LM2759SD_技术文档

June 10, 2010

LM2759

1A Switched Capacitor Flash LED Driver with I²C

Compatible Interface

General Description

Features

LM2759 is an integrated low-noise, high-current switched ca-

pacitor DC/DC converter with a regulated current source. The

device requires only four small ceramic capacitors making the

total solution area less than 22 mm²and the height less than

1 mm. The LM2759 is capable of driving loads up to 1A from

a single-cell Li-Ion battery. Maximum efficiency is achieved

over the input voltage range by actively selecting the proper

gain based on the LED forward voltage and current require-

ments.

Up to 1A Output Current

■

Solution Area < 22 mm²

No Inductor Required

90% Peak Efficiency

Adaptive 1x, 1.5x and 2x Gains for Maximum Efficiency

Load Disconnect in Shutdown

■

Accurate Input Current Control During Gain Transitions

Flash Time-Out

The LED current can be programmed up to 1A via an I²C-

compatible interface, along with eight selectable Flash Time-

Out durations. One high-current Flash LED can be driven

either in a high-power Flash mode or a low-power Torch

mode. The Strobe pin allows the flash to be toggled via a

Flash enable signal from a camera module. The TX input pin

limits the Flash LED current to the Torch current level during

a RF PA pulse, to reduce high loads on the battery. Internal

soft-start circuitry limits the amount of inrush current during

start-up.

TX Input Pin Ensures Synchronization with RF Power

Amplifier Pulse

Torch, Flash, and Indicator Modes

External Flash Enable via Strobe Input Pin

Strobe Input Disable via I²C

■

Programmable Flash Pulse Duration, and Torch and Flash

Currents via I²C-Compatible Interface

1MHz Constant Frequency Operation

■

Low Profile 12–Pin LLP (3mm x 3mm x 0.8mm)

LM2759 is offered in a small 12-pin thermally enhanced LLP

package.

Applications

Camera Flash in Cellular Phones

■

Typical Application Circuit

30069322

30069301

300693

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Connection Diagram

12-Pin LLP Package

3mm x 3mm x 0.8mm

NS Package Number SDF12A

30069302

Note 1: The actual physical placement of the package marking will vary from part to part. The package marking placeholder "XXXXX" is a code for the die

traceability. "YYYYY" identifies the device (part number, voltage option, etc.). See the Order Information table below for the device ID codes.

Pin Descriptions

10

3

Name

V_IN

Description

Input voltage connection.

V_OUT

C1−

Charge pump regulated output.

12

11

2

C1+

Flying capacitor connections.

C2+

1

C2−

4

GND

I_SINK

SDA

Strobe

Ground connection.

6

Regulated current sink input.

Serial data I/O pin.

8

7

Manual flash enable pin. Flash will remain on for the duration that the Strobe pin is held high

or when the Flash Timeout occurs, whichever comes first.

5

9

TX

Transmission pulse Flash interrupt pin. High = RF PA pulse active, LED current reduced to

Torch level, Low = RF PA pulse off, LED at full programmed current level.

SCL

Serial clock pin.

Ordering Information

Order Number

LM2759SD

Package Description

Package Marking

Supplied as Tape and Reel (Units)

1000 units, tape and reel

ZXYTT

L2759

No-Pullback

LLP-12

LM2759SDX

4500 units, tape and reel

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2

Absolute Maximum Ratings (Note 2, Note

3)

Operating Ratings (Note 2, Note 3)

Input Voltage Range

2.7V to 5.5V

LED Voltage Range

Junction Temperature Range (T_J)

2.0V to 4.0V

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

-30°C to +125°C

Ambient Temperature Range (T_A)

(Note 7)

-30°C to +85°C

V_INpin: Voltage to GND

-0.3V to 6.0V

-0.3V to (V_IN+ 0.3V)

w/ 6.0V max

Strobe, TX, SDA, SCL, I_SINKpins:

Voltage to GND

Continuous Power Dissipation

Thermal Information

Junction-to-Ambient Thermal Resistance

(Note 4)

Junction Temperature (T_J-MAX

Internally Limited

150°C

(θ_JA), Leadless Leadframe Package (Note 8)

36.7°C/W

)

Storage Temperature Range

Maximum Lead Temp. (Soldering)

ESD Rating(Note 6)

-65°C to 150°C

(Note 5)

Human Body Model

2.5KV

ESD Caution Notice

National Semiconductor recommends that all integrated circuits be handled with appropriate ESD precautions. Failure to observe

proper ESD handling techniques can result in damage to the device.

Electrical Characteristics (Note 3, Note 9)

Limits in standard typeface are for T_J= 25°C. Limits in boldface type apply over the full operating junction temperature range (-30°

C ≤ T_J≤ +125 °C). Unless otherwise noted, specifications apply to the LM2759 Typical Application Circuit (pg.1) with V_IN= 3.6V,

V_TX= 0V, V_STROBE= 0V, C_IN= C₁= C₂= 2.2 µF, C_OUT= 4.7 µF. (Note 10)

Symbol

I_LED

Parameter

Conditions

Min

Typ

Max

Units

LED Current Sink

Accuracy

Flash Mode

ADDR xB0 = 0x02

198

−10%

220

242

+10%

mA

I_FLASH

V_GDX

V_OUT

Max Flash Output Current Flash Mode

ADDR xB0 = 0x0F

I_LED= 500mA

1

A

Gain Transition Voltage

Threshold on I_SINK

350

mV

(V_ISINKfalling)

Output Voltage

1x Mode, I_OUT= 0 mA (V_IN>V_OUT)(Note 12)

1.5x Mode, I_OUT= 0 mA

2x Mode, I_OUT= 0 mA

4.7

4.9

5.4

V

5.1

R_OUT

x1 Mode Output

Impedance

I_OUT= 200mA, V_IN= 3.3V

0.33

Ω

1.5x Mode Output

Impedance

I_OUT= 500mA, V_IN= 3.3V

1.9

2.25

1

x2 Mode Output

Impedance

F_SW

Switching Frequency

MHz

0.7

1.3

2.7V ≤ V_IN≤ 5.5V

Pins: TX, Strobe

V_IH

V_IL

Input Logic High

Input Logic Low

1.26

V

Pins: TX, Strobe

0.7

0.9

4.0

7.0

9.7

I_OUT= 0 mA, 1x Mode

I_OUT= 0 mA, 1.5x Mode

I_OUT= 0 mA, 2x Mode

Device Disabled

0.6

3.4

5.9

5.8

I_Q

Quiescent Current

Shutdown Current

mA

µA

I_SD

2.7V ≤ V_IN≤ 5.5V

3

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Limits in standard typeface are for T_J= 25°C. Limits in boldface type apply over the full operating junction temperature range (-30°

C ≤ T_J≤ +125 °C). Unless otherwise noted, specifications apply to the LM2759 Typical Application Circuit (pg.1) with V_IN= 3.6V,

V_TX= 0V, V_STROBE= 0V, C_IN= C₁= C₂= 2.2 µF, C_OUT= 4.7 µF. (Note 10)

Symbol

Parameter

Conditions

Min

Typ

Max

Units

I²C Compatible Interface Voltage Specifications (SCL, SDA).

V_IL

Input Logic Low “0”

Input Logic High “1'

Output Logic Low “0”

0.72

V

2.7V ≤ V_IN≤ 5.5V

V_IH

V_OL

1.25

2.7V ≤ V_IN≤ 5.5V

I_LOAD= 3 mA

300

mV

I²C Compatible Interface Timing Voltage Specifications (SCL, SDA) (Note 11)

t₁

t₂

SCL (Clock Period)

2.5

µs

ns

Data in Setup Time to SCL

High

100

t₃

t₄

t₅

Data Out Stable After SCL

Low

0

ns

SDA Low Setup Time to

SCL Low (Start)

100

SDA High Hold Time After

SCL High (Stop)

Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation

of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions,

see the Electrical Characteristics tables.

Note 3: All voltages are with respect to the potential to the GND pin.

Note 4: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at T_J=150°C (typ.) and disengages at T_J

=

120°C (typ.).

Note 5: For detailed soldering specifications and information, please refer to National Semiconductor Application Note AN-1187.

Note 6: The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. (MIL-STD-883 3015.7)

Note 7: 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 (T_A-MAX) is dependent on the maximum operation junction temperature (T_J-MAX-OP= 125ºC), the maximum power

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

following equation: T_A-MAX= T_J-MAX-OP- (θ_JA× P_D-MAX).

Note 8: Junction-to-ambient thermal resistance (θ_JA) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the

JEDEC standard JESD51-7. The test board is a 4–layer FR-4 board measuring 102 mm x 76 mm x 1.6 mm with a 2x1 array of thermal vias. The ground plane

on the board is 50 mm x 50 mm. Thickness of copper layers are 53µm/35µm/35µm/53µm (1.5oz/1oz/1oz/1.5oz). Ambient temperature in simulation is 22°C, still

air. Power dissipation is 1W.

The value of θ_JAof this product in the LLP package could fall in a range as wide as 30ºC/W to 150ºC/W (if not wider), depending on PWB material, layout, and

environmental conditions. In applications where high maximum power dissipation exists (high V_IN, high I_OUT), special care must be paid to thermal dissipation

issues. For more information on these topics, please refer to Application Note 1187: Leadless Leadframe Package (LLP) and the Power Efficiency and Power

Dissipation section of this datasheet.

Note 9: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical (Typ) numbers are not guaranteed, but do represent the most likely

norm. Unless otherwise specified, conditions for Typ specifications are: V_IN= 3.6V and T_A= 25°C.

Note 10: C_IN, C_OUT, C₁, C₂: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics.

Note 11: SCL and SDA should be glitch-free in order for proper brightness control to be realized.

Note 12: For input voltage below the regulation target during the gain of 1x, the output voltage will typically be equal to the input voltage.

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4

Block Diagram

30069319

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Typical Performance Characteristics

Unless otherwise specified: T_A= 25°C, V_IN= 3.6V, C_IN= C₁= C₂= 2.2µF, C_OUT= 4.7µF. Capacitors are low-ESR multi-layer

ceramic capacitors (MLCC's). Luxeon PWF3 Flash LED.

Efficiency vs V_IN

Input Current vs V_IN

30069326

30069327

Quiescent Current vs V_IN, Gain = 1X

Quiescent Current vs V_IN, Gain = 2X

30069328

30069329

I_LEDvs V_ISINK

Shutdown Current vs V_IN

30069330

30069320

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6

Oscillator Frequency vs V_IN

Torch Code Levels

30069321

30069331

Flash Code Levels

Shutdown to Torch Mode, 100mA

30069333

CH1: SDA; Scale: 2V/Div, DC Coupled

CH2: V_OUT; Scale: 2V/Div, DC Coupled

CH3: I_IN; Scale: 100mA/Div, DC Coupled

CH4: I_LED; Scale: 100mA/Div, DC Coupled

Time scale: 400µs/Div

30069332

Shutdown to Flash Mode, 1A

Torch to Flash Mode, 100mA to 1A

30069334

30069335

CH1: SDA; Scale: 2V/Div, DC Coupled

CH2: V_OUT; Scale: 2V/Div, DC Coupled

CH3: I_IN; Scale: 1A/Div, DC Coupled

CH4: I_LED; Scale: 1A/Div, DC Coupled

Time scale: 1ms/Div

CH1: SDA; Scale: 2V/Div, DC Coupled

CH2: V_OUT; Scale: 2V/Div, DC Coupled

CH3: I_IN; Scale: 1A/Div, DC Coupled

CH4: I_LED; Scale: 1A/Div, DC Coupled

Time scale: 1ms/Div

7

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Flash Timeout, Timeout Code (x03) = 325ms

30069336

Torch Level (x0F) = 180mA, Flash Level (x05) = 410mA

CH1(bottom): I_IN; Scale: 200mA/Div, DC Coupled

CH2(middle): SDA; Scale: 2V/Div, DC Coupled

CH3(top): V_OUT; Scale: 2V/Div, DC Coupled

Time scale: 100ms/Div

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8

TX STROBE

FUNCTION

Current I²C programmed state (Off, Torch,

Flash, Indicator)

Application Information

0

CIRCUIT DESCRIPTION

The LM2759 is an adaptive CMOS charge pump with gains

of 1x, 1.5x, and 2x, optimized for driving Flash LEDs in cam-

era phones and other portable applications. It provides a

constant current of up to 1A (typ.) for Flash mode and 180 mA

(typ.) for Torch mode.

1

0

Current I²C programmed state (Off, Torch,

Flash, Indicator). If Flash is enabled via I²C

and TX is logic High, the LED current will be

at the programmed Torch level.

The LM2759 has selectable modes including Flash, Torch,

Indicator and Shutdown. Flash mode for the LM2759 can also

be enabled via the Strobe input pin. The LED is driven from

V_OUTand connected to the current sink. The LED drive current

and operating modes are programmed via an I²C compatible

interface. The LM2759 adaptively selects the next highest

gain mode when needed to maintain the programmed LED

current level.

0

1

Flash Mode (Total LED "ON" Duration limited

by Flash Timeout)

Torch Mode (Total LED "ON" Duration

limited by Flash Timeout)

I²C COMPATIBLE INTERFACE

START AND STOP CONDITIONS

To prevent a high battery load condition during a simultane-

ous RF PA transmission and Flash event, LM2759 has a

Flash interrupt pin (TX) to reduce the LED current to the pro-

grammed Torch current level for the duration of the RF PA

transmission pulse.

START and STOP conditions classify the beginning and the

end of the I²C session. A START condition is defined as SDA

signal transitioning from HIGH to LOW while SCL line is

HIGH. A STOP condition is defined as the SDA transitioning

from LOW to HIGH while SCL is HIGH. The I²C master always

generates START and STOP conditions. The I²C bus is con-

sidered to be busy after a START condition and free after a

STOP condition. During data transmission, the I²C master

can generate repeated START conditions. First START and

repeated START conditions are equivalent, function-wise.

CHARGE PUMP AND GAIN TRANSITIONS

The input to the 1x, 1.5x, 2x charge pump is connected to the

V_INpin, and the loosely regulated output of the charge pump

is connected to the V_OUTpin. In 1x mode, as long as the input

voltage is less than 4.7V (typ.), the output voltage is approx-

imately equal to the input voltage. When the input voltage is

over 4.7V (typ.) the output voltage is regulated to 4.7V (typ.).

In 1.5x mode, the output voltage is regulated to 4.7V (typ.)

over entire input voltage range. For the gain of 2x, the output

voltage is regulated to 5.1V (typ.). When under load, the volt-

age at V_OUTcan be less than the target regulation voltage

while the charge pump is still in closed loop operation. This is

due to the load regulation topology of the LM2759.

30069311

The charge pump’s gain is selected according to the head-

room voltage across the current sink of LM2759. When the

headroom voltage V_GDX(at the LED cathode) drops below

350 mV (typ.) the charge pump gain transitions to the next

available higher gain mode. Once the charge pump transi-

tions to a higher gain, it will remain at that gain for as long as

the device remains enabled. Shutting down and then re-en-

abling the device resets the gain mode to the minimum gain

required to maintain the load.

FIGURE 1. Start and Stop Conditions

DATA VALIDITY

The data on SDA line must be stable during the HIGH period

of the clock signal (SCL). In other words, state of the data line

can only be changed when SCL is LOW.

SOFT START

The LM2759 contains internal soft-start circuitry to limit inrush

currents when the part is enabled. Soft start is implemented

internally with a controlled turn-on of the internal voltage ref-

erence.

CURRENT LIMIT PROTECTION

The LM2759 charge pump contains current limit protection

circuitry that protects the device during V_OUTfault conditions

where excessive current is drawn. Output current is limited to

1.4A typically.

30069325

FIGURE 2. Data Validity Diagram

A pull-up resistor between the controller's VIO line and SDA

must be greater than [ (VIO-V_OL) / 3.5mA] to meet the V_OL

requirement on SDA. Using a larger pull-up resistor results in

lower switching current with slower edges, while using a

smaller pull-up results in higher switching currents with faster

edges.

LOGIC CONTROL PINS

LM2759 has two asynchronous logic pins, Strobe and TX.

These logic inputs function according to the table below:

TRANSFERING DATA

Every byte put on the SDA line must be eight bits long, with

the most significant bit (MSB) transferred first. Each byte of

9

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data has to be followed by an acknowledge bit. The acknowl-

edge related clock pulse is generated by the master. The

master releases the SDA line (HIGH) during the acknowledge

clock pulse. The LM2759 pulls down the SDA line during the

9th clock pulse, signifying an acknowledge. The LM2759 gen-

erates an acknowledge after each byte is received.

bit which is a data direction bit (R/W). The LM2759 address

is 53h. For the eighth bit, a “0” indicates a WRITE and a “1”

indicates a READ. The second byte selects the register to

which the data will be written. The third byte contains data to

write to the selected register.

After the START condition, the I²C master sends a chip ad-

dress. This address is seven bits long followed by an eighth

30069312

FIGURE 3. Write Cycle

w = write (SDA = "0")

r = read (SDA = "1")

ack = acknowledge (SDA pulled down by either master or slave)

id = chip address, 53h for LM2759

I²C COMPATIBLE CHIP ADDRESS

Internal Hex

Address

Power On Value

(lowest 4 bits)

Register

The chip address for LM2759 is 1010011, or 53h.

General Purpose

Register

10h

0000

1010

0111

1011

Flash Current

Register

B0h

A0h

C0h

Torch Current

Register

30069309

Flash Timeout

Duration Register

INTERNAL REGISTERS

GENERAL PURPOSE REGISTER AND STROBE INHIBIT

FUNCTION

The general purpose register (x10) is used set the mode of

operation for the LM2759. The selectable operating modes

using the lower 4 bits in the general purpose register are listed

in the table below.

The Strobe Input Pin can be disabled via I²C to ignore external

signals into this pin when desired. This function is implement-

ed through bit 3 of the General Purpose Register (See table

below). In the default state, input signals on the Strobe Input

are enabled. (Bit3 = “0”, inputs into the Strobe Pin are not

inhibited).

30069308

30069305

30069306

30069307

General Purpose Register (Reg x10)

Bit3 Bit2 Bit1 Bit0

Mode

Shutdown

X

1

X

0

X

0

1

0

X

0

1

X

Torch

X

1

Flash

Indicator (Lowest Torch Level)

Inhibit Inputs into the Strobe Pin

X

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10

SETTING LED CURRENT

FLASH TIME-OUT FEATURE

The current through the LED is set by programming the ap-

propriate register with the desired current level code for Flash

and Torch. The time that Flash mode is active is dependent

on the lesser of the duration that it is set to "ON" (via I²C or

the Strobe pin), or the duration of the Flash Timeout. Use the

tables below to select the desired current level.

Time-out Protection Circuitry disables the current sink when

either the Strobe pin is held at logic high or the Flash mode is

enabled via the I²C compatible interface longer than the pro-

grammed timeout duration. This prevents the device from

self-heating due to the high power dissipation during Flash

conditions. During the time-out condition, voltage will still be

present on V_OUTbut the current sink will be shut off, resulting

in no current through the Flash LED. When the device goes

into a time-out condition, disabling and then re-enabling the

device will reset the time-out. Use the table below to set the

desired Flash timeout duration.

Using the part in conditions where the junction temperature

might rise above the rated maximum requires that the oper-

ating ranges and/or conditions be de-rated. The printed circuit

board also must be carefully laid out to account for high ther-

mal dissipation in the part.

Flash Timeout Duration (Reg xC0)

Flash Current Table (Reg xB0)

CODE (Hex)

TIME (ms)

60

CODE (Hex)

FLASH CURRENT (mA)

00

01

02

03

04

05

06

07

00

01

02

03

04

05

06

07

08

09

0A

0B

0C

0D

0E

0F

80

150

220

280

350

410

470

530

590

650

710

770

830

890

950

1010

125

250

375

500

625

750

1100

CAPACITOR SELECTION

The LM2759 requires 4 external capacitors for proper opera-

tion. Surface-mount multi-layer ceramic capacitors are rec-

ommended. These capacitors are small, inexpensive and

have very low equivalent series resistance (ESR <20 mΩ

typ.). Tantalum capacitors, OS-CON capacitors, and alu-

minum electrolytic capacitors are not recommended for use

with the LM2759 due to their high ESR, as compared to ce-

ramic capacitors. For most applications, ceramic capacitors

with X7R or X5R temperature characteristic are preferred for

use with the LM2759. These capacitors have tight capaci-

tance tolerance (as good as ±10%) and hold their value over

temperature (X7R: ±15% over -55°C to 125°C; X5R: ±15%

over -55°C to 85°C). Capacitors with Y5V or Z5U temperature

characteristic are generally not recommended for use with the

LM2759. Capacitors with these temperature characteristics

typically have wide capacitance tolerance (+80%, -20%) and

vary significantly over temperature (Y5V: +22%, -82% over

-30°C to +85°C range; Z5U: +22%, -56% over +10°C to +85°

C range). Under some conditions, a nominal 1 μF Y5V or Z5U

capacitor could have a capacitance of only 0.1 μF. Such detri-

mental deviation is likely to cause Y5V and Z5U capacitors to

fail to meet the minimum capacitance requirements of the

LM2759. The voltage rating of the output capacitor should be

6.3V or more. For example, a 6.3V 0603 4.7 μF output ca-

pacitor (TDK C1608X5R0J475) is acceptable for use with the

LM2759, as long as the capacitance on the output does not

fall below a minimum of 3μF in the intended application. All

other capacitors should have a voltage rating at or above the

maximum input voltage of the application and should have a

minimum capacitance of 1 μF.

Torch Current Table (Reg xA0)

CODE (Hex)

TORCH CURRENT (mA)

00

01

02

03

04

05

06

07

08

09

0A

0B

0C

0D

0E

0F

15

30

40

50

65

80

90

100

110

120

130

140

150

160

170

180

11

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Suggested Capacitors and Suppliers

MFG Part No.

4.7 µF for C_OUT

Type

MFG

Voltage Rating

Case Size Inch (mm)

C1608X5R0J475

JMK107BJ475

Ceramic X5R

TDK

6.3V

0603 (1608)

Taiyo-Yuden

2.2 µF for C1, C2, C_IN

C1608X5R0J225

JMK107BJ225

Ceramic X5R

TDK

6.3V

0603 (1608)

Taiyo-Yuden

POWER EFFICIENCY

The junction temperature rating takes precedence over the

ambient temperature rating. The LM2759 may be operated

outside the ambient temperature rating, so long as the junc-

tion temperature of the device does not exceed the maximum

operating rating of 105°C. The maximum ambient tempera-

ture rating must be derated in applications where high power

dissipation and/or poor thermal resistance causes the junc-

tion temperature to exceed 105°C.

Efficiency of LED drivers is commonly taken to be the ratio of

power consumed by the LED (P_LED) to the power drawn at the

input of the part (P_IN). With a 1x, 1.5x, 2x charge pump, the

input current is equal to the charge pump gain times the output

current (total LED current). The efficiency of the LM2759 can

be predicted as follows:

P_LED= V_LED× I_LED

P_IN= V_IN× I_IN

MAXIMUM OUTPUT CURRENT

The maximum LED current that can be used for a particular

application depends on the rated forward voltage of the LED

used, the input voltage range of the application, and the Gain

mode of the LM2759’s charge pump. The following equation

can be used to approximate the relationship between the

maximum LED current, the LED forward voltage, the mini-

mum input voltage, and the charge pump gain:

P_IN= V_IN× (Gain × I_LED+ I_Q)

E = (P_LED÷ P_IN)

For a simple approximation, the current consumed by internal

circuitry (I_Q) can be neglected, and the resulting efficiency will

become:

E = V_LED÷ (V_IN× Gain)

(V_{IN_MIN}x Gain) > (V_F+ V_HR) + (I_LEDx R_{OUT_GAIN}

)

Neglecting I_Qwill result in a slightly higher efficiency predic-

tion, but this impact will be negligible due to the value of I_Q

being very low compared to the typical Torch and Flash cur-

rent levels (100mA - 1A). It is also worth noting that efficiency

as defined here is in part dependent on LED voltage. Variation

in LED voltage does not affect power consumed by the circuit

and typically does not relate to the brightness of the LED. For

an advanced analysis, it is recommended that power con-

sumed by the circuit (V_INx I_IN) be evaluated rather than power

efficiency.

V_HRor the voltage required across the current sink to remain

in regulation can be approximated by (I_LEDx K_HR), where

K_HRis 0.8 mV/mA (typ). R_{OUT_GAIN}is the output impedance of

the charge pump according to its gain mode. When using the

equation above, keep in mind that the (V_F+ V_HR) portion of

the equation can not be greater than the nominal output reg-

ulation voltage for a particular gain. In other words, when

making calculations for an application where the term (V_F+

V_HR) is higher than a particular gain’s regulation voltage, the

next higher gain level must be used for the calculation.

THERMAL PROTECTION

Example: V_F= 4V @ 1A, Charge Pump in the Gain of 2x with

a R_OUTof 2.25Ω (typ.)

V_{IN_MIN}> [(4V + 0.8V) + (1A x 2.25Ω) ] ÷ 2

V_{IN_MIN}> 3.53V (typ.)

Internal thermal protection circuitry disables the LM2759

when the junction temperature exceeds 150°C (typ.). This

feature protects the device from being damaged by high die

temperatures that might otherwise result from excessive pow-

er dissipation. The device will recover and operate normally

when the junction temperature falls below 120°C (typ.). It is

important that the board layout provide good thermal conduc-

tion to keep the junction temperature within the specified

operating ratings.

The maximum power dissipation in the LM2759 must also be

taken into account when selecting the conditions for an ap-

plication, such that the junction temperature of the device

never exceeds its rated maximum. The input voltage range,

operating temperature range, and/or current level of the ap-

plication may have to be adjusted to keep the LM2759 within

normal operating ratings.

POWER DISSIPATION

The power dissipation (P_DISSIPATION) and junction temperature

(T_J) can be approximated with the equations below. P_INis the

power generated by the 1x, 1.5x, 2x charge pump, P_LEDis the

power consumed by the LED, T_Ais the ambient temperature,

and θ_JAis the junction-to-ambient thermal resistance for the

12 pin LLP package. V_INis the input voltage to the LM2759,

V_LEDis the nominal LED forward voltage, and I_LEDis the pro-

grammed LED current.

BOARD LAYOUT CONSIDERATIONS

PC board layout is an important part of DC-DC converter de-

sign. Poor board layout can disrupt the performance of a DC-

DC converter and surrounding circuitry by contributing to EMI,

ground bounce, and resistive voltage loss in the traces. These

can send erroneous signals to the DC-DC converter IC, re-

sulting in poor regulation or instability. Poor layout can also

result in re-flow problems leading to poor solder joints be-

tween the LLP package and board pads. Poor solder joints

can result in erratic or degraded performance.

P_DISSIPATION= P_IN- P_LED

= (Gain × V_IN× I_LED) − (V_LED× I_LED

T_J= T_A+ (P_DISSIPATION× θ_JA

)

www.national.com

12

Physical Dimensions inches (millimeters) unless otherwise noted

12–Pin LLP Package

NS Package Number SDF12A

X1 = X2 = 3.0mm

X3 = 0.8mm

13

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Notes

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Clock and Timing

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Reference Designs

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Voltage References

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Serial Digital Interface (SDI) www.national.com/sdi

Temperature Sensors

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PowerWise® Design

University

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型号：	LM2759SD
厂家：	National Semiconductor
描述：	1A Switched Capacitor Flash LED Driver with I2C Compatible Interface 1A开关电容闪光灯LED驱动器带I2C兼容接口驱动器开关接口集成电路闪光灯
文件：	总14页 (文件大小：1304K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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