MAX1698AEUB+ - MAXIM INTEGRATED PRODUCTS

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General Description

The MAX1698 and MAX1698A are efficient drivers for

white or color LEDs. They are ideal for large LED backlit

displays in PDAs and laptop computers. Numerous

benefits include greater simplicity, lower cost, higher

efficiency, longer bulb life, and greater reliability when

compared to fluorescent (CCFL) and electrolumines-

cent (EL) lamps.

The MAX1698/MAX1698A are switch-mode boost con-

trollers in which LED current, rather than output voltage,

is regulated The devices drive series-connected LEDs

with a controlled current that is measured with a typical-

ly 15Ωsense resistor, not an expensive fractional-ohm

value. LED current control and dimming are accom-

plished with an adjust input (ADJ). For larger light out-

put, multiple LED banks can be connected in parallel

with up to 5W total output power.

The MAX1698A has a higher, and more accurately

specified, CS current limit than the MAX1698 for higher-

power applications of 5W or more. Both devices are

supplied in a space-saving 10-pin µMAX package that

occupies half the space of an 8-pin SO. An evaluation

kit (MAX1698EVKIT) is available to speed designs.

Applications

Battery-Powered Backlight Applications

Backlight for LCD Panels

Notebook PCs

Handy Terminals

PDAs

Features

♦Over 90% Efficiency

♦Lossless, Adjustable LED Brightness

♦Space-Saving 10-Pin µMAX Package

♦Simpler, Lower Cost, More Reliable Compared

to CCFL or EL Backlights

♦Up to 5W Output Power

MAX1698/MAX1698A

High-Efficiency Step-Up

Current Regulator for LEDs

________________________________________________________________ Maxim Integrated Products 1

EXT

PGND

GNDADJ

REF

VCC

MAX1698

MAX1698A

µMAX

TOP VIEW

FBGND

SHDN

VCC

VOUT

COUT

SHDN

GND

ADJ

REF

OFF

PGND

EXT

VCC

VBATT

ILED

RFB RN

MAX1698

MAX1698A

Typical Operating Circuit

19-1621; Rev 2; 2/07

Pin Configuration

Ordering Information

PART T EM P RA N G EPIN -

PA C K A G E

PKG

CODE

MAX1698EUB -40°C to +85°C 10µMAX U10-2

MAX1698AEUB -40°C to +85°C 10µMAX U10-2

MAX1698/MAX1698A

High-Efficiency Step-Up

Current Regulator for LEDs

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VCC = +3.3V, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional

operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to

absolute maximum rating conditions for extended periods may affect device reliability.

VCC, SHDN to GND..................................................-0.3V to +6V

EXT, FB, CS, ADJ, REF to GND..................-0.3V to (VCC + 0.3V)

GND to PGND.....................................................................±0.3V

Continuous Power Dissipation (TA= +70°C)

10-Pin µMAX (derate 5.6mW/°C above +70°C) ............444mW

Operating Temperature Range ...........................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-65°C to +150°C

Lead Temperature Range (soldering, 10s)......................+300°C

FB = GND, ADJ = REF, CS = GND

FB = GND, ADJ = REF

Rising edge

FB = GND

VFB = 0.3V

ADJ = REF

VADJ = 100mV

CONDITIONS

mV30 50 70ADJ Start Threshold

µs10 15 20tON(MAX)

Maximum On Time

µs0.8 1.0 1.2tOFF(MIN)

Minimum Off Time

ARMS

0.8ICS

CS Input Current

0.8 1.5 2.5

CS Trip Current

V2.25 2.4 2.55Undervoltage Lockout

µA260 500ICC

V2.7 5.5VCC

Supply Voltage

Quiescent Supply Current

mV80Undervoltage Hysteresis

285 300 315 mV

18 24 30

VFB

FB Regulation Voltage

UNITSMIN TYP MAXSYMBOLPARAMETER

VFB = 300mV nA-15 15IFB

FB Input Bias Current

V0.03 REFVADJ

ADJ Input Range

ADJ Input Bias Current IADJ ADJ = REF -50 50 nA

Shutdown Supply Current SHDN = GND 0.01 1 µA

REF Output Voltage VREF IREF = 0 1.20 1.25 1.30 V

REF Load Regulation ∆VREF IREF = 0 to 150µA -2 -25 mV

REF Short-Circuit Current REF = GND 0.45 1 mA

REF Power-Supply Rejection

Ratio PSRR VCC = 2.7V to 5.5V +0.3 +2 mV/V

EXT Driver Sink/Source Current VCC = 5V, EXT = 2V 0.2 A

EXT Driver On-Resistance VCC = 5V 8Ω

SHDN Input High Voltage VIH VCC = 2.7V to 5.5V 2 V

SHDN Input Low Voltage VIL VCC = 2.7V to 5.5V 0.8 V

SHDN Input Bias Current ISHDN VCC = 2.7V to 5.5V -1 1 µA

2.0 2.4 2.8

MAX1698

MAX1698A

MAX1698/MAX1698A

High-Efficiency Step-Up

Current Regulator for LEDs

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS

(VCC = +3.3V, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)

CONDITIONS

VVCC

Supply Voltage 2.7 5.5

UNITSMIN MAXSYMBOLPARAMETER

FB = GND 2.0 3.0

Minimum Off Time tOFF(MIN) FB = GND, ADJ = REF 0.6 1.4 µs

Maximum On Time tON(MAX) FB = GND, ADJ = REF, CS = GND 921µs

ADJ Start Threshold 25 75 mV

FB Input Bias Current IFB VFB = 300mV -20 20 nA

ADJ Input Bias Current IADJ ADJ = REF -50 50 nA

Shutdown Supply Current SHDN = GND 1µA

REF Output Voltage VREF IREF = 0 1.17 1.33 V

REF Load Regulation ∆VREF IREF = 0 to 150µA -30 mV

REF Short-Circuit Current REF = GND 1mA

REF Power-Supply Rejection

Ratio PSRR VCC = 2.7V to 5.5V +2 mV/V

EXT On-Resistance VCC = 5V 8Ω

SHDN Input High Voltage VIH VCC = 2.7V to 5.5V 2 V

SHDN Input Low Voltage VIL VCC = 2.7V to 5.5V 0.8 V

SHDN Input Bias Current ISHDN VCC = 2.7V to 5.5V -1 1 µA

Note 1: Specifications to -40°C are guaranteed by design, not production tested.

Rising edge VUndervoltage Lockout 2.20 2.65

VADJ = 100mV

ADJ = REF mV

13 35

VFB

FB Regulation Voltage 280 320

VFB = 0.3V µAICC

Quiescent Supply Current 500

CS Trip Current 0.8 3.0 A

MAX1698A

MAX1698

MAX1698/MAX1698A

High-Efficiency Step-Up

Current Regulator for LEDs

4 _______________________________________________________________________________________

045231 678910

MXA1698-01

VBATT, VCC (V)

OUTPUT CURRENT (mA)

OUTPUT CURRENT vs. VBATT AND VCC

1 CHAIN OF 4 LEDs

vs. VCC

vs. VBATT

0.4

0.2

0.8

0.6

1.0

1.2

0 0.6 0.80.2 0.4 1.0 1.2 1.4

NORMALIZED OUTPUT CURRENT

vs. ADJ VOLTAGE

MXA1698-02

ADJ VOLTAGE (V)

NORMALIZED OUTPUT CURRENT

20.0

20.5

21.5

21.0

22.0

0 468102 1214161820

MXA1698-03

INPUT VOLTAGE (V)

OUTPUT CURRENT (mA)

OUTPUT CURRENT

vs. INPUT VOLTAGE

1 CHAIN OF 4 LEDs

IBATT

IOUT

4 CHAINS OF 4 LEDs

200

100

400

300

600

500

700

900

800

1000

0 2341 567 9810

BATTERY CURRENT

vs. BATTERY VOLTAGE

MXA1698-04

VBATT (V)

IOUT, IBATT (mA)

100

100 400 700 1000 1300 1600 1900 2200

EXT RISE/FALL TIME vs. CEXT

MXA1698-05

CEXT

)

RISE/FALL TIME (ns)

FALL TIME

RISE TIME

100

0426810

EFFICIENCY vs. BATTERY VOLTAGE

MXA1698-06

BATTERY VOLTAGE (V)

EFFICIENCY (%)

IOUT = 80mA

4 CHAINS OF 4 LEDs

100

5253515 45 55 65 75 85

MXA1698-07

OUTPUT CURRENT (mA)

EFFICIENCY (%)

EFFICIENCY vs. OUTPUT CURRENT

VBATT = 5V

4 CHAINS OF 4 LEDs

MXA1698-08

IOUT

50mA/div

5ms/div

SHUTDOWN/SOFT-START TIMING

SHDN

5V/div

IBATT

100mA/div

3.3V

4 CHAINS OF 4 LEDs

IOUT SET TO 80mA,

MXA1698-09

LED AND INDUCTOR CURRENT

WAVEFORMS

INDUCTOR

CURRENT

500mA/div

LED

CURRENT

50mA/div

1µs/div

Typical Operating Characteristics

(Circuit of Figure 2, VCC = VBATT = 3.3V, VADJ = VREF, TA= +25°C, unless otherwise noted.)

MAX1698/MAX1698A

High-Efficiency Step-Up

Current Regulator for LEDs

_______________________________________________________________________________________ 5

NAME FUNCTION

1 VCC IC Supply Voltage Input. Power for internal circuitry. Input range is 2.7V to 5.5V.

2SHDN

Active-Low Shutdown Input. In shutdown, the MOSFET turns off, but a current path still exists between the

input and output. The minimum forward voltage of the LED array must exceed the maximum VBATT to ensure

that the LEDs are off in shutdown.

3REF 1.25V Reference Output. Capable of sourcing 150µA for external loads. This pin is internally compensated.

Do not connect any bypass capacitors at REF.

4ADJ Adjust Input. Allows dynamic adjustment of the output current. FB regulates to 300mV when ADJ = REF.

6FB Feedback Input. Connect to the external LED current-sense feedback resistor.

8PGND Power Ground

9CS FET Current-Sense Input

5, 7 GND Ground

10 EXT Gate Driver Output

Pin Description

UNDERVOLTAGE

LOCKOUT/

SHUTDOWN

TRIG

PGND

EXT

REF

GND

ADJ

SHDN

VCC

MAX tON

MIN tOFF

MAX1698

SOFT-START 1.25V

REFERENCE

Figure 1. Functional Diagram

MAX1698/MAX1698A

High-Efficiency Step-Up

Current Regulator for LEDs

6 _______________________________________________________________________________________

Detailed Description

The MAX1698/MAX1698As' high efficiency and small

size make them ideally suited to drive LEDs. They oper-

ate as a boost DC-DC converter that controls output

current rather than voltage. Losses are minimized by a

low, 300mV current-sense threshold. In the standard

configuration, a feedback resistor, RFB, sets the current

through the primary chain of LEDs. Additional chains of

matching LEDs can be added with an equivalent resis-

tor. In matched LED arrays, the secondary chain cur-

rents closely track the primary chain. An optional zener

diode, D2, prevents overvoltage in the event that one of

the LEDs in the primary chain becomes an open circuit.

The LED brightness can be adjusted dynamically by a

voltage input at ADJ.

Shutdown

In shutdown, the supply current is reduced below 1µA.

EXT goes low in shutdown, shutting off the external N-

channel FET. This leaves a current path between the

input and the LEDs through the boost inductor and

catch diode. The minimum forward voltage of the LED

array must exceed the maximum VBATT to ensure that

the LEDs remain off in shutdown. Typical shutdown tim-

ing characteristics are shown in the Typical Operating

Characteristics.

Soft-Start

The MAX1698/MAX1698A include a soft-start function

that eliminates input current surges at turn-on. They do

this by extending the external FET driver (EXT) mini-

mum off-time during start-up. During the first 512

switching cycles, the minimum off-time is 5µs. It is then

allowed to drop to 2µs for the next 1500 switching

cycles. After that time, the minimum off-time falls to the

1µs value used during normal operation. (See

Shutdown/Soft-Start Timing in the Typical Operating

Characteristics section.)

Design Procedure

Setting the Maximum LED Current

Resistor RFB sets the maximum current in the primary

chain of LEDs:

where ILEDMAX is the maximum LED current.

Adjusting LED Current

RFB sets the maximum LED current. This current can

be reduced proportional to the voltage at the ADJ pin

(see Normalized Output Current vs. ADJ Voltage in the

Typical Operating Characteristics section). Figure 3

RmV

FB LEDMAX

=300

VCC

SHDN

PGND

GND

RFB

OPTIONAL

LEDs

MBR0540

ADJ

REF

EXT

MAX1698

MAX1698A

VCC = 3.3V

10µF

VBATT = 3.3V

10µH

1µF

15Ω15Ω15Ω15Ω

1µF

Figure 2. Typical Operating Circuit

MAX1698/MAX1698A

High-Efficiency Step-Up

Current Regulator for LEDs

_______________________________________________________________________________________ 7

shows the standard method of setting the ADJ voltage.

Use the following equation to determine ILED:

where VADJ is the voltage at ADJ. Note that ADJ volt-

ages below 50mV turn the LEDs off.

Inductor Selection

Choose an inductor with low DC resistance (in the

neighborhood of 100mΩ) to minimize losses. A typical

inductance value for L is 10µH; however, values from

3.3µH to 100µH can also be used. Higher inductor val-

ues reduce the MAX1698’s switching frequency. The

typical operating frequency is given by:

The MAX1698/MAX1698A limit peak inductor current to

1.5A, but also contains a control loop that reduces

inductor current as a function of output power. For a

given output power, the required inductor peak current

rating is approximately set by:

IL(PEAK) = 1.0 ·POUT

where POUT is the output power to all LED banks in

watts and IL(PEAK) is in amperes.

Capacitor Selection

The exact value of output capacitance is not critical.

Typical values for the output capacitor are 0.1µF to

10µF. Larger values help reduce output ripple at the

expense of size and higher cost.

The requirements of the input capacitor depend on the

type of the input voltage source. However, in many

applications, the same capacitor type and value are

used for both the input and output capacitors.

Transistor Selection

The MAX1698/MAX1698A drive an external N-channel

MOSFET. Since the gate drive voltage is derived from

VCC, best performance is achieved with low-threshold

NFETs that specify on-resistance with gate-source volt-

ages (VGS) at the voltage supplied at VCC or less. For

best results, minimize the FET’s RDS(ON). The external

NFET’s maximum drain-to-source voltage (VDS(MAX))

must exceed the output voltage.

Catch Diode (D1) Selection

The MAX1698/MAX1698As' high-switching frequency

demands a high-speed rectifier. Schottky diodes are

recommended for most applications, due to their fast

recovery time and low forward-voltage drop. Ensure

that the diode’s average and peak current ratings

exceed the average output current and peak inductor

current, respectively. In addition, the diode’s reverse

breakdown voltage must exceed VOUT. For output volt-

ages exceeding 40V, high-speed silicon rectifiers may

be required for their higher breakdown voltages.

Zener Diode

For applications requiring open-circuit protection if one

of the LEDs in the primary chain opens, add a zener

diode as shown in Figure 2. The zener diode protects

the MOSFET and output capacitor if the current feed-

back signal is lost. The zener voltage should exceed

the maximum forward voltage of the LED network by at

least 2V.

Applications Information

PCB Layout

Due to fast switching waveforms and high-current

paths, careful PCB layout is required. Protoboards and

wire-wrap boards should not be used for evaluation. An

EV kit (MAX1698EVKIT) is available to aid most

designs.

When laying out a board, minimize trace lengths to CS,

the inductor, diode, input capacitor, and output capaci-

tor. Keep traces short, direct, and wide. Keep noisy

traces, such as the inductor’s traces, away from FB.

VCC’s bypass capacitor should be placed as close to

the IC as possible.

Refer to the MAX1698 EV kit for an example of proper

layout.

LED ADJ

=⋅416

GND

REF

ADJ

MAX1698

MAX1698A

500k

Figure 3. Adjusting LED Current

TRANSISTOR COUNT: 2180

Chip Information

BATT

=⋅067

MAX1698/MAX1698A

High-Efficiency Step-Up

Current Regulator for LEDs

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are

implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

8_____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

10LUMAX.EPS

PACKAGE OUTLINE, 10L uMAX/uSOP

21-0061 REV.DOCUMENT CONTROL NO.APPROVAL

PROPRIETARY INFORMATION

TITLE:

TOP VIEW

FRONT VIEW

0.498 REF

0.0196 REF

S6°

SIDE VIEW

BOTTOM VIEW

0° 0° 6°

0.037 REF

0.0078

MAX

0.006

0.043

0.118

0.120

0.199

0.0275

0.118

0.0106

0.120

0.0197 BSC

INCHES

0.0035

0.007

0.187

0.0157

0.114

DIM

0.116

0.114

0.116

0.002

MIN

-A

0.940 REF

0.500 BSC

0.090

0.177

4.75

2.89

0.40

0.200

0.270

5.05

0.70

3.00

MILLIMETERS

0.05

2.89

2.95

MIN

3.00

3.05

0.15

3.05

MAX

1.10

0.6±0.1

Ø0.50±0.1

4X S

A2 A

E1 L

GAGE PLANE

A2 0.030 0.037 0.75 0.95

Note: The MAX1698 does not have an exposed pad.

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information

go to www.maxim-ic.com/packages.)

Revision History

Pages changed at Rev 2: 1, 2, 8