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19-2731; Rev 0; 1/03
KIT ATION EVALU ILABLE AVA
High-Efficiency, 26V Step-Up Converter for Two to Six White LEDs
General Description Features
o Accurate Current Regulation for Uniform Illumination o High Efficiency Up to 84% o Flexible Analog or PWM Dimming Control o Up to 900mW Output Power with Internal 30V MOSFET Switch o Fast 1MHz PWM Operation o Small, Low-Profile External Components o 2.6V to 5.5V Input Range o 26V (max) Output with Overvoltage Protection o Optimized for Low 15mVP-P Input Ripple o Soft-Start with Zero Inrush Current o 0.3A Shutdown Current o Tiny 8-Pin, 3mm x 3mm Thin QFN Package
MAX1561
The MAX1561 step-up converter drives up to six white LEDs with a constant current to provide backlight in cell phones, PDAs, and other hand-held devices. The stepup converter topology allows series connection of the white LEDs so that the LED currents are identical for uniform brightness. This configuration eliminates the need for ballast resistors and expensive factory calibration. The MAX1561 includes an internal, high-voltage, lowRDS(ON) N-channel MOSFET switch for high efficiency and maximum battery life. A single Dual ModeTM input provides a simple means of brightness adjustment and on/off control. Fast 1MHz current-mode pulse-width modulated (PWM) operation allows for small input and output capacitors and a small inductor while minimizing ripple on the input supply/battery. Programmable softstart eliminates inrush current during startup. The MAX1561 is available in a space-saving, 8-pin 3mm x 3mm thin QFN package with exposed paddle (EP).
Applications
Cell Phones and Smart Phones PDAs, Palmtops, and Wireless Handhelds e-Books and Subnotebooks White LED Display Backlighting
PART MAX1561ETA
Ordering Information
TEMP RANGE -40C to +85C PIN-PACKAGE 8 Thin QFN-EP*
*EP = Exposed paddle.
Typical Operating Circuit
INPUT 2.6V TO 5.5V OUTPUT UP TO 6 LEDs IN LX OUT
OUT 1
Pin Configuration
TOP VIEW
MAX1561 8 LX 7 PGND 6 GND 5 COMP
PWM OR ANALOG DIMMING
MAX1561 CTRL PGND
IN 2 CTRL 3 CS 4
COMP
GND
CS
3mm x 3mm THIN QFN
Dual Mode is a trademark of Maxim Integrated Products, Inc. ________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
High-Efficiency, 26V Step-Up Converter for Two to Six White LEDs MAX1561
ABSOLUTE MAXIMUM RATINGS
IN to GND .................................................................-0.3V to +6V PGND to GND .......................................................-0.3V to +0.3V LX, OUT to GND .....................................................-0.3V to +30V CTRL to GND...................-0.3V to the lower of +6V or (VIN + 2V) COMP, CS to GND.........................................-0.3 to (VIN + 0.3V) ILX.............................................................................................1A Continuous Power Dissipation (TA = +70C) 3mm x 3mm 8-Pin Thin QFN (derate 24.4mW/C above +70C) .............................1950mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (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
(VIN = 3V, VOUT = 20V, L1 = 22H, COUT = 0.1F, CCOMP = 0.15F, RSENSE = 7.5, VCTRL = 1.5V, Figure 1, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Supply Voltage UVLO Threshold UVLO Hysteresis Quiescent Current Shutdown Supply Current OVLO Threshold OVLO Hysteresis OUT Input Bias Current Output Voltage Range ERROR AMPLIFIER CTRL to CS Regulation CS Input Bias Current CTRL Input Resistance CTRL Dual-Mode Threshold CTRL Dual-Mode Hysteresis CTRL Shutdown Enable Delay CS-to-COMP Transconductance OSCILLATOR Operating Frequency Minimum Duty Cycle Maximum Duty Cycle PWM mode Pulse skipping CTRL = IN, CS = GND 91 0.8 1.0 12 0 94 1.25 MHz % % (Note 2) VCOMP = 1.0V 6.5 32 VCTRL = 1V, VIN = 2.6V to 5.5V VCS = VCTRL/10 VCTRL 1.0V TA = +25C TA = +85C 290 100 0.095 0.100 0.01 0.03 500 170 5 8.2 50 10.5 82 780 240 0.105 1 V A k mV mV ms S VOUT = 26V, VCTRL > 0.24V VOUT = VIN, CTRL = GND (Note 1) (VIN - VD1) 10 No switching CTRL = GND, VOUT = VIN VOUT rising TA = +25C TA = +85C 26 VIN rising or falling CONDITIONS MIN 2.6 2.10 2.38 40 0.38 0.3 1 27 2 20 0.01 30 1 25.5 29 0.55 2 TYP MAX 5.5 2.55 UNITS V V mV mA A V V A V
2
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High-Efficiency, 26V Step-Up Converter for Two to Six White LEDs
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 3V, VOUT = 20V, L1 = 22H, COUT = 0.1F, CCOMP = 0.15F, RSENSE = 7.5, VCTRL = 1.5V, Figure 1, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER N-CHANNEL SWITCH LX On-Resistance LX Leakage Current LX Current Limit ILX = 190mA VLX = 28V, CTRL = GND Duty cycle = 90% TA = +25C TA = +85C 450 1.45 0.01 1 700 950 2.25 5 A mA CONDITIONS MIN TYP MAX UNITS
MAX1561
ELECTRICAL CHARACTERISTICS
(VIN = 3V, VOUT = 20V, L1 = 22H, COUT = 0.1F, CCOMP = 0.15F, RSENSE = 7.5, VCTRL = 1.5V, Figure 1, TA = -40C to +85C, unless otherwise noted.) (Note 3)
PARAMETER Supply Voltage UVLO Threshold Quiescent Current OVLO Threshold OUT Input Bias Current Output Voltage Range ERROR AMPLIFIER CTRL to CS Regulation CTRL Input Resistance CTRL Dual-Mode Threshold CTRL Shutdown Enable Delay CS-to-COMP Transconductance OSCILLATOR Operating Frequency Maximum Duty Cycle N-CHANNEL SWITCH LX On-Resistance LX Current Limit Duty cycle = 90% 450 2.25 950 mA CTRL = IN, CS = GND 0.75 91 1.25 MHz % (Note 2) VCOMP = 1.0V VCTRL = 1V, VIN = 2.6V to 5.5V VCTRL 1.0V 0.093 240 100 6 32 0.107 780 240 11 85 V k mV ms S VIN rising or falling No switching VOUT rising VOUT = 26V, VCTRL > 0.24V VOUT = VIN, CTRL = GND (Note 1) (VIN - VD1) 26 10 CONDITIONS MIN 2.6 2.10 TYP MAX 5.5 2.55 0.55 29 30 1 25.5 UNITS V V mA V A V
Note 1: VD1 is the diode forward voltage drop of diode D1 in Figure 1. Note 2: Time from CTRL going below the dual-mode threshold to IC shutdown. Note 3: Specifications to -40C are guaranteed by design and not production tested.
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3
High-Efficiency, 26V Step-Up Converter for Two to Six White LEDs MAX1561
Typical Operating Characteristics
(Circuit of Figure 1, VIN = 3.6V, ILED = 15mA, L1 = 22H, CIN = 2.2F, COUT = 0.1F, CCOMP = 0.1F, RSENSE = 7.5, 4 LEDs, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LED CURRENT (vs. NUMBER of LEDs)
MAX1561toc01
EFFICIENCY vs. LED CURRENT (vs. INPUT VOLTAGE)
MAX1561toc02
EFFICIENCY vs. LED CURRENT (vs. INDUCTANCE)
MAX1561toc03
100 90 6 LEDs EFFICIENCY (%) 80 70 60 50 40 0 5 10 LED CURRENT (mA) 15
100 90 EFFICIENCY (%) 80 70 60 50 40 4.2 VIN 3.6 VIN
100 90 10H EFFICIENCY (%) 80 70 60 50 40 47H 4.7H 22H
2.7 VIN
4 LEDs 2 LEDs
20
0
5
10 LED CURRENT (mA)
15
20
0
5
10 LED CURRENT (mA)
15
20
INPUT RIPPLE vs. INDUCTANCE
35 INPUT RIPPLE (mVP-P) 30 25 20 15 10 1 5 0 0 5 10 15 20 25 30 35 40 45 50 INDUCTANCE (H) 0 0 1
MAX1561toc04
INPUT RIPPLE SPECTRUM
MAX1561toc05
LED CURRENT vs. DIRECT-PWM DIMMING DUTY CYCLE
MAX1561toc06
40
6 5 INPUT RIPPLE (mVRMS) 4 3 2
25 20 LED CURRENT (mA)
15
10
5 VCTRL = SQUARE WAVE 200Hz < FREQ < 200kHz 0 2 3 4 5 6 7 8 9 10 0 10 20 30 40 50 60 70 80 90 100 DIRECT-PWM DIMMING DUTY CYCLE (%) FREQUENCY (MHz)
SWITCHING WAVEFORMS
MAX1561toc07
SOFT-START AND SHUTDOWN
MAX1561toc08
CTRL STEP RESPONSE
MAX1561toc09
VIN 10mV/div
VCTRL 1V/div 0V VIN 50mV/div IIN 50mA/div 0mA VOUT 5V/div VIN = 3.8V Li+ BATTERY 0V VIN = 3.8V Li+ BATTERY
VCTRL 1V/div 0V VIN 50mV/div IIN 50mA/div 0mA VOUT 5V/div 0V 20ms/div
VOUT 100mV/div
VLX 10V/div
500ns/div
20ms/div
4
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High-Efficiency, 26V Step-Up Converter for Two to Six White LEDs
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 3.6V, ILED = 15mA, L1 = 22H, CIN = 2.2F, COUT = 0.1F, CCOMP = 0.1F, RSENSE = 7.5, 4 LEDs, TA = +25C, unless otherwise noted.) DIRECT-PWM DIMMING
MAX1561toc10
MAX1561
LINE TRANSIENT RESPONSE
MAX1561toc11
VCTRL 32kHz 1V/div 0V VIN 50mV/div IIN 50mA/div 0mA VOUT 5V/div VIN = 3.8V Li+ BATTERY 0V 50s/div 50s/div
4V VIN 500mV/div 3.5V
IIN 50mA/div 0mA VCS 100mV/div 0V
Pin Description
PIN 1 2 3 NAME OUT IN CTRL FUNCTION Overvoltage Sense. When VOUT is greater than 27V, the internal N-channel MOSFET turns off until VOUT drops below 25V, then the IC reenters soft-start. Connect a 0.1F capacitor from OUT to PGND. Input Voltage Supply. Input voltage range is 2.6V to 5.5V. Connect a 2.2F capacitor from IN to PGND. Brightness Control Input. LED brightness is controlled by the voltage applied to CTRL. Varying the voltage from 0.24V to 1.62V adjusts the brightness from dim to bright, respectively. Any voltage above 1.62V does not increase brightness. If CTRL is held below 100mV for more than 8.2ms, the IC shuts down. Current-Sense Feedback Input. Connect a resistor from CS to GND to set the LED bias current. The voltage at CS regulates to VCTRL/10 or 0.162V, whichever is lower. Compensation Input. Connect a 0.1F capacitor (CCOMP) from COMP to GND. CCOMP stabilizes the converter and controls soft-start. CCOMP discharges to GND when in shutdown. Ground. Connect to PGND and the exposed pad directly under the IC. Power Ground. Connect to GND and the exposed pad directly under the IC. Inductor Connection. This pin is high impedance during shutdown. Ground. Connect directly to GND and PGND under the IC.
4 5 6 7 8 Exposed Pad
CS COMP GND PGND LX EP
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5
High-Efficiency, 26V Step-Up Converter for Two to Six White LEDs MAX1561
INPUT 2.6V TO 5.5V 2.2F L1 22H IN LX COUT 0.1F PGND fOSC 1MHz
PWM CONTROL
SLOPE COMP CURRENT SENSE
OVERVOLTAGE PROTECTION
OUT
UP TO 26V
2 TO 6 LEDS
PWM OR ANALOG DIMING
CTRL
450k COMP CCOMP 0.1F CS
50k
gm
8.2ms (TYP) SHUTDOWN TIMER 170mV GND
SHUTDOWN
RSENSE 7.5
Figure 1. Functional Diagram
Detailed Description
The MAX1561's high efficiency and small size make it ideally suited to drive up to six series-connected LEDs. The device operates as a boost DC-to-DC converter that regulates output current rather than voltage. The MAX1561 provides even illumination by sourcing the same output current through each LED, eliminating the need for expensive factory calibration. The fast 1MHz internal oscillator allows for a small inductor and small input and output capacitors while minimizing input and output ripple. The single analog control input allows easy adjustment of LED brightness and on/off control. This allows either simple logic-level on/off control, analog voltage control, or PWM duty-cycle control of both brightness and shutdown. In shutdown, supply current is reduced to a low 0.3A. A soft-start gradually illuminates the LEDs, eliminating the inrush current during startup.
6
Soft-Start
The MAX1561 attains soft-start by charging CCOMP gradually with a current source. When VCOMP rises above 1.25V, the internal MOSFET begins switching at a reduced duty cycle. When VCOMP rises above 2.25V, the duty cycle is at its maximum. See the Typical Operating Characteristics for an example of soft-start operation.
Shutdown
The MAX1561 enters shutdown when V CTRL is less than 100mV for more than 8.2ms. In shutdown, supply current is reduced to 0.3A by powering down the entire IC except for the CTRL voltage-detection circuitry. CCOMP is discharged during shutdown, allowing the device to reinitiate soft-start when it is enabled. Although the internal N-channel MOSFET does not switch in shutdown, there is still a DC current path between the input and the LEDs through the inductor
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High-Efficiency, 26V Step-Up Converter for Two to Six White LEDs
and Schottky diode. The minimum forward voltage of the LED array must exceed the maximum input voltage to ensure that the LEDs remain off in shutdown. However, with two or more LEDs, the forward voltage is large enough to keep leakage current low, less than 1A (typ). Typical shutdown timing characteristics are shown in the Typical Operating Characteristics. to GND. For stable operation, COUT must not exceed 10 times CCOMP.
MAX1561
Inductor Selection
Inductor values range from 10H to 47H. A 22H inductor optimizes the efficiency for most applications while maintaining low 15mVP-P input ripple. With input voltages near 5V, a larger value of inductance can be more efficient. To prevent core saturation, ensure that the inductor saturation current rating exceeds the peak inductor current for the application. Calculate the peak inductor current with the following formula: IPEAK = VOUT(MAX) x ILED(MAX) 0.8 x VIN(MIN) + VIN(MIN) x 0.8s 2 xL
Overvoltage Protection
Overvoltage lockout (OVLO) occurs when V OUT is above 27V. The protection circuitry stops the internal MOSFET from switching and causes VCOMP to decay to 0V. The device comes out of OVLO and into soft-start when VOUT falls below 25V.
Design Procedure
Adjusting LED Current
Adjusting the MAX1561's output current changes the brightness of the LEDs. An analog input (CTRL) and the sense-resistor value set the output current. Output current is given by: ILED = VCTRL 10 x RSENSE
Schottky Diode Selection
The MAX1561's high switching frequency demands a high-speed rectification diode (D1) for optimum efficiency. A Schottky diode is recommended due to its fast recovery time and low forward-voltage drop. Ensure that the diode's average and peak current rating exceed the average output current and peak inductor current. In addition, the diode's reverse breakdown voltage must exceed VOUT. The RMS diode current can be calculated from: IDIODE(RMS) IOUT x IPEAK
The VCTRL voltage range for adjusting output current is 0.24V to 1.62V. To set the maximum current, calculate RSENSE when VCTRL is at its maximum as follows: RSENSE = 1.62V 10 x ILED(MAX)
Applications Information
PC Board Layout
Due to fast switching waveforms and high current paths, careful PC board layout is required. An evaluation kit (MAX1561EVKIT) is available to speed design. When laying out a board, minimize trace lengths between the IC and RSENSE, the inductor, the diode, the input capacitor, and the output capacitor. Keep traces short, direct, and wide. Keep noisy traces, such as the LX node trace, away from CS. The IN bypass capacitor (CIN) should be placed as close to the IC as possible. PGND and GND should be connected directly to the exposed paddle underneath the IC. The ground connections of CIN and COUT should be as close together as possible. The traces from IN to the inductor and from the Schottky diode to the LEDs may be longer.
Power dissipation in RSENSE is typically less than 5mW, making a standard chip resistor sufficient.
PWM Dimming Control
CTRL is also used as a digital input allowing LED brightness control with a logic-level PWM signal applied directly to CTRL. The frequency range is from 200Hz to 200kHz, while 0% duty cycle corresponds to zero current and 100% duty cycle corresponds to full current. The error amplifier and compensation capacitor form a lowpass filter such that PWM dimming results in DC current to the LEDs without the need for any additional RC filters; see the Typical Operating Characteristics.
Capacitor Selection
The exact values of input and output capacitors are not critical. The typical value for the input capacitor is 2.2F and the typical value for the output capacitor is 0.1F. Larger value capacitors can be used to reduce input and output ripple, but at the expense of size and higher cost. CCOMP stabilizes the converter and controls soft-start. Connect a 0.1F capacitor from COMP
Chip Information
TRANSISTOR COUNT: 2895 PROCESS: BiCMOS
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7
High-Efficiency, 26V Step-Up Converter for Two to Six White LEDs MAX1561
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.) 6, 8, &10L, QFN THIN.EPS
PACKAGE OUTLINE, 6, 8 & 10L, QFN THIN (DUAL), EXPOSED PAD, 3x3x0.80 mm
21-0137
C
8
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High-Efficiency, 26V Step-Up Converter for Two to Six White LEDs
Package Information (continued)
(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.)
MAX1561
COMMON DIMENSIONS SYMBOL A D E A1 L k A2 MIN. 0.70 2.90 2.90 0.00 0.20 MAX. 0.80 3.10 3.10 0.05 0.40
0.25 MIN 0.20 REF.
PACKAGE VARIATIONS PKG. CODE T633-1 T833-1 T1033-1 N 6 8 10 D2 1.50-0.10 1.50-0.10 1.50-0.10 E2 2.30-0.10 2.30-0.10 2.30-0.10 e 0.95 BSC 0.65 BSC 0.50 BSC JEDEC SPEC MO229 / WEEA MO229 / WEEC MO229 / WEED-3 b 0.40-0.05 0.30-0.05 0.25-0.05 [(N/2)-1] x e 1.90 REF 1.95 REF 2.00 REF
PACKAGE OUTLINE, 6, 8 & 10L, QFN THIN (DUAL), EXPOSED PAD, 3x3x0.80 mm
21-0137
C
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________ 9 (c) 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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