19-2028; Rev 2; 8/05
White LED Step-Up Converter in SOT23
General Description
The MAX1848 drives white LEDs with a constant cur-
rent 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, high-
voltage, low R
DSON
N-channel MOSFET switch for high
efficiency and maximum battery life. A single analog
voltage Dual Mode
™
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 soft-
start eliminates inrush current during startup.
The MAX1848 is available in space-saving 8-pin thin
QFN (3mm
✕
3mm) and 8-pin SOT23 packages.
Features
♦
Constant Current Regulation for Uniform
Illumination
♦
High 87% Efficiency
♦
Analog or Logic Control of LED Intensity
♦
0.8W Output Power with Internal High-Voltage
MOSFET Switch
♦
Small, Low-Profile External Components
♦
2.6V to 5.5V Input Range
♦
13V Maximum Output with Overvoltage Protection
♦
Optimized for Low Input Ripple
♦
Programmable Soft-Start
♦
0.3µA Shutdown Current
♦
Small 8-pin Thin QFN (3mm
✕
3mm) and
8-Pin SOT23 Packages
MAX1848
Ordering Information
PART
MAX1848EKA-T
MAX1848ETA-T
MAX1848ETA+T
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
PIN-
PACKAGE
8 SOT23
8 Thin QFN
(3mm
✕
3mm)
8 Thin QFN
(3mm
✕
3mm)
TOP
MARK
AAIM
ACR
ACR
Applications
Cell Phones and Smart Phones
PDAs, Palmtops, and Wireless Handhelds
e-Books and Subnotebooks
White LED Display Backlighting
+
Denotes lead-free package.
Note:
Hand soldering is not recommended for the MAX1848
SOT23 package.
Typical Application Circuit
2.6V to 5.5V
C
IN
3.3μF
V+
MAX1848
LOGIC
OR DAC
C
COMP
0.15μF
CTRL
COMP
PGND
GND
SOT23
Pin Configuration
TOP VIEW
L1
33μH
D1
LX
OUT
C
OUT
1μF
+
COMP
1
CTRL
OUT
2
8
7
V+
CS
GND
PGND
COMP
CTRL
OUT
LX
1
2
+
8
7
V+
CS
GND
PGND
MAX1848
3
6
5
3
4
LX
4
MAX1848
6
5
CS
R
SENSE
5Ω
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 ....................................................................0.45A
RMS
Continuous Power Dissipation (T
A
= +70°C)
8-Pin SOT23 (derate 9.7mW/°C above +70°C).............777mW
8-Pin Thin QFN 3mm
✕
3mm (derate 24.4mW/°C
above +70°C)..............................................................1951mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-60°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°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.
DC ELECTRICAL CHARACTERISTICS
°
°
(V+ = 3V, V
OUT
= 11V, L = 33µH, C
OUT
= 1µF, C
COMP
= 0.15µF, R
SENSE
= 5Ω, V
CTRL
= 1V,
T
A
=
0°
C to +85°C,
unless otherwise
noted. Typical values are at T
A
= +25°C.)
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
V
CTRL
= V+, V
CS
= GND
85
1.0
1.2
12
0
90
97
1.5
MHz
%
%
Hysteresis = 25mV typical
V+ = 2.6V to 5.5V, V
CTRL
= 3V
Device in shutdown or overvoltage
7.5
0.3
V
CTRL
= 2V, V+ = 2.6V to 5.5V
V
CS
= V
CTRL
/13.33
450
100
71
75
0.01
670
170
0.05
15
50
2.4
81
1
1100
240
mV/V
µA
kΩ
mV
%/V
kΩ
V
V+ rising, 40mV hysteresis typical
V+ falling
Not switching, V
CTRL
= V
CS
= V+
Switching, V
CTRL
= V+, V
CS
= GND
V
CTRL
= GND
T
A
= +25°C
T
A
= +85°C
12.5
11.5
V
CTRL
> 0.25V
V
CTRL
= GND
V+ -
V
DIODE
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
µA
V
µA
V
V+ rising, 1V hysteresis typical
V+ falling
V
OUT
= 13V
2
_______________________________________________________________________________________
White LED Step-Up Converter in SOT23
DC ELECTRICAL CHARACTERISTICS (continued)
°
°
(V+ = 3V, V
OUT
= 11V, L = 33µH, C
OUT
= 1µF, C
COMP
= 0.15µF, R
SENSE
= 5Ω, V
CTRL
= 1V,
T
A
=
0°C to +85°C,
unless otherwise
noted. Typical values are at T
A
= +25°C.)
PARAMETER
N-CHANNEL SWITCH
LX On-Resistance
LX Leakage Current
LX Current Limit
I
LX
= 100mA
V
OUT
= V
LX
= 13V
Duty cycle = 65%
T
A
= +25°C
T
A
= +85°C
300
1.4
0.01
0.05
500
750
2.2
2
Ω
µA
mA
CONDITIONS
MIN
TYP
MAX
UNITS
MAX1848
DC ELECTRICAL CHARACTERISTICS
°
°
(V+ = 3V, V
OUT
= 11V, L = 33µH, C
OUT
= 1µF, C
COMP
= 0.15µF, R
SENSE
= 5Ω, V
CTRL
= 1V,
T
A
=
-40°C to +85°C,
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
I
LX
= 100mA
Duty cycle = 65%
225
2.2
850
Ω
mA
V
CTRL
= V+, V
CS
= GND
0.9
85
1.6
97
MHz
%
Hysteresis = 25mV typical
Device in shutdown or overvoltage
V
CTRL
= 2V, V+ = 2.6V to 5.5V
V
CS
= V
CTRL
/13.33
450
100
7.5
0.3
65
85
1
1100
240
50.0
2.4
mV/V
µA
kΩ
mV
kΩ
V
V+ rising, 40mV hysteresis typical
V+ falling
Not switching, V
CTRL
= V
CS
= V+
Switching, V
CTRL
= V+, V
CS
= GND
V+ rising, 1V hysteresis typical
V+ falling
V
OUT
= 13V
V
CTRL
> 0.25V
V
CTRL
= GND
V+ -
V
DIODE
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
µA
V
Note 1:
Limits are 100% production tested at T
A
= +25°C. Limits over the operating temperature range are guaranteed through cor-
relation using statistical quality control (SQC) methods.
_______________________________________________________________________________________
3
White LED Step-Up Converter in SOT23
MAX1848
Typical Operating Characteristics
(See
Typical Application Circuit,
V+ = 3V, I
LED
= 15mA, L = 33µH, C
OUT
= 1µF, C
COMP
= 0.15µF, R
SENSE
= 5Ω, CTRL = 1V, 2 LEDs,
T
A
= +25°C, 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
I
OUT
= 15mA
88
EFFICIENCY (%)
2 LEDs
86
90
1.4
T
A
= +85°C
1.3
T
A
= +25°C
T
A
= -40°C
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
V
CTRL
0
38mA
I
IN
0
15mA
I
LED
t
RISE
= 15ms
C
COMP
= 0.15μF
50ms/div
0
50ms/div
V
CTRL
I
IN
1V
0
38mA
0
I
IN
V
CTRL
1V
0
38mA
0
I
LED
15mA
t
RISE
= 110ms
C
COMP
= 1μF
0
20μs/div
I
LED
15mA
0
SHUTDOWN WAVEFORMS
MAX1848 toc07
LINE-TRANSIENT RESPONSE
MAX1848 toc08
V
CTRL
TRANSIENT RESPONSE
3.50V
MAX1848 toc09
1V
V
CTRL
0
V
IN
V
OUT
V
OUT
V
OUT
EVENTUALLY
DECAYS TO V+ = 3V
1.5V
V
COMP
0
2μs/div
I
LED
C
COMP
= 0.15μF
5ms/div
6.3V
4.9V
V
CTRL
2.0V
1.0V
6.7V
6.3V
C
COMP
= 0.15μF
30mA
3.00V
6.25V
500mV/div
V
OUT
15mA
10mA/div
I
LED
15mA
20ms/div
4
_______________________________________________________________________________________
White LED Step-Up Converter in SOT23
MAX1848
Typical Operating Characteristics (continued)
(See
Typical Application Circuit,
V+ = 3V, I
LED
= 15mA, L = 33µH, C
OUT
= 1µF, C
COMP
= 0.15µF, R
SENSE
= 5Ω, CTRL = 1V, 2 LEDs,
T
A
= +25°C, unless otherwise noted.)
V
CTRL
TRANSIENT RESPONSE
SWITCHING WAVEFORMS
MAX1848 toc11
MAX1848 toc10
V
CTRL
2.0V
1.0V
V
LX
6.5V
0
61mA
I
L
V
OUT
6.7V
6.3V
V
IN_RIPPLE
30mA
I
LED
C
COMP
= 1μF
20ms/div
15mA
V
OUT_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 0.01µF). V
COMP
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 V
OUT
is greater than 13.25V, the internal N-channel MOSFET is turned
off and V
COMP
decays to GND. When V
OUT
drops below 12.25V, the IC will re-enter soft-start.
Connect a 1µF 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 V
CTRL
.
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+
_______________________________________________________________________________________
5
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