

DAP GND

Dimming & shutdown control input. Logic high enables operation. Duty Cycle from 0 to 100%. Do not allow this pin to float or be greater than VIN + 0.3V.

Signal ground pin. Place the bottom resistor of the feedback network as close as possible to this pin & pin

Signal & Power ground. Connect to pin 1 & pin 5 on top layer. Place 4-6 vias from DAP to bottom layer GND plane.

LM3410

Pin Descriptions - 8-Pin eMSOP

Pin Name Function

1 - No Connect

2 PGND Power ground pin. Place PGND and output capacitor GND close together.

3 VIN Supply voltage for power stage, and input supply voltage.

4 DIM

5 FB Feedback pin. Connect FB to external resistor divider to set output voltage.

6 AGND Signal ground pin. Place the bottom resistor of the feedback network as close as possible to this pin & pin 5

7 SW Output switch. Connect to the inductor, output diode.

8 - No Connect

DAP GND

Dimming & shutdown control input. Logic high enables operation. Duty Cycle from 0 to 100%. Do not allow this pin to float or be greater than VIN + 0.3V.

Signal & Power ground. Connect to pin 2 & pin 6 on top layer. Place 4-6 vias from DAP to bottom layer GND plane.

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Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

LM3410

please contact the National Semiconductor Sales Office/ Distributors for availability and specifications.

SW Voltage -0.5V to 26.5V FB Voltage -0.5V to 3.0V DIM Voltage -0.5V to 7.0V ESD Susceptibility (Note 4) Human Body Model 2kV Junction Temperature (Note 2) 150°C

-0.5V to 7V

Storage Temp. Range -65°C to 150°C Soldering Information Infrared/Convection Reflow (15sec) 220°C

Operating Ratings (Note 1)

(Note 5) 0V to V

DIM

Junction Temperature Range -40°C to 125°C Power Dissipation

(Internal) SOT23-5 400 mW

2.7V to 5.5V

3V to 24V

Electrical Characteristics Limits in standard type are for T

= 25°C only; limits in boldface type apply over the

junction temperature (TJ) 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 at TJ = 25°C, and are provided for reference purposes only. VIN = 5V, unless otherwise indicated under the Conditions column.

Symbol Parameter Conditions Min Typ Max Units

ΔVFB/V

MAX

MIN

DS(ON)

Feedback Voltage 178 190 202 mV

Feedback Voltage Line Regulation

VIN = 2.7V to 5.5V

- 0.06 - %/V

Feedback Input Bias Current - 0.1 1 µA

Switching Frequency

Maximum Duty Cycle

Minimum Duty Cycle

Switch On Resistance

LM3410-X 1200 1600 2000

LM3410-Y 360 525 680

LM3410-X 88 92 -

LM3410-Y 90 95 -

LM3410-X - 5 -

LM3410-Y - 2 -

SOT23-5 and eMSOP-8 - 170 330

LLP-6 190 350

Switch Current Limit 2.1 2.80 - A

kHz

mΩ

SU Start Up Time - 20 - µs

Quiescent Current (switching)

Quiescent Current (shutdown)

UVLO Undervoltage Lockout

DIM_H

DIM

Shutdown Threshold Voltage - - 0.4

Enable Threshold Voltage 1.8 - -

Switch Leakage

Dimming Pin Current Sink/Source - 100 - nA

LM3410-X VFB = 0.25

LM3410-Y VFB = 0.25

All Options V

DIM

= 0V

VIN Rising

VIN Falling

VSW = 24V

- 7.0 11

- 3.4 7

- 80 - nA

- 2.3 2.65

1.7 1.9 -

- 1.0 - µA

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Symbol Parameter Conditions Min Typ Max Units

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but does not guarantee specific performance limits. For guaranteed specifications and conditions, see the Electrical Characteristics.

Note 2: Thermal shutdown will occur if the junction temperature exceeds the maximum junction temperature of the device.

Note 3: Applies for packages soldered directly onto a 3” x 3” PC board with 2oz. copper on 4 layers in still air.

Note 4: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Test method is per JESD22-A114.

Note 5: Do not allow this pin to float or be greater than VIN +0.3V.

Junction to Ambient 0 LFPM Air Flow (Note 3)

Junction to Case (Note 3)

Thermal Shutdown Temperature (Note 2) - 165 - °C

LLP-6 and eMSOP-8 Package - 80 -

SOT23-5 Package - 118 -

LLP-6 and eMSOP-8 Package - 18 -

SOT23-5 Package - 60 -

°C/W

LM3410

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Typical Performance Characteristics All curves taken at V

= 5.0V with configuration in typical

application circuit shown in Application Information section of this datasheet. TJ = 25C, unless otherwise specified.

LM3410

LM3410-X Efficiency vs VIN (R

SET

= 4Ω)

LM3410-X Start-Up Signature

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4 x 3.3V LEDs 500 Hz DIM FREQ D = 50%

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Current Limit vs Temperature

DIM Freq & Duty Cycle vs Avg I-LED

vs Temperature

DSON

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LM3410

Oscillator Frequency vs Temperature - "X"

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VFB vs Temperature

Oscillator Frequency vs Temperature - "Y"

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Simplified Internal Block Diagram

LM3410

FIGURE 1. Simplified Block Diagram

Application Information

THEORY OF OPERATION

The LM3410 is a constant frequency PWM, boost regulator IC. It delivers a minimum of 2.1A peak switch current. The device operates very similar to a voltage regulated boost converter except that it regulates the output current through LEDs. The current magnitude is set with a series resistor. This series resistor multiplied by the LED current creates the feedback voltage (190 mV) which the converter regulates to. The regulator has a preset switching frequency of either 525 kHz or 1.60 MHz. This high frequency allows the LM3410 to operate with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a minimum amount of board space. The LM3410 is internally compensated, so it is simple to use, and requires few external components. The LM3410 uses current-mode control to regulate the LED current. The following operating description of the LM3410 will refer to the Simplified Block Diagram (Figure 1) the simplified schematic (Figure 2), and its associated waveforms (Figure 3). The LM3410 supplies a regulated LED current by switching the internal NMOS control switch at constant frequency and variable duty cycle. A switching cycle

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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 control switch. During this on-time, the SW pin voltage (VSW) decreases to approximately GND, and the inductor current (IL) increases with a linear slope. IL is measured by the current sense amplifier, which generates an output proportional to the switch current. The sensed signal is summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage and V output switch turns off until the next switching cycle begins. During the switch off-time, inductor current discharges through diode D1, which forces the SW pin to swing to the output voltage plus the forward voltage (VD) of the diode. The regulator loop adjusts the duty cycle (D) to maintain a regulated LED current.

. When the PWM comparator output goes high, the

REF

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FIGURE 2. Simplified Boost Topology Schematic

LM3410

Design Guide

SETTING THE LED CURRENT

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FIGURE 4. Setting I

LED

The LED current is set using the following equation:

where R

is connected between the FB pin and GND.

SET

DIM PIN / SHUTDOWN MODE

The average LED current can be controlled using a PWM signal on the DIM pin. The duty cycle can be varied between 0 & 100% to either increase or decrease LED brightness. PWM frequencies in the range of 1 Hz to 25 kHz can be used. For controlling LED currents down to the µA levels, it is best to use a PWM signal frequency between 200-1 kHz. The maximum LED current would be achieved using a 100% duty cycle, i.e. the DIM pin always high.

LED-DRIVE CAPABILITY

When using the LM3410 in the typical application configuration, with LEDs stacked in series between the VOUT and FB pin, the maximum number of LEDs that can be placed in series is dependent on the maximum LED forward voltage (VF

MAX

(VF

x N

MAX

When inserting a value for maximum VF voltage variation over the operating temperature range

) + 190 mV < 24V

LEDs

the LED forward

MAX

should be considered.

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FIGURE 3. Typical Waveforms

CURRENT LIMIT

The LM3410 uses cycle-by-cycle current limiting to protect the internal NMOS switch. It is important to note that this current limit will not protect the output from excessive current during an output short circuit. The input supply is connected to the output by the series connection of an inductor and a diode. If a short circuit is placed on the output, excessive current can damage both the inductor and diode.

THERMAL SHUTDOWN

Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature exceeds 165°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature drops to approximately 150°C.

INDUCTOR SELECTION

The inductor value determines the input ripple current. Lower inductor values decrease the physical size of the inductor, but increase the input ripple current. An increase in the inductor value will decrease the input ripple current.

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LM3410

From the previous equations, the inductor value is then obtained.

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FIGURE 5. Inductor Current

The Duty Cycle (D) for a Boost converter can be approximated by using the ratio of output voltage (V (VIN).

) to input voltage

OUT

Therefore:

Power losses due to the diode (D1) forward voltage drop, the voltage drop across the internal NMOS switch, the voltage drop across the inductor resistance (R losses must be included to calculate a more accurate duty

) and switching

DCR

cycle (See Calculating Efficiency and Junction Tempera- ture for a detailed explanation). A more accurate formula for calculating the conversion ratio is:

Where η equals the efficiency of the LM3410 application. Or:

Therefore:

Where

1/TS = f

One must also ensure that the minimum current limit (2.1A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (Lpk I) in the inductor is calculated by:

= IIN + ΔIL or I

Lpk

= I

OUT

/D' + Δi

When selecting an inductor, make sure that it is capable of supporting the peak input current without saturating. Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating correctly. Because of the speed of the internal current limit, the peak current of the inductor need only be specified for the required maximum input current. For example, if the designed maximum input current is 1.5A and the peak current is 1.75A, then the inductor should be specified with a saturation current limit of >1.75A. There is no need to specify the saturation or peak current of the inductor at the 2.8A typical switch current limit.

Because of the operating frequency of the LM3410, ferrite based inductors 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 provide better operating efficiency. For recommended inductors see Example Circuits.

INPUT CAPACITOR

An input capacitor is necessary to ensure that VIN does not drop excessively during switching transients. The primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and ESL (Equivalent Series Inductance). The recommended input capacitance is 2.2 µF to 22 µF depending on the application. The capacitor manufacturer specifically states the input voltage rating. Make sure to check any recommended deratings and also verify if there is any significant change in capacitance at the operating input voltage and the operating temperature. The ESL of an input capacitor is usually determined by the effective cross sectional area of the current path. At the operating frequencies of the LM3410, certain capacitors may have an ESL so large that the resulting impedance (2πfL) will be higher than that required to provide stable operation. As a result, surface mount capacitors are strongly recommended. Multilayer ceramic capacitors (MLCC) are good choices for both input and output capacitors and have very low ESL. For MLCCs it is recommended to use X7R or X5R dielectrics. Consult capacitor manufacturer datasheet to see how rated capacitance varies over operating conditions.

Inductor ripple in a LED driver circuit can be greater than what would normally be allowed in a voltage regulator Boost & Sepic design. A good design practice is to allow the inductor to produce 20% to 50% ripple of maximum load. The increased ripple shouldn’t be a problem when illuminating LEDs.

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OUTPUT CAPACITOR

The LM3410 operates at frequencies allowing the use of ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple. The output capacitor is selected based upon the desired output ripple and transient response. The initial current of a load transient is provided mainly by the output capacitor. The output impedance will therefore determine the maximum voltage perturbation. The output ripple of the converter is a function

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