Datasheet

Application information LED7707
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6.2 Thermal considerations
In order to prevent the device from exceeding the thermal shutdown threshold (150 °C), it is
important to estimate the junction temperature through the following equation:
Equation 17
where T
A
is the ambient temperature, R
th,JA
is the equivalent thermal resistance junction to
ambient and P
D,tot
is the power dissipated by the device.
The R
th,JA
measured on the application demonstration board (described in Section 6.5) is
42 °C/W.
The P
D,tot
has several contributions, listed below.
a) Conduction losses due to the R
DS(on)
of the internal power switch, equal to:
Equation 18
where D is defined as:
Equation 19
and D
DIM
is the duty cycle of the PWM dimming signal.
b) Switching losses due to the power MOSFET turn on and off, calculated as:
Equation 20
where t
r
and t
f
are the power MOSFET rise time and fall time respectively.
c) Current generators losses. This contribution is strictly related to the LEDs used in
the application. Only the contribution of the leading current generator (“master”
current generator) can be predicted, regardless of the LEDs forward voltage:
Equation 21
where I
ROW
is the current flowing through the row, whereas V
IFB
is the voltage across the
master current generator (typically 700 mV).
The voltages across the other current generators depend on the spread of the LEDs forward
voltage. The worst case for power dissipation (maximum forward voltage LEDs in the master
row, minimum forward voltage LEDs in all other rows) can be estimated as:
tot,DJA,thAJ
PRTT
⋅
+
=
DIM
2
INDSoncond,D
DDIRP ⋅⋅⋅=
OUT
IN
V
V
1D −=
DIM
fr
swINOUTsw,D
D
2
)tt(
fIVP ⋅
+
⋅⋅⋅=
DIMIFBROWMaster,GEN
DVIP
⋅
⋅
=