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19-1621; Rev 0; 1/00
KIT ATION EVALU ILABLE AVA
High-Efficiency Step-Up Current Regulator for LEDs
Features
o Over 90% Efficiency o Lossless, Adjustable LED Brightness o Space-Saving 10-Pin MAX Package o Simpler, Lower Cost, More Reliable Compared to CCFL or EL Backlights o Up to 5W Output Power
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 15, 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 MAX package that occupies half the space of an 8-pin SO. An evaluation kit (MAX1698EVKIT) is available to speed designs.
MAX1698
Ordering Information
PART MAX1698EUB TEMP. RANGE -40C to +85C PIN-PACKAGE 10 MAX
Pin Configuration
TOP VIEW
VCC 1 SHDN REF ADJ GND 2 3 4 5 10 EXT 9 CS PGND GND FB
MAX1698
8 7 6
Applications
Battery-Powered Backlight Applications Backlight for LCD Panels Cell Phones Handy Terminals PDAs
MAX
Typical Operating Circuit
L VBATT D1 VOUT COUT VCC VCC EXT
ON OFF SHDN REF ADJ
MAX1698
CS PGND ILED
FB GND RFB RN
________________________________________________________________ Maxim Integrated Products
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High-Efficiency Step-Up Current Regulator for LEDs MAX1698
ABSOLUTE MAXIMUM RATINGS
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 = +70C) 10-Pin MAX (derate 5.6mW/C above +70C) ............444mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature Range (soldering, 10s)......................+300C
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.
ELECTRICAL CHARACTERISTICS
(VCC = +3.3V, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Supply Voltage Quiescent Supply Current Undervoltage Lockout Undervoltage Hysteresis FB Regulation Voltage CS Trip Current CS Input Current Minimum Off Time Maximum On Time ADJ Start Threshold ADJ Input Range FB Input Bias Current ADJ Input Bias Current Shutdown Supply Current REF Output Voltage REF Load Regulation REF Short-Circuit Current REF Power-Supply Rejection Ratio EXT Driver Sink/Source Current EXT Driver On-Resistance SHDN Input High Voltage SHDN Input Low Voltage SHDN Input Bias Current VIH VIL I SHDN PSRR VREF VREF VADJ IFB IADJ VFB = 300mV ADJ = REF SHDN = GND IREF = 0 IREF = 0 to 150A REF = GND VCC = 2.7V to 5.5V VCC = 5V, EXT = 2V VCC = 5V VCC = 2.7V to 5.5V VCC = 2.7V to 5.5V VCC = 2.7V to 5.5V -1 2 0.8 1 1.20 ICS tOFF(MIN) tON(MAX) FB = GND, ADJ = REF FB = GND, ADJ = REF, CS = GND 0.8 10 30 0.03 -15 -50 0.01 1.25 -2 0.45 +0.3 0.2 8 1.0 15 50 VFB ADJ = REF VADJ = 100mV FB = GND 285 18 0.8 SYMBOL VCC ICC VFB = 0.3V Rising edge 2.25 CONDITIONS MIN 2.7 260 2.4 80 300 24 1.5 315 30 2.5 0.8 1.2 20 70 REF 15 50 1 1.30 -25 1 +2 TYP MAX 5.5 500 2.55 UNITS V A V mV mV A ARMS s s mV V nA nA A V mV mA mV/V A V V A
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High-Efficiency Step-Up Current Regulator for LEDs
ELECTRICAL CHARACTERISTICS
(VCC = +3.3V, TA = -40C to +85C, unless otherwise noted.) (Note 1) PARAMETER Supply Voltage Quiescent Supply Current Undervoltage Lockout FB Regulation Voltage CS Trip Current Minimum Off Time Maximum On Time ADJ Start Threshold FB Input Bias Current ADJ Input Bias Current Shutdown Supply Current REF Output Voltage REF Load Regulation REF Short-Circuit Current REF Power-Supply Rejection Ratio EXT On-Resistance SHDN Input High Voltage SHDN Input Low Voltage SHDN Input Bias Current VIH VIL I SHDN PSRR VREF VREF IFB IADJ VFB = 300mV ADJ = REF SHDN = GND IREF = 0 IREF = 0 to 150A REF = GND VCC = 2.7V to 5.5V VCC = 5V VCC = 2.7V to 5.5V VCC = 2.7V to 5.5V VCC = 2.7V to 5.5V -1 2 0.8 1 1.17 tOFF(MIN) tON(MAX) VFB SYMBOL VCC ICC VFB = 0.3V Rising edge ADJ = REF VADJ = 100mV FB = GND FB = GND, ADJ = REF FB = GND, ADJ = REF, CS = GND 2.20 280 13 0.8 0.6 9 25 -20 -50 CONDITIONS MIN 2.7 MAX 5.5 500 2.65 320 35 3.0 1.4 21 75 20 50 1 1.33 -30 1 +2 8 UNITS V A V mV A s s mV nA nA A V mV mA mV/V V V A
MAX1698
Note 1: Specifications to -40C are guaranteed by design, not production tested.
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High-Efficiency Step-Up Current Regulator for LEDs MAX1698
Typical Operating Characteristics
(Circuit of Figure 2, VCC = VBATT = 3.3V, VADJ = VREF, TA = +25C, unless otherwise noted.)
OUTPUT CURRENT vs. VBATT AND VCC
MXA1698-01
NORMALIZED OUTPUT CURRENT vs. ADJ VOLTAGE
MXA1698-02
OUTPUT CURRENT vs. INPUT VOLTAGE
MXA1698-03
25 vs. VBATT 20 OUTPUT CURRENT (mA) vs. VCC
1.2 NORMALIZED OUTPUT CURRENT 1.0 0.8 0.6 0.4 0.2
22.0
15
OUTPUT CURRENT (mA)
21.5
21.0
10
5 1 CHAIN OF 4 LEDs 0 0 1 2 3 4 5 6 7 8 9 10 VBATT, VCC (V)
20.5 1 CHAIN OF 4 LEDs 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 ADJ VOLTAGE (V) 20.0 0 2 4 6 8 10 12 14 16 18 20 INPUT VOLTAGE (V)
BATTERY CURRENT vs. BATTERY VOLTAGE
MXA1698-04
EXT RISE/FALL TIME vs. CEXT
MXA1698-05
EFFICIENCY vs. BATTERY VOLTAGE
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 IOUT = 80mA 4 CHAINS OF 4 LEDs 0 2 4 6 8 10
MXA1698-06
1000 900 800 4 CHAINS OF 4 LEDs
100 90 80 RISE/FALL TIME (ns) RISE TIME
100
700 IOUT, IBATT (mA) 600 500 400 300 200 100 0 0 1 2
IBATT
70 60 50 40 30 20 FALL TIME
IOUT
10 0 4 5 6 7 8 9 10 100 400 700 1000 1300 1600 1900 2200 CEXT (pF)
3
VBATT (V)
BATTERY VOLTAGE (V)
EFFICIENCY vs. OUTPUT CURRENT
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 5 15 25 35 45 55 65 75 85 OUTPUT CURRENT (mA) VBATT = 5V 4 CHAINS OF 4 LEDs 3.3V 0
MXA1698-07
SHUTDOWN/SOFT-START TIMING
MXA1698-08
LED AND INDUCTOR CURRENT WAVEFORMS
MXA1698-09
100
IOUT 50mA/div
LED CURRENT 50mA/div
IBATT 100mA/div INDUCTOR CURRENT 500mA/div
IOUT SET TO 80mA, 4 CHAINS OF 4 LEDs 5ms/div
SHDN 5V/div 1s/div
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High-Efficiency Step-Up Current Regulator for LEDs
Pin Description
PIN 1 2 NAME VCC SHDN FUNCTION IC Supply Voltage Input. Power for internal circuitry. Input range is 2.7V to 5.5V. 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. 1.25V Reference Output. Capable of sourcing 150A for external loads. This pin is internally compensated. Do not connect any bypass capacitors at REF. Adjust Input. Allows dynamic adjustment of the output current. FB regulates to 300mV when ADJ = REF. Ground Feedback Input. Connect to the external LED current-sense feedback resistor. Power Ground FET Current-Sense Input Gate Driver Output
MAX1698
3 4 5, 7 6 8 9 10
REF ADJ GND FB PGND CS EXT
CS SHDN UNDERVOLTAGE LOCKOUT/ SHUTDOWN
MAX1698
VL VCC MAX tON EXT
VCC
ADJ PGND
MIN tOFF
FB 1.25V REFERENCE REF
SOFT-START GND
Figure 1. Functional Diagram
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High-Efficiency Step-Up Current Regulator for LEDs MAX1698
10H VBATT = 3.3V MBR0540 10F 1F VCC = 3.3V 1F VCC EXT
MAX1698
SHDN REF ADJ FB CS LEDs OPTIONAL DZ
GND
PGND
RFB
15
15
15
15
Figure 2. Typical Operating Circuit
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.
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 5s. It is then allowed to drop to 2s for the next 1500 switching cycles. After that time, the minimum off-time falls to the 1s value used during normal operation. (See Shutdown/SoftStart 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: RFB = 300mV I LEDMAX
Shutdown
In shutdown, the MAX1698's supply current is reduced below 1A. 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 timing characteristics are shown in the Typical Operating Characteristics.
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
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High-Efficiency Step-Up Current Regulator for LEDs
Transistor Selection
REF ADJ GND 500k
MAX1698
MAX1698
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 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
Figure 3. Adjusting LED Current
shows the standard method of setting the ADJ voltage. Use the following equation to determine ILED: VADJ I LED = 4.16 RFB where VADJ is the voltage at ADJ. Note that ADJ voltages below 50mV turns the LEDs off.
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 VOUT. For output voltages exceeding 40V, high-speed silicon rectifiers may be required for their higher breakdown voltages.
Inductor Selection
Choose an inductor with low DC resistance (in the neighborhood of 100m) to minimize losses. A typical inductance value for L is 10H; however, values from 3.3H to 100H can also be used. Higher inductor values will reduce the MAX1698's switching frequency. The typical operating frequency is given by: f= 0.67 VBATT L
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.
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: 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 0.1F to 10F. 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.
Chip Information
TRANSISTOR COUNT: 2180
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High-Efficiency Step-Up Current Regulator for LEDs MAX1698
Package Information
10LUMAX.EPS
Note: The MAX1698 does not have an exposed pad.
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 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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