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White LED Charge Pump with Mono Class D
Audio Amp and Dual LDO
full-scale output current while automatically reducing
current gradually above +40°C in accordance with
LED_ current not returning to 0mA during the off time.
See the following equations to ensure proper operation:
popular LED ratings. The on-chip derating feature can
be enabled, or disabled, using the I 2 C interface.
t ON _ BLINK ≥
t LED _ RU
32
( LED _[ 4 : 0 ] + 1 )
Current Regulator Switchover
When V IN is higher than the forward voltage of the white
LED plus the 0.15V headroom of the current regulator,
t OFF _ BLINK ≥
t LED _ RD
32
( LED _[ 4 : 0 ] + 1 )
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 IN . Each current
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.
Ramp-Up and Ramp-Down Function
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 RAMP )
LED current times are controlled by the LED_RU and
where LED_[4:0] is the programmed current set by I 2 C
(see Tables 6 and 7).
Figure 6 shows combining ramp function and blink timer.
LED Short- and Open-Circuit Protection
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.
Temperature Derating
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 2 C interface and by default
is disabled.
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
I LED_ = 25.6mA
t RAMP = t RAMP =
262ms 524ms
t RAMP =
1048ms
t RAMP =
2097ms
every t RAMP /32 until the desired current is reached.
Blink Timer
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 2 C interface (Table 11). See
Figure 5.
Combining Blink Timer and Ramp Function
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
I LED_ = 0
Figure 3. Ramp-Up Behavior
I LED_ = 25.6mA
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
I LED_ = 0
t RAMP =
262ms
t RAMP =
524ms
t RAMP =
1048ms
t RAMP =
2097ms
(LED_[4:0], Tables 6 and 7) during the ON time and the
Figure 4. Ramp-Down Behavior
______________________________________________________________________________________
17
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功能描述:LED照明驱动器 White LED Charge Pump RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
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功能描述:LED照明驱动器 Negative Charge Pump LED Driver RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
MAX8822ETE+T
功能描述:LED照明驱动器 Negative Charge Pump LED Driver RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
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制造商:Maxim Integrated Products 功能描述:
MAX882CPA
功能描述:低压差稳压器 - LDO 5/3.3/AdjV 200mA Linear Regulator RoHS:否 制造商:Texas Instruments 最大输入电压:36 V 输出电压:1.4 V to 20.5 V 回动电压（最大值）:307 mV 输出电流:1 A 负载调节:0.3 % 输出端数量: 输出类型:Fixed 最大工作温度:+ 125 C 安装风格:SMD/SMT 封装 / 箱体:VQFN-20
MAX882CPA+
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