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19-2028; Rev 1; 9/02
White LED Step-Up Converter in SOT23
General Description
The MAX1848 drives white LEDs with a constant current to provide backlight in cell phones, PDAs, and other hand-held devices. The step-up 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. Other benefits include greater simplicity, lower cost, higher efficiency, and greater reliability. This step-up PWM converter includes an internal, highvoltage, low RDSON N-channel MOSFET switch for high efficiency and maximum battery life. A single analog voltage Dual ModeTM input provides a simple means of brightness adjustment and on/off control. Fast 1.2MHz current-mode PWM control 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 MAX1848 is available in space-saving 8-pin thin QFN (3mm 3mm) and 8-pin SOT23 packages.
Features
o Constant Current Regulation for Uniform Illumination o High 87% Efficiency o Analog or Logic Control of LED Intensity o 0.8W Output Power with Internal High-Voltage MOSFET Switch o Small, Low-Profile External Components o 2.6V to 5.5V Input Range o 13V Maximum Output with Overvoltage Protection o Optimized for Low Input Ripple o Programmable Soft-Start o 0.3A Shutdown Current o Small 8-pin Thin QFN (3mm 3mm) and 8-Pin SOT23 Packages
MAX1848
Applications
Cell Phones and Smart Phones PDAs, Palmtops, and Wireless Handhelds e-Books and Subnotebooks White LED Display Backlighting
PART MAX1848EKA MAX1848ETA
Ordering Information
TEMP RANGE -40C to +85C -40C to +85C PIN-PACKAGE 8 SOT23 8 Thin QFN (3mm 3mm) TOP MARK AAIM ACR
Note: Hand soldering is not recommended for the MAX1848 SOT23 package.
Typical Application Circuit
L1 33H
TOP VIEW
Pin Configuration
2.6V to 5.5V CIN 3.3F V+
D1 LX OUT MAX1848 COUT 1F
COMP 1 CTRL 2 8 7 V+ CS GND PGND COMP CTRL OUT LX 1 2 8 7 V+ CS GND PGND
MAX1848
OUT 3 6 5 3 4 LX 4
MAX1848
6 5
LOGIC OR DAC CCOMP 0.15F
CTRL COMP
PGND GND CS RSENSE 5
SOT23
THIN QFN 3mm x 3mm
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.
White LED Step-Up Converter in SOT23 MAX1848
ABSOLUTE MAXIMUM RATINGS
V+ to GND ................................................................-0.3V to +6V PGND to GND .......................................................-0.3V to +0.3V LX, OUT to GND .....................................................-0.3V to +14V LX to OUT ...............................................................-14V to +0.3V CTRL to GND.......................................-0.3V to +6V or (V+ + 2V) COMP, CS to GND .......................................-0.3V to (V+ + 0.3V) LX Current ....................................................................0.45ARMS Continuous Power Dissipation (TA = +70C) 8-Pin SOT23 (derate 9.7mW/C above +70C).............777mW 8-Pin Thin QFN 3mm 3mm (derate 24.4mW/C above +70C)..............................................................1951mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-60C 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.
DC ELECTRICAL CHARACTERISTICS
(V+ = 3V, VOUT = 11V, L = 33H, COUT = 1F, CCOMP = 0.15F, RSENSE = 5, VCTRL = 1V, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Supply Voltage Undervoltage Lockout Threshold Quiescent Current Shutdown Supply Current Overvoltage Threshold OUT Input Bias Current Output Voltage Range ERROR AMPLIFIER CTRL to CS Regulation CS Input Bias Current CTRL Input Resistance CTRL Dual Mode Threshold CS Line Regulation COMP Pin Resistance to Ground COMP Output Voltage Swing OSCILLATOR Operating Frequency Minimum Duty Cycle Maximum Duty Cycle PWM mode Pulse skipping VCTRL = V+, VCS = GND 85 1.0 1.2 12 0 90 97 1.5 MHz % % Hysteresis = 25mV typical V+ = 2.6V to 5.5V, VCTRL = 3V Device in shutdown or overvoltage 7.5 0.3 VCTRL = 2V, V+ = 2.6V to 5.5V VCS = VCTRL /13.33 450 100 71 75 0.01 670 170 0.05 15 50 2.4 81 1 1100 240 mV/V A k mV %/V k V V+ rising, 40mV hysteresis typical V+ falling Not switching, VCTRL = VCS = V+ Switching, VCTRL = V+, VCS = GND VCTRL = GND TA = +25C TA = +85C 12.5 11.5 VCTRL > 0.25V VCTRL = GND V+ VDIODE 10 CONDITIONS MIN 2.6 2.15 2.10 2.38 2.34 0.25 1 0.3 0.3 13.25 12.25 20 0.01 14.0 13 30 1 12.5 TYP MAX 5.5 2.59 2.56 0.40 2 1 UNITS V V mA A V A V
V+ rising, 1V hysteresis typical V+ falling VOUT = 13V
2
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White LED Step-Up Converter in SOT23
DC ELECTRICAL CHARACTERISTICS (continued)
(V+ = 3V, VOUT = 11V, L = 33H, COUT = 1F, CCOMP = 0.15F, RSENSE = 5, VCTRL = 1V, 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 = 100mA VOUT = VLX = 13V Duty cycle = 65% TA = +25C TA = +85C 300 1.4 0.01 0.05 500 750 2.2 2 A mA CONDITIONS MIN TYP MAX UNITS
MAX1848
DC ELECTRICAL CHARACTERISTICS
(V+ = 3V, VOUT = 11V, L = 33H, COUT = 1F, CCOMP = 0.15F, RSENSE = 5, VCTRL = 1V, TA = -40C to +85C, unless otherwise noted. (Note 1)
PARAMETER Supply Voltage Undervoltage Lockout Threshold Quiescent Current Overvoltage Threshold OUT Input Bias Current Output Voltage Range ERROR AMPLIFIER CTRL to CS Regulation CS Input Bias Current CTRL Input Resistance CTRL Dual Mode Threshold COMP Pin Resistance to Ground COMP Output Voltage Swing OSCILLATOR Operating Frequency Maximum Duty Cycle N-CHANNEL SWITCH LX On-Resistance LX Current Limit ILX = 100mA Duty cycle = 65% 225 2.2 850 mA VCTRL = V+, VCS = GND 0.9 85 1.6 97 MHz % Hysteresis = 25mV typical Device in shutdown or overvoltage VCTRL = 2V, V+ = 2.6V to 5.5V VCS = VCTRL/13.33 450 100 7.5 0.3 65 85 1 1100 240 50.0 2.4 mV/V A k mV k V V+ rising, 40mV hysteresis typical V+ falling Not switching, VCTRL = VCS = V+ Switching, VCTRL = V+, VCS = GND V+ rising, 1V hysteresis typical V+ falling VOUT = 13V VCTRL > 0.25V VCTRL = GND V+ VDIODE 12.25 11.25 10 CONDITIONS MIN 2.6 2.15 2.10 MAX 5.5 2.59 2.56 0.4 2 14.00 13.25 30 1 12.25 UNITS V V mA V A V
Note 1: Limits are 100% production tested at TA = +25C. Limits over the operating temperature range are guaranteed through correlation using statistical quality control (SQC) methods.
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3
White LED Step-Up Converter in SOT23 MAX1848
Typical Operating Characteristics
(See Typical Application Circuit, V+ = 3V, ILED = 15mA, L = 33H, COUT = 1F, CCOMP = 0.15F, RSENSE = 5, CTRL = 1V, 2 LEDs, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. SUPPLY VOLTAGE
MAX1848 toc01
EFFICIENCY vs. LOAD CURRENT
2 LEDs 85 EFFICIENCY (%) 3 LEDs 80
MAX1848 toc02
SWITCHING FREQUENCY vs. SUPPLY VOLTAGE
MAX1848 toc03
90 IOUT = 15mA 88 EFFICIENCY (%) 2 LEDs 86
90
1.4 TA = +85C 1.3 TA = +25C TA = -40C 1.2
84 3 LEDs 82
75
SWITCHING FREQUENCY (MHz)
80 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
70 0 10 20 30 40 50 60 LOAD CURRENT (mA)
1.1 2.5 3.0 3.5 4.0 4.5 SUPPLY VOLTAGE (V) 5.0 5.5
POWER-UP WAVEFORMS
MAX1848 toc04
POWER-UP WAVEFORMS
MAX1848 toc05
SHUTDOWN WAVEFORMS
MAX1848 toc06
1V VCTRL 0 38mA IIN 0 15mA ILED tRISE = 15ms CCOMP = 0.15F 50ms/div 0 50ms/div VCTRL IIN
1V 0 38mA 0 IIN VCTRL
1V 0 38mA 0
ILED
15mA tRISE = 110ms CCOMP = 1F 0 20s/div ILED
15mA 0
SHUTDOWN WAVEFORMS
MAX1848 toc07
LINE-TRANSIENT RESPONSE
MAX1848 toc08
VCTRL TRANSIENT RESPONSE
3.50V
MAX1848 toc09
1V VCTRL 0 VIN VOUT VOUT VOUT EVENTUALLY DECAYS TO V+ = 3V 1.5V VCOMP 0 2s/div ILED CCOMP = 0.15F 5ms/div 6.3V 4.9V
VCTRL
2.0V 1.0V 6.7V 6.3V CCOMP = 0.15F 30mA
3.00V 6.25V 500mV/div
VOUT
15mA 10mA/div
ILED 15mA
20ms/div
4
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White LED Step-Up Converter in SOT23
Typical Operating Characteristics (continued)
(See Typical Application Circuit, V+ = 3V, ILED = 15mA, L = 33H, COUT = 1F, CCOMP = 0.15F, RSENSE = 5, CTRL = 1V, 2 LEDs, TA = +25C, unless otherwise noted.)
VCTRL TRANSIENT RESPONSE SWITCHING WAVEFORMS
MAX1848 toc11
MAX1848
MAX1848 toc10
VCTRL
2.0V 1.0V
VLX
6.5V 0 61mA
IL VOUT 6.7V 6.3V VIN_RIPPLE 30mA ILED CCOMP = 1F 20ms/div 15mA VOUT_RIPPLE
17mA
10mV/div
10mV/div
200ns/div
Pin Description
PIN 1 NAME COMP FUNCTION Compensation Pin for Error Amplifier. Connect capacitor from COMP to GND. Startup time is set by the capacitance connected to this pin (0.833ms for each 0.01F). VCOMP passively discharges to GND when in shutdown. Brightness/Shutdown Dual Mode Control Input. LED brightness and IC shutdown are controlled by the voltage on CTRL. Voltages between 250mV and 5.5V or (V+ + 2V), whichever is less, adjust the brightness from dim to bright, respectively. To put the IC into shutdown, drive below 100mV or connect to GND. Overvoltage Sense. When VOUT is greater than 13.25V, the internal N-channel MOSFET is turned off and VCOMP decays to GND. When VOUT drops below 12.25V, the IC will re-enter soft-start. Connect a 1F capacitor from OUT to GND. Inductor Connection. Drain of the internal high-voltage N-channel MOSFET. Power Ground. Source of the internal high-voltage N-channel MOSFET. Ground Current-Sense Feedback Input. Connect a resistor from this pin to GND to set the LED bias current. This pin regulates to 7.5% of VCTRL. Supply Voltage Input. The IC is powered from this pin. Input range is 2.6V to 5.5V. Bypass with a ceramic capacitor to GND.
2
CTRL
3 4 5 6 7 8
OUT LX PGND GND CS V+
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5
White LED Step-Up Converter in SOT23 MAX1848
Detailed Description
The MAX1848's high efficiency and small size make it ideally suited to drive series-connected LEDs. It operates as a boost DC-DC converter that controls output current rather than voltage. The MAX1848 provides even illumination by sending the same output current through each LED, eliminating the need for expensive factory calibration. The fast 1.2MHz 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 or a DAC to control both brightness and on/off. In shutdown, supply current is reduced to a low 0.3A. A programmable soft-start gradually illuminates the LEDs, reducing the inrush current during startup. startup time will increase by 0.833ms. The start time can be calculated by: t SOFT- START (MAX) = CCOMP x 1V 12A
Shutdown
The MAX1848 is put into shutdown when VCTRL is less than 100mV. In shutdown, supply current is reduced to 0.3A by powering down the entire IC except for the CTRL voltage detection circuitry. CCOMP is passively discharged during shutdown, allowing the device to reinitiate a soft-start whenever the device is enabled. When in shutdown, the internal N-channel FET does not switch, which leaves a current path between the input and the LEDs through the boost inductor and Schottky diode. The minimum forward voltage of the LED array must exceed the maximum V+ to ensure that the LEDs remain off in shutdown. Typical shutdown timing characteristics are shown in the Typical Operating Characteristics.
Soft-Start
The MAX1848 attains soft-start by charging C COMP gradually with a constant 12A current. When VCOMP rises above 1.25V, the internal MOSFET begins switching, but at a reduced duty cycle. When VCOMP rises above 2.25V, the duty cycle will be at its maximum. The maximum startup time is determined by the value of CCOMP. For every 0.01F connected to COMP, the
V+ LX PWM CONTROL PGND 13.25V
Overvoltage Protection
Overvoltage protection occurs when VOUT is above 13.25V. The protection circuitry stops the internal MOSFET from switching and causes VCOMP to decay to GND. The device comes out of overvoltage lockout and into soft-start when VOUT falls below 12.25V.
Design Procedure
Adjusting LED Current
Adjusting the MAX1848's output current will change 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 13.33 x RSENSE
OUT 1.2MHz 8 CS
SLOPE CURRENT SENSE CTRL 617k 50k SHUTDOWN 170mV COMP
The VCTRL voltage range for adjusting output current is 250mV to (V+ + 2V) or 5.5V, whichever is less. To set the maximum current, calculate RSENSE when VCTRL is at its maximum. Power dissipation in RSENSE is typically less than 5mW; therefore, a standard chip resistor is sufficient.
Capacitor Selection
The exact values of input and output capacitors are not critical. The typical value for the input capacitor is 3.3F, and the typical value for the output capacitor is 1.0F. Larger value capacitors can be used to reduce input and output ripple, but at the expense of size and higher cost.
MAX1848
GND
Figure 1. Functional Diagram 6 _______________________________________________________________________________________
White LED Step-Up Converter in SOT23
The output current and the number of LEDs in each leg affect the capacitance of CCOMP. Table 1 shows the minimum CCOMP values needed to stabilize the converter in worst-case conditions. If further stability analysis is required, note that the error amplifier has 50A/V transconductance.
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 evaluation kit (MAX1848EV kit) is available to aid 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 inductor's traces, away from CS. V+'s bypass capacitor (CIN) should be placed as close to the IC as possible. PGND and GND should be connected in only one place as close to the IC as possible. The ground connections of C IN and C OUT should be as close together as possible. The traces from V+ to the inductor and from the Schottky diode to the LEDs may be longer. Refer to the MAX1848 EV kit for an example of proper layout.
MAX1848
Inductor Selection
The value of the inductor depends on the maximum output current to the LEDs. See Table 1 for inductance values and peak current ratings for the inductor.
Schottky Diode Selection
The MAX1848's high-switching frequency demands a high-speed rectification diode. A Schottky diode is required due to their 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, respectively. In addition, the diode's reverse breakdown voltage must exceed VOUT. IDIODE(RMS) IOUT x IPEAK
Applications Information
Connecting Four or Six LEDs
The MAX1848 can drive one, two, or three legs of LEDs (Figure 2) as long as the total number of LEDs does not exceed six. Each leg must contain the same number of LEDs and the same sense-resistor value. Adding the second or third leg does not affect the sense-resistor value (see the Adjusting LED Current section). Three legs of two LEDs is more efficient than two legs of three LEDs (see Efficiency Graphs in the Typical Operating Characteristics); however, a third sense resistor is needed. Multiple legs can have slight current mismatches due to component tolerances.
2.6V to 5.5V
10H
3.3F LX V+ MAX1848 LOGIC OR DAC CTRL COMP 0.01F PGND GND CS OUT
D1
1F
25
RSENSE 5
30
Figure 2. Six LEDs in 3 x 2 Configuration
Table 1. Component Selection
ILED (mA) 12 20 40 60 NO. OF LEDs 3 2 3 2 3 2 3 2 CCOMP (F) 0.220 0.100 0.150 0.068 0.100 0.047 0.068 0.01 INDUCTOR L (H) 56 33 15 10 IPEAK(mA) 80 130 260 375
Chip Information
TRANSISTOR COUNT: 1290
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7
White LED Step-Up Converter in SOT23 MAX1848
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.)
SOT23, 8L.EPS
8
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White LED Step-Up Converter in SOT23
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.)
6, 8, &10L, QFN THIN.EPS
MAX1848
PACKAGE OUTLINE, 6, 8 & 10L, QFN THIN (DUAL), EXPOSED PAD, 3x3x0.80 mm
21-0137
C
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) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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