MSL3085 datasheet(13/20 Pages) ATMEL

13 / 20 page

Atmel LED Driver-MSL3085

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:

LED

, where I

LED is in amperes and RS is in ohms.

For example, a full-scale LED current of 100mA returns

S = 5Ω. 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 VOUT(MAX), the

minimum and maximum string supply voltage limits,

using:

OUT(MIN) = (Vf (MIN) *[#ofLEDs])+ 0.5 ,

and

OUT(MAX) = (Vf (MAX) *[#ofLEDs])+ 0.5 ,

where V

f(MIN) and Vf(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 Vf(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 VOUT(MAX) to

38.5V. Then, determine R

TOP using:

)

(

)

(

)

(

MAX

FBOn

MIN

OUT

MAX

OUT

TOP

−

where I

FBOn(MAX) is the 255µA maximum

output current of the efficiency optimizer

outputs FBOn. Finally, determine R

BOTTOM using:

MAX

OUT

TOP

BOTTOM

−

∗

)

(

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.

TOP

FBO

OUT

∗

∆

Atmel LED Driver-MSL3085

8-string White and RGB LED Driver with Adaptive Power

Control, EEPROM, and I2C/SMBus Serial Interface

MSL3085 датащи(PDF) 13 Page - ATMEL Corporation

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