MAX1910/MAX1912
1.5x/2x High-Efficiency White LED
Charge Pumps
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charge-pump topologies because the charge trans-
ferred per cycle is only the amount required to supply
the output load.
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.
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
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.
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:
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.
