Maxim MAX1698EUB Datasheet

General Description

The MAX1698 is the most efficient driver for white or
color LEDs. The device is ideal for LED backlit displays
in PDAs and digital cameras, and can also be used for
larger displays like those in laptop computers. Its
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 is a switch-mode boost controller in
which LED current, rather than output voltage, provides
the feedback signal. It can operate with battery inputs
as low as 0.8V. The device drives series-connected
LEDs with a controlled current that is measured at a
sense resistor connected at the feedback pin. The
required sense resistor is typically 15Ω, not an expen­sive fractional-ohm value. LED current control and dim­ming are accomplished with an adjust input (ADJ), not
with lossy current-limiting resistors. For larger light out­put, multiple LED banks can be connected in parallel
with up to 5W total output power.

The MAX1698 is 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

Cell Phones

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

High-Efficiency Step-Up

Current Regulator for LEDs

________________________________________________________________ Maxim Integrated Products 1

Typical Operating Circuit

19-1621; Rev 0; 1/00

PART

MAX1698EUB

-40°C to +85°C

TEMP. RANGE PIN-PACKAGE

10 µMAX

EVALUATION KIT

AVAILABLE

Pin Configuration

Ordering Information

For free samples and the latest literature, visit www.maxim-ic.com or phone [phone].
For small orders, phone [phone].

TOP VIEW

V

SHDN

REF

1

CC

2

MAX1698

3

4

5

10

9

8

7

6

EXT

CS

PGND

GNDADJ

FBGND

V

CC

SHDN

REF

ADJ

GND

L

EXT

MAX1698

CS

PGND

FB

V

BATT

V

CC

OFF

ON

µMAX

D1

I

LED

R

FB

R

N

V

OUT

C

OUT

MAX1698

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

A0.8 1.5 2.5CS 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

MAX1698

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

CS Trip Current FB = GND 0.8 3.0 A

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

MAX1698

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

vs. V

CC

15

10

OUTPUT CURRENT (mA)

5

0

045231 678910

V

BATT

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

I

BATT

I

OUT

0 2341 567 9810

V

BATT

EFFICIENCY vs. OUTPUT CURRENT

5253515 45 55 65 75 85

OUTPUT CURRENT (mA)

AND V

BATT

1 CHAIN OF 4 LEDs

, VCC (V)

4 CHAINS OF 4 LEDs

(V)

V

= 5V

BATT

4 CHAINS OF 4 LEDs

NORMALIZED OUTPUT CURRENT

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

100

90

MXA1698-04

80

70

60

50

40

RISE/FALL TIME (ns)

30

20

10

0

100 400 700 1000 1300 1600 1900 2200

vs. ADJ VOLTAGE

ADJ VOLTAGE (V)

EXT RISE/FALL TIME vs. C

RISE TIME

FALL TIME

C

EXT

EXT

MXA1698-02

MXA1698-05

22.0

21.5

21.0

OUTPUT CURRENT (mA)

20.5

20.0
0 468102 1214161820

100

90

80

70

60

50

40

EFFICIENCY (%)

30

20

10

0

0426810

OUTPUT CURRENT

vs. INPUT VOLTAGE

1 CHAIN OF 4 LEDs

INPUT VOLTAGE (V)

EFFICIENCY vs. BATTERY VOLTAGE

I

= 80mA

OUT

4 CHAINS OF 4 LEDs

BATTERY VOLTAGE (V)

MXA1698-03

MXA1698-06

LED AND INDUCTOR CURRENT

MXA1698-07

SHUTDOWN/SOFT-START TIMING

3.3V
0

I

SET TO 80mA,

OUT

5ms/div

MXA1698-08

4 CHAINS OF 4 LEDs

I

OUT

50mA/div

I

BATT

100mA/div

SHDN
5V/div

WAVEFORMS

1µs/div

MXA1698-09

LED
CURRENT
50mA/div

INDUCTOR
CURRENT
500mA/div

MAX1698

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

PGND

EXT

REF

GND

ADJ

FB

SHDN

V

L

V

CC

V

CC

MAX t

ON

MIN t

OFF

MAX1698

SOFT-START

1.25V REFERENCE

Figure 1. Functional Diagram

MAX1698

High-Efficiency Step-Up
Current Regulator for LEDs

6 _______________________________________________________________________________________

Detailed Description

The MAX1698’s high efficiency and small size make it
ideally suited to drive LEDs. It operates as a boost DC­DC 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 MAX1698’s 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 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 includes a soft-start function that elimi­nates input current surges at turn-on. It does this by
extending the external FET driver (EXT) minimum off­time 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/Soft­Start 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

SHDN

REF

ADJ

MAX1698

GND

EXT

CS

FB

PGND

MBR0540

OPTIONAL

DZ

15Ω 15Ω 15Ω 15Ω

R

FB

1µF

LEDs

R

=

FB

mV

300

I

LEDMAX

MAX1698

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 turns 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 will reduce the MAX1698’s switching frequency.
The typical operating frequency is given by:

The MAX1698 limits 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 drives 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 (VGS)
at the voltage supplied at VCCor less. For best results,
minimize the FET’s R

DS(ON)

. The external NFET’s maxi-

mum drain-to-source voltage (V

DS(MAX)

) must exceed

the output voltage.

Catch Diode (D1) Selection

The MAX1698’s 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

PC Board Layout

Due to fast switching waveforms and high-current
paths, careful PC board 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

I

LED

ADJ

GND

V

ADJ

.

=⋅416

R

FB

500k

f

=

⋅067

.

V

L

BATT

MAX1698

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

© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

Package Information

Note: The MAX1698 does not have an exposed pad.

10LUMAX.EPS