Datasheet

19
Atmel MSL3080 Datasheet
8 String 60mA LED Driver with Integrated Boost Controller
9.6 The Efciency Optimizer (EO)
A voltage divider from the boost regulator output voltage to FB sets the regulation voltage (R
TOP
and R
BOTTOM
in Figure 8.1 on page 14).
The EO improves power efciency by dynamically adjusting the power supply output voltage to the minimum required to power the LEDs.
This ensures that there is sufcient voltage available for LED current regulation, and good power supply noise rejection, while minimizing
power dissipation. It does this by injecting a current into the FB input over 256 steps (8-bit resolution).
When turned on, either by applying input voltage to VIN while EN is high, or by driving EN high with voltage applied to VIN, the EO begins
an initial calibration cycle by monitoring the LED current regulators. If all the current regulators maintain LED current regulation the EO
output current is increased to reduce the boost output voltage. After the 4ms power supply settling time, it rechecks the regulators, and
if they continue to maintain regulation the process repeats until one or more current regulator loses regulation. This step requires that
the strings are turned on for a minimum of 2 µs to detect current regulation. The EO then decreases the output current to increase boost
output voltage, giving the regulator enough headroom to maintain regulation with minimal power dissipation. The oscilloscope picture
Figure 9.2 shows this procedure. The EO automatically re-calibrates V
OUT
every 1 second, and at any time increases the boost voltage
when detects an LED string with insufcient current.
Figure 9.2 Efciency Optimizer (EO)
MSL3050/MSL3060/MSL3080 Datasheet
Preliminary
MSL3050/60/80 preliminary datasheet revision 0.1 Page 19 of 24
© Atmel Inc., 2011. All rights reserved.
power supply noise rejection, while minimizing power dissipation. It does this by injecting a current into the FB input over
256 steps (8-bit resolution).
When turned on, either by applying input voltage to VIN while EN is high, or by driving EN high with voltage applied to
VIN, the EO begins an initial calibration cycle by monitoring the LED current regulators. If all the current regulators
maintain LED current regulation the EO output current is increased to reduce the boost output voltage. After the 4ms
power supply settling time, it rechecks the regulators, and if they continue to maintain regulation the process repeats until
one or more current regulator looses regulation. This step requires that the strings are turned on for a minimum of 3 µs to
detect current regulation. The EO then decreases the output current to increase boost output voltage, giving the regulator
enough headroom to maintain regulation with minimal power dissipation. The oscilloscope picture Figure 7 shows this
procedure. The EO automatically re-calibrates V
OUT
every 1 second, and at any time increases the boost voltage when
detects an LED string with insufficient current.
Figure 7. Efficiency Optimizer (EO)
SETTING THE BOOST REGULATOR OUTPUT VOLTAGE
Select the voltage divider resistors (R
TOP
and R
BOTTOM
in Figure 5 on page 14) to set the boost regulator output voltage by
first determining V
OUT(MIN)
and V
OUT(MAX)
, the required minimum and maximum LED string anode power supply (boost
regulator) voltage, using: