Datasheet

18
Atmel MSL3086/MSL3088 Datasheet
8-String 60mA LED Drivers with Integrated Boost Controller and Phase Shifted Dimming
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 13).
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 control, and good power supply noise rejection, while minimizing
power dissipation. It does this by injecting a small 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 external 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 are maintaining 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 2µs 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 VOUT every 1 second, and always increases
the string voltage when a string is detected with insufcient current.
Figure 9.2 Efciency Optimizer (EO)
MSL3086/MSL3087/MSL3088 Datasheet
Page 19 of 26
© Atmel Inc., 2011. All rights reserved.
required to power the LEDs. This ensures that there is sufficient voltage available for LED current control, and good power
supply noise rejection, while minimizing power dissipation. It does this my injecting a small 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 external 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 are maintaining 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 5 shows this
pro
cedure. The EO automatically re-calibrates V
OUT
every 1 second, and always increases the string voltage when a
string is detected with insufficient current.
Figure 5. Efficiency Optimizer (EO)
SETTING THE BOOST REGULATOR OUTPUT VOLTAGE
Select the voltage divider resistors (R
TOP
and R
BOTTOM
in Figure 3 on page 13) by first determining V
OUT(MIN)
and V
OUT(MAX)
,
the minimum and maximum LED string anode power supply (boost regulator) voltage, using: