Efficiency Measurements - STMicroelectronics LED7707 Application Note

Getting started
Figure 9.
9.4

Efficiency measurements

Figure 10
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
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
current delivered to the load (the sum of the current of the six channels). Since all the
14/17
LED7707 synchronization setup
700kHz – 30% 700kHz – 30% 700kHz – 30%
Clock Generator Clock Generator Clock Generator
FSW FSW
SYNC SYNC
CH1 CH1 CH2 CH2
Scope Scope
shows the setup used to perform the efficiency measurements. The efficiency in
P
is the voltage across the LEDs in row i, whereas I
is the voltage across the parallelized channels, whereas I
Doc ID 14893 Rev 1
GND GND
DIM DIM
VBOOST VBOOST
6x10 – 75mA LEDs ARRAY 6x10 – 75mA LEDs ARRAY
6
∑
⋅
= V I
LOAD STRINGi
i = 1
⋅
P = V I
LOAD STRING STRING
PWM Generator PWM Generator
VIN- VIN-
4.5V – 32V @ 2A 4.5V – 32V @ 2A
Power Supply Power Supply
+ +
VIN+ VIN+
STRINGi
STRING_i
STRING
AN2810
is the current
is the total