LTC3605
15
3605fh
For more information www.linear.com/LTC3605
operaTion
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.
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
measure the junction temperature directly is to use the
internal junction diode on one of the pins (PGOOD) to
measure its diode voltage change based on ambient
temperature change. First remove any external passive
component on the PGOOD pin, then pull out 100µA from
the PGOOD pin to turn on its internal junction diode and
bias the PGOOD pin to a negative voltage. With no output
current load, measure the PGOOD voltage at an ambient
temperature of 25°C, 75°C and 125°C to establish a slope
relationship between the delta voltage on PGOOD and
delta ambient temperature. Once this slope is established,
then the junction temperature rise can be measured as a
function of power loss in the package with corresponding
output load current. Keep in mind that doing so will violate
absolute maximum voltage ratings on the PGOOD pin,
however, with the limited current, no damage will result.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3605 (refer to Figure 3). Check the following in
your layout:
1. Do the capacitors C
IN
connect to the power PV
IN
and
power PGND as close as possible? These capacitors
provide the AC current to the internal power MOSFETs
and their drivers.
2. Are C
OUT
and L1 closely connected? The (–) plate of
C
OUT
returns current to PGND and the (–) plate of C
IN
.
3. The resistive divider, R1 and R2, must be connected
between the (+) plate of C
OUT
and a ground line termi-
nated near SGND. The feedback signal V
FB
should be
routed away from noisy components and traces, such
as the SW line, and its trace should be minimized. Keep
R1 and R2 close to the IC.
AMBIENT TEMPERATURE (°C)
20
0
LOAD CURRENT (A)
1
2
3
4
6
40
60 80 100
120 140
5
V
IN
= 12V
V
OUT
= 3.3V
f
SW
= 1MHz
DC1215 DEMO BOARD
Figure 2. Load Current vs Ambient Temperature
