LT3667
27
3667fb
For more information www.linear.com/LT3667
APPLICATIONS INFORMATION
The power dissipation of each LDO is comprised of two
components. Each power device dissipates:
P
PASS
= (V
IN
− V
OUT
) • I
OUT
where P
PASS
is the power, V
IN
the input voltage, V
OUT
the output voltage, and I
OUT
the output current. The base
currents of the LDO power PNP transistors flow to ground
internally and are the major component of the ground
current. For each LDO, this causes a power dissipation
P
GND
of:
P
GND
= V
IN
• I
GND
where V
IN
is the input voltage and I
GND
the ground current
generated by the corresponding power device. GND pin
current is determined by the current gain of the power
PNP, which has a typical value of 40 for the purpose of
this calculation:
I
GND
=
OUT
The total power dissipation equals the sum of the power
loss in the switching regulator and the two LDO compo-
nents listed above.
The
LT3667 has internal thermal limiting that protects
the device during overload conditions. If the junction
temperature reaches the thermal shutdown threshold, the
LT3667 will shut down the LDOs and stop switching to
prevent internal damage due to overheating. For continuous
normal conditions, do not exceed the maximum operat
-
ing junction temperature. Carefully consider all sources
of
thermal resistance from junction-to-ambient including
other nearby heat sources. Both LT3667 packages have
exposed pads that must be soldered to a ground plane to
act as heat sink. To keep thermal resistance low, extend the
ground plane as much as possible, and add thermal vias
under and near the LT3667 to additional ground planes
within the circuit board and on the bottom side.
The die temperature rise is calculated by multiplying the
power dissipation of the LT3667 by the thermal resistance
from junction to ambient. Example: Given the front page
application with maximum output current, an input voltage
of 12V and a maximum ambient temperature of 85°C, what
will the maximum junction temperature be?
As can
be seen from the Typical Performance Characteris-
tics,
the switching regulator efficiency approaches 85% at
400mA
output current. This leads to a power loss, P
LOSS
, of:
P
LOSS
= 5V • 400mA •
0.85
–1
= 353mW
(For the sake of simplicity and as a conservative estimate
assume that all of this power is dissipated in the LT3667.)
The power dissipations of the LDO power devices are:
P
PASS2
= (5V − 2.5V) • 100mA = 250mW
P
PASS3
= (5V − 3.3V) • 100mA = 170mW
For 100mA load current a maximum ground current of
2.5mA is to be expected. Thus, the corresponding power
dissipations are:
P
GND2
= P
GND3
= 5V • 2.5mA = 12.5mW
Finally, the total power dissipation is:
P
TOT
= P
LOSS
+ P
PASS2
+ P
PASS3
+ P
GND2
+ P
GND3
= 786mW
Using the MSOP package, which has a thermal resistance
of approximately 40°C/W, this total power dissipation
would raise the junction temperature above ambient by:
0.786W • 40°C/W = 32°C
With the assumed maximum ambient temperature of 85°C,
this puts the maximum junction temperature at:
T
JMAX
= 85°C + 32°C = 117°C
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 318
shows how to generate a bipolar output supply using a
buck regulator.
