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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
1
2
3
4
5
10
9
8
7
6
EXT
CS
PGND
GNDADJ
REF
VCC
MAX1698
MAX1698A
µMAX
TOP VIEW
FBGND
SHDN
VCC
VOUT
COUT
SHDN
FB
GND
ADJ
REF
CS
ON
OFF
PGND
EXT
D1
VCC
VBATT
L
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
A
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 _______________________________________________________________________________________
0
5
10
15
20
25
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
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
0
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)
0
30
20
10
50
40
90
80
70
60
100
100 400 700 1000 1300 1600 1900 2200
EXT RISE/FALL TIME vs. CEXT
MXA1698-05
CEXT
(p
F
)
RISE/FALL TIME (ns)
FALL TIME
RISE TIME
0
30
20
10
40
50
60
70
80
90
100
0426810
EFFICIENCY vs. BATTERY VOLTAGE
MXA1698-06
BATTERY VOLTAGE (V)
EFFICIENCY (%)
IOUT = 80mA
4 CHAINS OF 4 LEDs
0
20
10
40
30
60
50
70
90
80
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
0
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.
PIN
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
CS
TRIG
Q
TRIG
Q
PGND
EXT
REF
GND
ADJ
FB
SHDN
VCC
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
I
FB LEDMAX
=300
VCC
SHDN
PGND
GND
FB
RFB
OPTIONAL
DZ
LEDs
MBR0540
ADJ
REF
CS
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.
IV
R
LED ADJ
FB
.
=⋅416
GND
REF
ADJ
MAX1698
MAX1698A
500k
Figure 3. Adjusting LED Current
TRANSISTOR COUNT: 2180
Chip Information
fV
L
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
© 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products. Inc.
10LUMAX.EPS
PACKAGE OUTLINE, 10L uMAX/uSOP
1
1
21-0061 REV.DOCUMENT CONTROL NO.APPROVAL
PROPRIETARY INFORMATION
TITLE:
TOP VIEW
FRONT VIEW
1
0.498 REF
0.0196 REF
S6°
SIDE VIEW
α
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
1
10
L1
0.0035
0.007
e
c
b
0.187
0.0157
0.114
H
L
E2
DIM
0.116
0.114
0.116
0.002
D2
E1
A1
D1
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
2.95
-
MIN
3.00
3.05
0.15
3.05
MAX
1.10
10
0.6±0.1
0.6±0.1
Ø0.50±0.1
H
4X S
e
D2
D1
b
A2 A
E2
E1 L
L1
c
α
GAGE PLANE
A2 0.030 0.037 0.75 0.95
A1
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
