Datasheet

ADP3050 Data Sheet
Rev. C | Page 16 of 20
85.0
)(
SATOUT
MININ
VV
V
+
=
(12)
THERMAL CONSIDERATIONS
Several factors contribute to IC power dissipation: ac and dc
switch losses, boost current, and quiescent current. The following
formulas are used to calculate these losses to determine the power
dissipation of the IC. These formulas assume continuous mode
operation, but they provide a reasonable estimate for disconti-
nuous mode systems (do not use these formulas to calculate
efficiency at light loads).
Switch loss
( )
SW
IN
OUT
OV
IN
OUT
SATOUT
SW
fVIt
V
V
VIP ×××+








××=
(13)
Boost current loss
IN
OUT
SW
OUT
BOOST
V
V
β
I
P
2
×=
(14)
Quiescent current loss
( )
( )
BIAS
OUT
Q
IN
Q
IVIVP ×+×=
(15)
where:
V
SAT
is ~0.6 V at I
OUT
= 800 mA (taken from Figure 15).
f
SW
is the switch frequency (200 kHz).
t
OV
is the switch current/voltage overlap time (~50 ns).
β
SW
is the current gain of the NPN power switch (~50).
I
Q
is the quiescent current drawn from V
IN
(~1 mA).
I
BIAS
is the quiescent current drawn from V
OUT
(~4 mA).
For example, a 5 V to 3.3 V system with I
OUT
= 800 mA
( )
mW357102000.5 8.01050
0.5
3.3
6.08.0
39
=×××××
+






××=
−
SW
P
mW35
0.5
3.3
50
8.0
2
=×=
BOOST
P
( ) ( )
mW181043.3105
33
=××+×=
−−
Q
P
For a total IC power dissipation of
mW410
=++=
Q
BOOST
SW
TOTAL
PPPP
(16)
The ADP3050 is offered in a thermally enhanced (not Pb-free)
8-lead SOIC package with a thermal resistance, θ
JA
, of 60.6°C/W,
and in a standard Pb-free 8-lead SOIC package with θ
JA
of
87.5°C / W.
The maximum die temperature, T
J
, is calculated using the
thermal resistance and the maximum ambient temperature
TOTAL
JAA
J
PθTT
×+=
(17)
For the previous example (5 V to 3.3 V at 800 mA system, Pb-
free 8-lead SOIC package using good layout techniques) with a
worst-case ambient temperature of 70°C
T
J
= 70°C + 87.5°C/W × 0.41 = 105.9°C
The maximum operating junction (die) temperature is 125°C,
therefore this system operates within the safe limits of the
ADP3050. Check the die temperature at minimum and
maximum supply voltages to ensure proper operation under all
conditions. Although the PCB and its copper traces provide
sufficient heat sinking, it is important to follow the layout
suggestions in the Board Layout Guidelines section. For any
design that combines high output current with high duty cycle
and/or high input voltage, the junction temperature must be
calculated to ensure normal operation. Always use the
equations in this section to estimate the power dissipation.