19-2290; Rev 2; 3/04
1.5x/2x High-Efficiency White LED
Charge Pumps
General Description
The MAX1910/MAX1912 power LEDs with a regulated
output voltage or current (up to 120mA) from an unreg-
ulated input supply (2.7V to 5.3V). These are complete
DC-DC converters requiring only four small ceramic
capacitors and no inductors. Input ripple is minimized
by a unique regulation scheme that maintains a fixed
750kHz switching frequency over a wide load range.
Also included are logic-level shutdown and soft-start to
reduce input current surges at startup.
The MAX1910 has two automatically selected operating
modes: 1.5x and 2x. 1.5x mode improves efficiency at
higher input voltages, while 2x mode maintains regula-
tion at lower input voltages. The MAX1912 operates
only in 1.5x mode.
The MAX1910 and the MAX1912 are available in a
space-saving 10-pin µMAX package.
Features
♦
High-Efficiency 1.5x/2x Charge Pumps
♦
Low Input Ripple with 750kHz Operation
♦
200mV Current-Sense Threshold Reduces
Power Loss
♦
Current- or Voltage-Regulated Charge Pump
♦
Up to 120mA Output Current
♦
No Inductors Required
♦
Small Ceramic Capacitors
♦
Regulated ±5% LED Current
♦
Load Disconnected in Shutdown
♦
1µA Shutdown Current
♦
Small 10-Pin µMAX Package
MAX1910/MAX1912
Applications
White LED Backlighting
Cellular Phones
PDAs
Digital Still Cameras
MP3 Players
Backup-Battery Boost Converters
PART
MAX1910EUB
MAX1912EUB
Ordering Information
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
10 µMAX
10 µMAX
Typical Operating Circuit
TOP VIEW
V
IN
IN1
C1+
C1
C
IN
C1-
C2+
C2
C2-
GND
IN2
SHDN
OUT
Pin Configuration
GND 1
IN1
C2-
2
3
4
5
10 SET
9
C1-
IN2
C2+
SHDN
MAX1910
MAX1912
8
7
6
MAX1910
MAX1912
SET
C
OUT
C1+
OUT
µMAX
________________________________________________________________
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.
1.5x/2x High-Efficiency White LED
Charge Pumps
MAX1910/MAX1912
ABSOLUTE MAXIMUM RATINGS
IN1, IN2, OUT,
SHDN,
SET to GND …………………-0.3V to +6V
C1-, C2-, to GND..................................................-0.3V, V
IN
+ 1V
C1+, C2+ to GND..........-0.3V, greater of V
OUT
+ 1V or V
IN
+ 1V
OUT Short-Circuit to GND ..........................................Continuous
Continuous Power Dissipation (T
A
= +70°C)
10-Pin µMAX (derate 5.6 mW/°C above +70°C) ..........444mW
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-65°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.
ELECTRICAL CHARACTERISTICS
(V
IN
= 3.6V, GND = 0,
SHDN
= SET = IN, C
IN
= 2.2µF, C1 = C2 = 0.47µF, C
OUT
= 2.2µF,
T
A
= 0°C to +85°C.
Typical values are at
T
A
= +25°C, unless otherwise noted.)
PARAMETER
Input Voltage Operating Range
Undervoltage Lockout Threshold
Undervoltage Lockout Hysteresis
SET Regulation Point
MAX1910 Current Regulation
MAX1912 Current Regulation
Maximum Output Current
No Load Input Current
Supply Current in Shutdown
Output Leakage Current in Shutdown
Switching Frequency
Switching Frequency Temperature
Coefficient
SET Input Current
SHDN
Input Current
SHDN
Input Voltage Low
SHDN
Input Voltage High
Thermal-Shutdown Threshold
SHDN
= 0 or 5.5V
2.7V < V
IN
< 5.3V
2.7V < V
IN
< 5.3V
Rising temperature, 15°C hysteresis typical
1.6
160
Output current change for 2.7V < V
OUT
< 5V
Output current change for 3V < V
OUT
< 5V
MAX1910 V
IN
= 2.7V
MAX1912 V
IN
= 3.6V
V
IN
= 3.6V
V
IN
= 5.3V, V
OUT
= 0,
SHDN
= 0
V
IN
= 3.6V,
SHDN
= 0
V
IN
= 3.6V
f = 750kHz
625
80
120
1.5
0.1
0.1
750
250
1
100
1
0.4
2.5
10
10
875
0.19
Both rising and falling edges
CONDITIONS
MIN
2.7
2.2
35
0.2
0.5
0.5
0.21
TYP
MAX
5.3
2.5
UNITS
V
V
mV
V
%/V
%/V
mA
mA
µA
µA
kHz
ppm/°C
nA
µA
V
V
°C
ELECTRICAL CHARACTERISTICS
(V
IN
= 3.6V, GND = 0,
SHDN
= SET = IN, C
IN
= 2.2µF, C1 = C2 = 0.47µF, C
OUT
= 2.2µF,
T
A
= -40°C to +85°C,
unless otherwise
noted.) (Note 1)
PARAMETER
Input Voltage Operating Range
Undervoltage Lockout Threshold
Maximum Output Current
Supply Current in Shutdown
Both rising and falling edges
MAX1910 V
IN
= 2.7V
MAX1910 V
IN
= 3.6V
V
IN
= 5.3V, V
OUT
= 0,
SHDN
= 0
CONDITIONS
MIN
2.7
2.2
80
120
10
MAX
5.3
2.5
UNITS
V
V
mA
µA
2
_______________________________________________________________________________________
1.5x/2x High-Efficiency White LED
Charge Pumps
ELECTRICAL CHARACTERISTICS (continued)
(V
IN
= 3.6V, GND = 0,
SHDN
= SET = IN, C
IN
= 2.2µF, C1 = C2 = 0.47µF, C
OUT
= 2.2µF,
T
A
= -40°C to +85°C,
unless otherwise
noted.) (Note 1)
PARAMETER
Output Leakage Current in Shutdown
SET Regulation Point
SET Input Current
SHDN
Input Current
SHDN
Input Voltage Low
SHDN
Input Voltage High
SHDN
= 0 or 5.5V
2.7V < V
IN
< 5.3V
2.7V < V
IN
< 5.3V
1.6
CONDITIONS
V
IN
= 3.6V,
SHDN
= 0
0.19
MIN
MAX
10
0.21
100
1
0.4
UNITS
µA
V
nA
µA
V
V
MAX1910/MAX1912
Note 1:
Limits to -40°C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 2, V
IN
= 3.3V, T
A
= +25°C, unless otherwise noted.)
INPUT AND OUTPUT VOLTAGE RIPPLE
MAX1910/12 toc01
INPUT AND OUTPUT VOLTAGE RIPPLE
MAX1910/12 toc02
START-UP INPUT CURRENT AND
OUTPUT VOLTAGE
MAX1910/12 toc03
5V/div
V
SHDN
2V/div
V
IN
CIRCUIT OF FIGURE 7
DRIVING 4 LEDS (60mA)
V
IN
20mV/div
20mV/div
V
OUT
50mA/div
V
OUT
V
OUT
I
IN
I
OUT
= 60mA
1µs/div
1µs/div
1ms/div
QUIESCENT CURRENT vs. INPUT VOLTAGE
MAX1910/12 toc04
LED CURRENT vs. INPUT VOLTAGE
MAX1910/12 toc05
INTENSITY CHANGE STEP RESPONSE
MAX1910/12 toc06
4.0
3.5
QUIESCENT CURRENT (mA)
3.0
2.5
2.0
1.5
1.0
0.5
0
0
140
120
LED CURRENT (mA)
100
80
V
LOGIC
2V/div
V
SET
60
40
20
I
OUT
0
CIRCUIT OF FIGURE 9
100mV/div
60mA
20mA
40µs/div
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
INPUT VOLTAGE (V)
2.7
3.0
3.3
3.6
3.9
4.2
4.5
INPUT VOLTAGE (V)
_______________________________________________________________________________________
3
1.5x/2x High-Efficiency White LED
Charge Pumps
MAX1910/MAX1912
Typical Operating Characteristics (continued)
(Circuit of Figure 2, V
IN
= 3.3V, T
A
= +25°C, unless otherwise noted.)
EFFICIENCY vs. INPUT VOLTAGE
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
2.7
3.0
3.3
3.6
3.9
4.2
4.5
INPUT VOLTAGE (V)
CIRCUIT OF FIGURE 2
MAX1910 4 WHITE LEDs
I
OUT
= 60mA
MAX1910/12 toc07
INPUT CURRENT vs. INPUT VOLTAGE
DRIVING 4 LEDS
MAX1910/12 toc08
100
140
120
100
CURRENT (mA)
80
60
40
20
0
2.7
3.0
3.3
3.6
3.9
4.2
CIRCUIT OF FIGURE 2
MAX1910
I
LOAD
= 60mA
4.5
INPUT VOLTAGE (V)
Pin Description
PIN
1
2
3
4
5
6
7
8
9
10
NAME
GND
IN1
C2-
C1+
OUT
SHDN
C2+
IN2
C1-
SET
Ground
Supply Voltage Input. Connect to IN2. Bypass to GND with a 2.2µF ceramic capacitor.
Transfer Capacitor 2 Connection, Negative Side
Transfer Capacitor 1 Connection, Positive Side
Output. Bypass to GND with a 2.2µF ceramic capacitor.
Shutdown Input. Drive low to turn off the device and disconnect the load from the input. OUT is high
impedance in shutdown. Drive high or connect to IN for normal operation.
Transfer Capacitor 2 Connection, Positive Side
Supply Voltage Input. Connect to IN1.
Transfer Capacitor 1 Connection, Negative Side
SET programs the output current with a resistor from SET to GND. SET can also program the output
voltage with a resistor-divider between OUT and GND.
FUNCTION
Detailed Description
The MAX1910/MAX1912 are complete charge-pump
boost converters requiring only four small ceramic
capacitors. They employ a 750kHz fixed-frequency
50% duty-cycle clock.
The MAX1910 has two modes of operation: 1.5x and
2x. Each mode has two phases: charge and transfer
(see Figure 1). In 1.5x mode charge phase, transfer
capacitors C1 and C2 charge in series from the input
voltage. In transfer phase, C1 and C2 are configured in
parallel and connected from OUT to IN, transferring
charge to C
OUT
. If this system were allowed to operate
unregulated and unloaded, it would generate an output
voltage 1.5 times the input voltage (hence the terms
4
“fractional charge pump” and “1.5x mode”). When the
input voltage drops sufficiently, the operating mode
shifts from a 1.5x fractional charge pump to a 2x dou-
bler. C2 is not used in doubler mode. The device transi-
tions out of doubler mode when V
IN
is greater than
~75% of V
OUT
for more than 32 clock cycles (at full
load). The MAX1912 operates only in 1.5x charge-
pump mode.
Output Regulation
The output is regulated by controlling the rate at which
the transfer capacitors are charged. The switching fre-
quency and duty cycle are constant, so the output
noise spectrum is predictable. Input and output ripple
are much smaller in value than with other regulating
_______________________________________________________________________________________
1.5x/2x High-Efficiency White LED
Charge Pumps
charge-pump topologies because the charge trans-
ferred per cycle is only the amount required to supply
the output load.
LED current matching by raising the ballast resistance
while maintaining a 200mV V
SET
. The increased ballast
resistance tolerates wider LED mismatch, but reduces
efficiency and raises the minimum input voltage
required for regulation.
Yet another method of biasing LEDs is shown in Figure
5. In this case, the current through the complete paral-
lel combination of LEDs is set by R5. R1–R4 are only
used to compensate for LED variations. This method of
biasing is useful for parallel LED arrays that do not
allow connection to individual LEDs.
MAX1910/MAX1912
Soft-Start
The MAX1910/MAX1912 include soft-start circuitry to
limit inrush current at turn-on. When starting up with the
output voltage at zero, the output capacitor charges
through a ramped current source, directly from the
input with no charge-pump action until the output volt-
age is near the input voltage. If the output is shorted to
ground, the part remains in this mode without damage
until the short is removed.
Once the output capacitor charges to the input voltage,
the charge-pumping action begins. Startup surge cur-
rent is minimized by ramping up charge on the transfer
capacitors. As soon as regulation is reached, soft-start
ends and the part operates normally. If the SET voltage
reaches regulation within 2048 clock cycles (typically
2.7ms), the part begins to run in normal mode. If the
SET voltage is not reached by 2048 cycles, the soft-
start sequence is repeated. The devices continue to
repeat the soft-start sequence until the SET voltage
reaches the regulation point.
Setting Output Voltage
The MAX1910 has a SET voltage threshold of 0.2V.
Output voltage can be set by connecting a resistor volt-
age-divider as shown in Figure 6. The output voltage is
adjustable from V
IN
to 5V. To set the output voltage,
select a value for R2 that is less than 20kΩ, then solve
for R1 using the following equation:
V
R1
=
R2
OUT
- 1
0.2
Capacitor Selection
Use low-ESR ceramic capacitors. Recommended values
are 0.47µF for the transfer capacitors, 2.2µF to 10µF for
the input capacitor, and 2.2µF to 4.7µF for the output
capacitor. To ensure stability over a wide temperature
range, ceramic capacitors with an X7R dielectric are rec-
ommended. Place these capacitors as close to the IC as
possible. Increasing the value of the input and output
capacitors further reduces input and output ripple. With
a 10µF input capacitor and a 4.7µF output capacitor,
input ripple is less than 5mV peak-to-peak and output
ripple is less than 15mV peak-to-peak for 60mA of output
current. A constant 750kHz switching frequency and
fixed 50% duty cycle create input and output ripple with
a predictable frequency spectrum.
Decoupling the input with a 1Ω resistor (as shown in
Figures 2–9) improves stability when operating from low-
impedance sources such as high-current laboratory
bench power supplies. This resistor can be omitted
when operating from higher impedance sources such
as lithium or alkaline batteries.
For some designs, such as an LED driver, input ripple is
more important than output ripple. Input ripple depends
on the source supply’s impedance. Adding a lowpass fil-
ter to the input further reduces ripple. Figure 7 shows a C-
R-C filter used to reduce input ripple. With 10µF-1Ω-10µF,
input ripple is less than 1mV when driving a 60mA load.
Shutdown Mode
When driven low,
SHDN
turns off the charge pump.
This reduces the quiescent current to approximately
0.1µA. The output is high impedance in shutdown.
Drive
SHDN
high or connect to IN for normal operation.
Thermal Shutdown
The MAX1910/MAX1912 shut down when their die tem-
perature reaches +160°C. Normal operation continues
after the die cools by 15°C. This prevents damage if an
excessive load is applied or the output is shorted to
ground.
Design Procedure
Setting Output Current
The MAX1910/MAX1912 have a SET voltage threshold
of 0.2V, used for LED current regulation (Figure 2). The
current through the resistor and LED is:
I
LED
= 0.2/R
SET
If additional matching LEDs with ballast resistors are
connected to the output as in Figure 2, the current
through each additional LED is the same as that in the
regulated LED.
In Figure 2, total LED current depends somewhat on
LED matching. Figure 3 shows a connection that regu-
lates the average of all the LED currents to reduce the
impact of mismatched LEDs. Figure 4’s circuit improves
_______________________________________________________________________________________
5
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