AN2810 Datasheet, PDF (14/17 Pages) STMicroelectronics – 6-row 85 mA LED driver with boost

Getting started
Figure 9. LED7707 synchronization setup
700kHz – 30%
Clock Generator
FSW
SYNC
GND
DIM
AN2810
PWM Generator
CH1 CH2
Scope
VIN-
VIN+
4.5V – 32V @ 2A
Power Supply
+
VBOOST
6x10 – 75mA LEDs ARRAY
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
6
∑ ^PLOAD ⁼
V_STRINGi ⋅ I_STRINGi
i=1
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
P_LOAD = V_STRING ⋅ I_STRING
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
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