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

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

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

i1

=

6

∑

I

STRINGi

⋅

=

P

LOAD

V

STRING

I

STRING

⋅

=