1.5x/2x High-Efficiency White LED
Charge Pumps
Typical Operating Characteristics (continued)
(Circuit of Figure 2, V IN = 3.3V, T A = +25°C, unless otherwise noted.)
INPUT CURRENT vs. INPUT VOLTAGE
100
90
80
70
60
50
40
30
EFFICIENCY vs. INPUT VOLTAGE
140
120
100
80
60
40
DRIVING 4 LEDS
20
10
0
CIRCUIT OF FIGURE 2
MAX1910 4 WHITE LEDs
I OUT = 60mA
20
0
CIRCUIT OF FIGURE 2
MAX1910
I LOAD = 60mA
2.7
3.0
3.3
3.6
3.9
4.2
4.5
2.7
3.0
3.3
3.6
3.9
4.2
4.5
INPUT VOLTAGE (V)
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
FUNCTION
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.
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
“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
4
_______________________________________________________________________________________
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