LED7706 Application information
As explained in Section 6.2
, there are several contributions to the total power dissipation.
Neglecting the power dissipated by the LDO (surely less significant compared with the other contributions), equation (19), (21), (22) and (23) help to estimate the overall power dissipation.
Before starting the power dissipation estimate it is important to highlight that the following calculations are considering the worst case (the actual value of the dissipated power would require measurements). Therefore the power dissipation is estimated according to the following assumptions:
1.
Minimum input voltage (9.6 V), which leads to maximum input current (and also D will have the higher value, see
Section 6.4.4
);
R
DS(on)
of the internal power MOSFET;
3. LEDs in the row of the leading generator will have the maximum forward voltage, whereas all other LEDs in the other rows will have the minimum forward voltage.
4. 100 % dimming signal duty cycle is considered.
The conduction and switching losses on the internal power switch can be calculated as:
Equation 42
P
D , cond
=
R
DSon
⋅
I
2
IN
⋅
D
⋅
D
DIM
=
34 mW
Equation 43
P
D , sw
=
V
OUT
⋅
I
IN
⋅ f sw
⋅
( t r
+
2 t f
)
⋅
D
DIM
=
111 mW where t r
= t f
= 15 ns
The power dissipation related to the current generators is given by:
Equation 44
P
GEN , Master
=
I
ROW
⋅
V
IFB
⋅
D
DIM
=
8 mW
Equation 45
P
GEN
=
I
ROW
⋅
( n
ROWs
−
1
)
⋅
(
V
IFB
+ Δ
V f , LEDs
⋅ n
LEDs
)
⋅
D
DIM
=
360 mW
Equation 46
P
D , tot
≅
P
D , cond
+
P
D , sw
+
P
GEN , Master
+
P
GEN
=
513 mW
The junction temperature can be estimated by equation (18) considering T
A
= 25 °C:
Equation 47
T
J
=
T
Amb
+
R th , JA
⋅
P
D , tot
=
46 .
5
°
C
35/46
Application information LED7706
In order to estimate also the efficiency, other contributions to the power dissipation must be added to P
D, tot
(which represents only the power dissipated by the device), that is:
Equation 48
P
DISS , Diode
=
V
F , Diode
⋅
I
IN
⋅
D
2
=
34 .
5 mW where V
F, Diode
= 0.4 V
Equation 49
P
DISS , Ind
=
DCR
⋅
I
2
Ind , RMS
≅
DCR
⋅
I
2
IN
=
11 .
2 mW where DCR = 80 mΩ (typical DCR of the recommended inductors).
Therefore the total dissipated power is:
Equation 50
P
DISS , TOTAL
=
P
D , tot
+
P
DISS , Diode
+
P
DISS , Ind
=
559 .
1 mW
Considering the input power as the result of input voltage multiplied by the input current, the estimated efficiency is:
Equation 51
η =
P
IN
−
P
DISS , TOT
P
IN
=
0 .
845
Note: It is important to remind that the previous calculations consider the worst case, especially for the power dissipated on the current generators.
Statistical analysis (confirmed by bench measurements) shows that the series connection of more LEDs on each channel leads to compensation effects.
The hypothesis 3 above mentioned is thus rather unlikely.
Therefore P
GEN
is significantly lower and the overall efficiency is typically around 90 %.
36/46
