MAX8822 Maxim Integrated Products, MAX8822 Datasheet - Page 11

• Manufacturer Part Number: MAX8822
• Description: Ultra-Efficient Negative Charge-Pump LED Driver
• Manufacturer: Maxim Integrated Products

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Table 1. ENLED Pulse Count and Programmed LED_ Current
* Rising edge.
** If ENLED is pulsed more than 31 times, the pulse count restarts at 1 on the 32
The MAX8822 internally generates a PWM signal to
obtain higher resolution at lower currents. As LED cur-
rent is set below 6.4mA, the IC adjusts not only LED DC
current, but the duty cycle is controlled by the PWM
signal. The frequency of the PWM dimming signal is set
at 1kHz with a minimum duty cycle of 1/16 to avoid the
LED flickering effect to human eyes. Table 2 shows the
current level and the corresponding duty cycle.
Similar to the LED dimming control structure, the
MAX8822 uses serial-pulse control to program the out-
put voltages of LDO1 and LDO2 to one of sixteen differ-
ent combinations. To program the LDO voltages, pulse
ENLDO low (500ns to 500µs pulse width), as shown in
Figure 2. An internal register accumulates the pulse
count on the rising edge of the ENLDO pulse. An initial
pulse (t
down. See Table 3 for LDO voltage combinations and
the corresponding ENLDO pulse count. When the
desired pulse count is met, hold ENLDO high for at
least 2.5ms (t
value and program the LDO output voltages at the
desired level. If ENLDO is pulsed for more than 15
counts, LDO1 and LDO2 are off when the count is
Driver with Dual LDOs in 3mm x 3mm Thin QFN
ENLED PULSE
COUNT*
INIT
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
9
LDO1/LDO2 Output Voltage Control
) is required to enable the LDOs from shut-
Ultra-Efficient Negative Charge-Pump LED
HOLD
______________________________________________________________________________________
) to internally latch the pulse-count
PROGRAMMED LED_ CURRENT
Low LED Current Levels
(mA)
24.0
22.4
20.8
19.2
17.6
16.0
14.4
12.8
11.2
9.6
8.0
6.4
5.6
4.8
4.0
3.2
ENLED PULSE COUNT*
latched (the count does not roll over). To program new
LDO_ output voltages, follow the previous sequence
from the beginning. Do not include the initial t
when programming LDO voltages unless enabling the
LDOs from shutdown (the OFF setting is not shutdown).
Pulling ENLDO low for greater than 2.5ms places the
LDOs in shutdown.
Table 2. Internal PWM Duty Cycle vs. LED
Set Current
DUTY CYCLE
(n/16th)
31**
17
18
19
20
21
22
23
24
25
26
27
28
29
30
—
16
14
12
10
16
14
12
10
16
14
nd
rising edge.
I
LED
6.4
5.6
4.8
4.0
3.2
2.8
2.4
2.0
1.6
1.4
(mA)
PROGRAMMED LED_ CURRENT
DUTY CYCLE
(n/16th)
12
10
8
7
6
5
4
3
2
1
(mA)
2.8
2.4
2.0
1.6
1.4
1.2
1.0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
—
I
LED
INIT
1.2
1.0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
(mA)
pulse
11