Application information LED7706
Equation 23
P
GEN
=
I
ROW
⋅
( n
ROWs
−
1
)
⋅
(
V
IFB
+ Δ
V f , LEDs
⋅ n
LEDs
)
⋅
D
DIM where n
ROWs
is the number of active rows, voltage and n
LEDs
ΔV f,LEDs
is the number of LEDs per row.
is the spread of the LEDs forward d) LDO losses, due to the dissipation of the 5 V linear regulator:
Equation 24
P
D , LDO
=
(
V
IN
−
V
LDO
)
⋅
I
LDO
The LED7706 is housed in a 24 leads 4x4-VFQFPN package with exposed pad that allows good thermal performance. However it is also important to design properly the demonstration board layout in order to assure correct heat dissipation.
Figure 19
shows a picture of the LED7706 application demonstration board taken using an infrared camera. The chip temperature, in those application conditions, is kept below 50 °C.
Figure 19.
Demonstration board thermographic analysis
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Being the LED7706 mostly dedicated to backlighting, real-estate applications dictate severe constrain in selecting the optimal inductor. The inductor choice must take into account different parameters like conduction losses (DCR), core losses (ferrite or iron-powder), saturation current and magnetic-flux shielding (core shape and technology).
The switching frequency of the LED7706 can be set in the 200 kHz-1 MHz range, allowing a wide selecting room for the inductance value. Low switching frequencies takes to high inductance value, resulting in significant DCR and size. On the other hand, high switching frequencies result in significant core losses. The suggested range is 4.7-22 µH, even if the
