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Atmel LED Driver-MSL3085
8-string White and RGB LED Driver with Adaptive Power
Control, EEPROM, and I 2 C/SMBus Serial Interface
R TOP =
I FBOn ( MAX )
The MSL3085 enable input, EN, enables the device.
Drive EN low to enter low power operation, which
lowers quiescent current draw to less than 20μA. With
EN low, the serial interface is ignored, the efficiency
optimizer is disabled, and all strings are turned off. Drive
EN high to turn on the device. When EN is driven high,
the contents of the EEPROM are boot loaded into the
control registers, simulating a cold start-up, and all bits in
the fault registers are cleared to 0.
Setting the Maximum LED String Current
With a Source Resistor
The maximum string current, I LED , for each string is set
by a resistor, R S , connected to ground from the source
terminal of the external string drive MOSFET. The
feedback threshold is 500mV. Determine the resistor
value using:
where V f(MIN) and V f(MAX) are the LED minimum and
maximum forward voltage drops at the peak current
set by R S . For example, if the LED data are V f(MIN) = 3.5V
and V f(MAX) = 3.8V, and ten LEDs are used in a string,
then the total minimum and maximum voltage drops
across a string are 35V and 38V, respectively. Adding
an allowance of 0.5V for the string drive MOSFET
headroom brings V OUT(MIN) to 35.5V and V OUT(MAX) to
38.5V. Then, determine R TOP using:
V OUT ( MAX) ? V OUT ( MIN )
.
where I FBOn(MAX) is the 255μ A maximum
output current of the efficiency optimizer
outputs FBOn. Finally, determine R BOTTOM using:
R S =
0 . 5
I LED
, where I LED is in amperes and R S is in ohms.
R BOTTOM = R TOP ?
V FB
V OUT ( MAX ) ? V FB
,
? V OUT = ? I FBO ? R TOP
For example, a full-scale LED current of 100mA returns
R S = 5 Ω . The current for all LED strings is reduced from
the full-scale value with 8-bit resolution using ISTR, the
string current control register 0x31.
Connecting the Efficiency Optimizer to an LED
String Power Supply and Selecting Resistors
The MSL3085 is designed to control an external LED
string power supply that uses a voltage divider (R TOP
and R BOTTOM in Figure 4) to set output voltage, and
whose regulation feedback voltage is not more than
3.5V. The efficiency optimizer improves power efficiency
by injecting a current of between 0 and 255μA into
the voltage divider of the external power supply,
dynamically adjusting the power supply's output to the
minimum voltage required by the LED strings. To select
the resistors, first determine V OUT(MIN) and V OUT(MAX) , the
minimum and maximum string supply voltage limits,
using:
V OUT(MIN) = ( V f (MIN) *[# ofLEDs ] ) + 0.5 ,
and
V OUT(MAX) = ( V f (MAX) *[# ofLEDs ] ) + 0.5 ,
Atmel LED Driver-MSL3085
where V FB is the regulation feedback voltage of the
power supply. Place a diode (1N4148 or similar)
between FBOn and the supply’s feedback node to
protect the MSL3085 against current flow into FBOn.
Assign all strings powered by a common supply to the
proper FBO output using string set registers 0x40 -
0x5D.
Once configured, determine the change in power
supply output voltage in response to a change in FBOn
output current using:
.
Figure 4. FBOn Connects to the Power Supply
Voltage Divider through a Diode.
13
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MSL3086-IUR 功能描述:LED照明驱动器 8-Str LED Driver Internal Boost Cntr 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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