STMicroelectronics LED7707, Application Note, Page: 14 / 17

Getting started AN2810
14/17 Doc ID 14893 Rev 1
Figure 9. LED7707 synchronization setup
9.4 Efficiency measurements
Figure 10 shows the setup used to perform the efficiency measurements. The efficiency in
this device is typically defined as the ratio between the power provided to the load (current
flowing through the LEDs multiplied by the voltage across the LEDs) and the total input
power. The power dissipated in the current generators is correctly considered as a power
loss. This way of calculating the efficiency implies that the voltage across the LEDs is the
same for all the strings. However this is not true. The power delivered to the load should be
calculated as follows:
Equation 2
where V
STRING_i
is the voltage across the LEDs in row i, whereas I
STRING_i
is the current
flowing through the row i. To facilitate the measurement, the voltage drop of all the
generators is equalized by connecting them together. In this condition, the power provided to
the LEDs is simple to calculate:
Equation 3
where V
STRING
is the voltage across the parallelized channels, whereas I
STRING
is the total
current delivered to the load (the sum of the current of the six channels). Since all the
PWM Generator
4.5V – 32V @ 2A
Power Supply
6x10 – 75mA LEDs ARRAY
VBOOST
VIN+
VIN-
GND
DIM
+
CH2 CH1
Scope
700kHz – 30%
Clock Generator
FSW
SYNC
PWM Generator
4.5V – 32V @ 2A
Power Supply
6x10 – 75mA LEDs ARRAY
VBOOST
VIN+
VIN-
GND
DIM
+
CH2 CH1
Scope
700kHz – 30%
Clock Generator
700kHz – 30%
Clock Generator
FSW
SYNC
P
LOAD
V
STRINGi
i 1 =
6
∑
I
STRINGi
⋅
=
P
LOAD
V
STRING
I
STRING
⋅
=