MAX8821 MAXIM [Maxim Integrated Products], MAX8821 Datasheet - Page 17

• Manufacturer Part Number: MAX8821
• Description: White LED Charge Pump with Mono Class D Audio Amp and Dual LDO
• Manufacturer: MAXIM [Maxim Integrated Products]

full-scale output current while automatically reducing
current gradually above +40°C in accordance with
popular LED ratings. The on-chip derating feature can
be enabled, or disabled, using the I
When V
LED plus the 0.15V headroom of the current regulator,
the LED current returns through ground. If this condi-
tion is satisfied for all six white LEDs the charge pump
remains inactive. When the input voltage drops so that
the current regulator head room cannot be maintained
for any of the individual white LEDs, the inverting
charge pump activates and generates a voltage on
NEG that is no greater than 5V below V
regulator contains circuitry that detects when it is in
dropout and switches that current regulator return path
from GND to NEG. Since this is done on an LED-by-
LED basis, the LED current is switched for only the indi-
vidual LED requiring higher voltage, thus minimizing
power consumption.
The LED drivers in the MAX8821 provide ramp-up and
ramp-down of LED current for smooth transitions
between different brightness settings. A controlled
ramp is used when the LED current level is changed,
when the LEDs are enabled, and when the LEDs are
disabled. The LED currents ramp up and down smooth-
ly on a pseudo-log scaling of the LED current sources
(Figures 3 and 4). Each LED source has an individual
ramp control making it possible to ramp different LEDs
at different rates. The ramp-up and ramp-down (t
LED current times are controlled by the LED_RU and
LED_RD control bits (Tables 8, 9, and 10). The
LED_RP_EN bit enables and disables the ramps. The
MAX8821 increases/decreases the current one step
every t
The current regulators for LED5 and LED6 feature a
blink function. The OFF and ON time for LED5 and
LED6 can be set using the I
Figure 5.
To combine the ramp function together with the blink
timer for LED5 and/or LED6, special timing considera-
tions need to be fulfilled. It is recommended to keep the
ramp-up timer shorter than the ON blink timer and the
ramp-down timer shorter than the OFF blink timer.
Failing to comply with these timing constraints results in
the LED_ not reaching the programmed current
(LED_[4:0], Tables 6 and 7) during the ON time and the
RAMP
IN
Combining Blink Timer and Ramp Function
is higher than the forward voltage of the white
White LED Charge Pump with Mono Class D
/32 until the desired current is reached.
Ramp-Up and Ramp-Down Function
______________________________________________________________________________________
Current Regulator Switchover
2
C interface (Table 11). See
2
C interface.
IN
. Each current
Blink Timer
RAMP
)
Audio Amp and Dual LDO
LED_ current not returning to 0mA during the off time.
See the following equations to ensure proper operation:
where LED_[4:0] is the programmed current set by I
(see Tables 6 and 7).
Figure 6 shows combining ramp function and blink timer.
If any LED fails as an open circuit, the corresponding
LED_ is internally connected to ground and the charge
pump is enabled. To disable the corresponding current
regulator, short any unused LED_ to IN1. The MAX8821
contains special circuitry to detect this condition and
disables the corresponding current regulator to avoid
wasting battery power.
The MAX8821 contains a derating function that auto-
matically limits the LED current at high temperatures in
accordance with the recommended derating curve of
popular white LEDs. The derating function enables the
safe usage of higher LED current at room temperature,
thus reducing the number of LEDs required to backlight
the display. The derating circuit limits the LED current
by reducing the LED current above +40°C at approxi-
mately 1.67%/°C. The temperature derating function is
enabled/disabled using the I
is disabled.
Figure 3. Ramp-Up Behavior
Figure 4. Ramp-Down Behavior
I
I
LED_
LED_
= 25.6mA
I
= 25.6mA
I
LED_
LED_
= 0
= 0
t
t
ON BLINK
OFF BLINK
_
LED Short- and Open-Circuit Protection
t
262ms
_
t
262ms
RAMP
RAMP
=
=
t
524ms
RAMP
t
524ms
RAMP
≥
≥
t
LED RU
t
=
=
LED RD
32
32
_
_
2
C interface and by default
t
1048ms
t
1048ms
RAMP
RAMP
(
Temperature Derating
LED
(
LED
=
=
_[ : ]
_[ : ]
4 0 1
4 0 1
+
+
)
)
t
2097ms
t
2097ms
RAMP
RAMP
2
17
=
=
C