MAXIM MAX1698, MAX1698A Technical data

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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 electrolumines­cent (EL) lamps.

The MAX1698/MAX1698A are switch-mode boost con­trollers 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 typical­ly 15Ω sense resistor, not an expensive fractional-ohm
value. LED current control and dimming are accom­plished with an adjust input (ADJ). For larger light out­put, 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 higher­power 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

Typical Operating Circuit

19-1621; Rev 2; 2/07

Pin Configuration

Ordering Information

PART T EM P RA N G E

MAX1698EUB -40°C to +85°C

MAX1698AEUB -40°C to +85°C

PIN ­PA C K A G E

10µMAX

10µMAX

PKG

CODE

U10-2

U10-2

TOP VIEW

V

SHDN

REF

1

CC

2

MAX1698

3

MAX1698A

4

5

µMAX

10

EXT

9

CS

8

PGND

7

GNDADJ

6

FBGND

V

CC

SHDN

REF

ADJ

GND

L

MAX1698

MAX1698A

EXT

CS

PGND

FB

V

BATT

V

CC

OFF

ON

D1

I

LED

R

FB

R

N

V

OUT

C

OUT

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 (V

CC

+ 0.3V)

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

Continuous Power Dissipation (T

A

= +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

V

ADJ

= 100mV

CONDITIONS

mV30 50 70ADJ Start Threshold

µs10 15 20t

ON(MAX)

Maximum On Time

µs0.8 1.0 1.2t

OFF(MIN)

Minimum Off Time

A

RMS

0.8I

CS

CS Input Current

A

0.8 1.5 2.5

CS Trip Current

V2.25 2.4 2.55Undervoltage Lockout

µA260 500I

CC

V2.7 5.5V

CC

Supply Voltage

Quiescent Supply Current

mV80Undervoltage Hysteresis

285 300 315

mV

18 24 30

V

FB

FB Regulation Voltage

UNITSMIN TYP MAXSYMBOLPARAMETER

VFB= 300mV nA-15 15I

FB

FB Input Bias Current

V0.03 REFV

ADJ

ADJ Input Range

ADJ Input Bias Current I

ADJ

ADJ = REF -50 50 nA

Shutdown Supply Current

SHDN = GND

0.01 1 µA

REF Output Voltage V

REFIREF

= 0 1.20 1.25 1.30 V

REF Load Regulation ∆V

REFIREF

= 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 8 Ω
SHDN Input High Voltage

V

IH

VCC= 2.7V to 5.5V 2 V

SHDN Input Low Voltage

V

IL

VCC= 2.7V to 5.5V 0.8 V

SHDN Input Bias Current

I

SHDN

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

VV

CC

Supply Voltage 2.7 5.5

UNITSMIN MAXSYMBOLPARAMETER

FB = GND

2.0 3.0

Minimum Off Time t

OFF(MIN)

FB = GND, ADJ = REF 0.6 1.4 µs

Maximum On Time t

ON(MAX)

FB = GND, ADJ = REF, CS = GND 921µs

ADJ Start Threshold 25 75 mV

FB Input Bias Current I

FB

VFB= 300mV -20 20 nA

ADJ Input Bias Current I

ADJ

ADJ = REF -50 50 nA

Shutdown Supply Current

SHDN = GND

1 µA

REF Output Voltage V

REFIREF

= 0 1.17 1.33 V

REF Load Regulation ∆V

REFIREF

= 0 to 150µA -30 mV

REF Short-Circuit Current REF = GND 1 mA

REF Power-Supply Rejection
Ratio

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

EXT On-Resistance VCC= 5V 8 Ω
SHDN Input High Voltage

V

IH

VCC= 2.7V to 5.5V 2 V

SHDN Input Low Voltage

V

IL

VCC= 2.7V to 5.5V 0.8 V

SHDN Input Bias Current

I

SHDN

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

V

ADJ

= 100mV

ADJ = REF

mV

13 35

V

FB

FB Regulation Voltage

280 320

VFB= 0.3V µAI

CC

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 _______________________________________________________________________________________

(pF)

Typical Operating Characteristics

(Circuit of Figure 2, VCC= V

BATT

= 3.3V, V

ADJ

= V

REF

, TA= +25°C, unless otherwise noted.)

OUTPUT CURRENT vs. V

25

vs. V

BATT

20

15

10

OUTPUT CURRENT (mA)

5

0

045231 678910

vs. V

CC

V

, VCC (V)

BATT

AND V

BATT

1 CHAIN OF 4 LEDs

CC

1.2

MXA1698-01

1.0

0.8

0.6

0.4

NORMALIZED OUTPUT CURRENT

0.2

0

0 0.6 0.80.2 0.4 1.0 1.2 1.4

BATTERY CURRENT

1000

900

800

700

600

(mA)

500

BATT

I

400

OUT,

I

300

200

100

0

100

90

80

70

60

50

40

EFFICIENCY (%)

30

20

10

0

vs. BATTERY VOLTAGE

4 CHAINS OF 4 LEDs

MXA1698-04

I

BATT

I

OUT

0 2341 567 9810

V

(V)

BATT

EFFICIENCY vs. OUTPUT CURRENT

MXA1698-07

V

= 5V

BATT

4 CHAINS OF 4 LEDs

5253515 45 55 65 75 85

OUTPUT CURRENT (mA)

100

90

80

70

60

50

40

RISE/FALL TIME (ns)

30

20

10

0

100 400 700 1000 1300 1600 1900 2200

3.3V
0

NORMALIZED OUTPUT CURRENT

vs. ADJ VOLTAGE

ADJ VOLTAGE (V)

EXT RISE/FALL TIME vs. C

RISE TIME

FALL TIME

C

EXT

SHUTDOWN/SOFT-START TIMING

I

SET TO 80mA,

OUT

4 CHAINS OF 4 LEDs

5ms/div

EXT

MXA1698-08

MXA1698-02

MXA1698-05

I

OUT

50mA/div

I

BATT

100mA/div

SHDN
5V/div

OUTPUT CURRENT

vs. INPUT VOLTAGE

22.0

21.5

21.0

OUTPUT CURRENT (mA)

20.5

20.0
0 468102 1214161820

INPUT VOLTAGE (V)

1 CHAIN OF 4 LEDs

EFFICIENCY vs. BATTERY VOLTAGE

100

90

80

70

60

50

40

EFFICIENCY (%)

30

20

I

= 80mA

10

0

0426810

BATTERY VOLTAGE (V)

OUT

4 CHAINS OF 4 LEDs

LED AND INDUCTOR CURRENT

WAVEFORMS

1µs/div

MXA1698-09

MXA1698-03

MXA1698-06

LED
CURRENT
50mA/div

INDUCTOR
CURRENT
500mA/div

MAX1698/MAX1698A

High-Efficiency Step-Up

Current Regulator for LEDs

_______________________________________________________________________________________ 5

NAME FUNCTION

1 V

CC

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

2

SHDN

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 V

BATT

to ensure

that the LEDs are off in shutdown.

PIN

3 REF

1.25V Reference Output. Capable of sourcing 150µA for external loads. This pin is internally compensated.
Do not connect any bypass capacitors at REF.

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

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

8 PGND Power Ground

9 CS FET Current-Sense Input

5, 7 GND Ground

10 EXT Gate Driver Output

Pin Description

UNDERVOLTAGE

LOCKOUT/

SHUTDOWN

CS

TRIG

Q

TRIG

Q

PGND

EXT

REF

GND

ADJ

FB

SHDN

V

CC

V

CC

MAX t

ON

MIN t

OFF

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 oper­ate 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 resis­tor. In matched LED arrays, the secondary chain cur­rents 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 N­channel 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 V

BATT

to ensure that
the LEDs remain off in shutdown. Typical shutdown tim­ing 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) mini­mum 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 RFBsets the maximum current in the primary
chain of LEDs:

where I

LEDMAX

is the maximum LED current.

Adjusting LED Current

RFBsets 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

Figure 2. Typical Operating Circuit

10µH

V

= 3.3V

BATT

10µF

VCC = 3.3V

1µF

V

CC

MAX1698A

SHDN

REF

ADJ

GND

MAX1698

PGND

EXT

CS

OPTIONAL

FB

MBR0540

DZ

15Ω 15Ω 15Ω 15Ω

R

FB

1µF

LEDs

R

=

FB

mV

300

I

LEDMAX

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 I

LED

:

where V

ADJ

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 val­ues 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:

I

L(PEAK)

= 1.0 · P

OUT

where P

OUT

is the output power to all LED banks in

watts and I

L(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 volt­ages (V

GS

) at the voltage supplied at VCCor less. For

best results, minimize the FET’s R

DS(ON)

. The external

NFET’s maximum drain-to-source voltage (V

DS(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 V

OUT

. For output volt­ages 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 feed­back 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 capaci­tor. 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.

Figure 3. Adjusting LED Current

TRANSISTOR COUNT: 2180

Chip Information

REF

MAX1698

MAX1698A

I

LED

ADJ

GND

V

ADJ

.

=⋅416

R

FB

500k

⋅067

V

.

f

=

BATT

L

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

© 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products. Inc.

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

e

10

Ø0.50±0.1

0.6±0.1

1

0.6±0.1

A2

TOP VIEW

D2

b

D1

4X S

H

GAGE PLANE

A

A1

α

FRONT VIEW

BOTTOM VIEW

SIDE VIEW

10

1

E2

E1

L

L1

PROPRIETARY INFORMATION

TITLE:

INCHES

DIM

MIN

-A

0.002

A1
A2 0.030 0.037 0.75 0.95

0.116

D1

0.114

D2

0.116

E1

0.114

E2

0.187

H

0.0157

L
L1

0.037 REF

0.007

b

e

0.0197 BSC

0.0035

c

0.0196 REF

S

α

0° 0° 6°

c

MAX

0.043

0.006

0.120

0.118

0.120

0.118

0.199

0.0275

0.0106

0.0078

6°

MILLIMETERS

MAX

MIN

1.10

-

0.15

0.05

3.05

2.95

3.00

2.89

3.05

2.95

2.89

3.00

4.75

5.05

0.40

0.70

0.940 REF

0.177

0.270

0.500 BSC

0.090

0.200

0.498 REF

10LUMAX.EPS

PACKAGE OUTLINE, 10L uMAX/uSOP

REV.DOCUMENT CONTROL NO.APPROVAL

21-0061

1

1