Datasheet

LTC3605
14
3605fc
OPERATION
system. It is very important to include these “system”
level losses in the design of a system. Transition loss
arises from the brief amount of time the top power
MOSFET spends in the saturated region during switch
node transitions. The LTC3605 internal power devices
switch quickly enough that these losses are not signifi-
cant compared to other sources. Other losses including
diode conduction losses during dead-time and inductor
core losses which generally account for less than 2%
total additional loss.
Thermal Considerations
In a majority of applications, the LTC3605 does not dis-
sipate much heat due to its high efficiency and low thermal
resistance of its exposed-back QFN package. However, in
applications where the LTC3605 is running at high ambi-
ent temperature, high V
IN
, high switching frequency and
maximum output current load, the heat dissipated may
exceed the maximum junction temperature of the part.
If the junction temperature reaches approximately 160°C,
both power switches will be turned off until the temperature
drops about 15°C cooler.
To avoid the LTC3605 from exceeding the maximum junc-
tion temperature, the user will need to do some thermal
analysis. The goal of the thermal analysis is to determine
whether the power dissipated exceeds the maximum
junction temperature of the part. The temperature rise is
given by:
T
RISE
= P
D
• θ
JA
As an example, consider the case when the LTC3605 is
used in applications where V
IN
= 12V, I
OUT
= 5A, f = 1MHz,
V
OUT
= 1.8V. The equivalent power MOSFET resistance
R
SW
is:
R
SW
= R
DS(ON)
Top •
V
OUT
V
IN
+ R
DS(ON)
Bot 1–
V
OUT
V
IN






= 70mW •
1.8
12
+ 35mW •
10.2
12
=
40.25m
W
The V
IN
current during 1MHz force continuous operation
with no load is about 11mA, which includes switching
and internal biasing current loss, transition loss, inductor
core loss and other losses in the application. Therefore,
the total power dissipated by the part is:
P
D
= I
OUT
2
• R
SW
+ V
IN
• I
VIN
(No Load)
= 25A
2
• 40.25mΩ + 12V • 11mA = 1.14W
The QFN 4mm × 4mm package junction-to-ambient thermal
resistance, θ
JA
, is around 37°C/W. Therefore, the junction
temperature of the regulator operating in a 25°C ambient
temperature is approximately:
T
J
= 1.14W • 37°C/W + 25°C = 67°C
Remembering that the above junction temperature is
obtained from an R
DS(ON)
at 25°C, we might recalculate
the junction temperature based on a higher R
DS(ON)
since
it increases with temperature. Redoing the calculation
assuming that R
SW
increased 15% at 67°C yields a new
junction temperature of 72°C. If the application calls for
a higher ambient temperature and/or higher switching
frequency, care should be taken to reduce the temperature
rise of the part by using a heat sink or air flow. Figure 2
is a temperature derating curve based on the DC1215
demo board.
AMBIENT TEMPERATURE (°C)
20
0
LOAD CURRENT (A)
1
2
3
4
6
40
60 80 100
3605 F02
120 140
5
V
IN
= 12V
V
OUT
= 3.3V
f
SW
= 1MHz
DC1215 DEMO BOARD
Figure 2. Load Current vs Ambient Temperature
Junction Temperature Measurement
The junction-to-ambient thermal resistance will vary de-
pending on the size and amount of heat sinking copper
on the PCB board where the part is mounted, as well as
the amount of air flow on the device. One of the ways to