LTC3621/LTC3621-2
13
3621fc
For more information www.linear.com/LTC3621
applicaTions inForMaTion
2. The switching current is the sum of the MOSFET driver
and control currents. The power MOSFET driver current
results from switching the gate capacitance of the power
MOSFETs. Each time a power MOSFET gate is switched
from low to high to low again, a packet of charge dQ
moves from V
IN
to ground. The resulting dQ/dt is a
current out of V
IN
that is typically much larger than the
DC control bias current. In continuous mode, I
GATECHG
= f(Q
T
+ Q
B
), where Q
T
and Q
B
are the gate charges of
the internal top and bottom power MOSFETs and f is
the switching frequency. The power loss is thus:
Switching Loss = I
GATECHG
• V
IN
The gate charge loss is proportional to V
IN
and f and
thus their effects will be more pronounced at higher
supply voltages and higher frequencies.
3. Other “hidden” losses such as transition loss and cop
-
per trace and internal load resistances can account for
additional efficiency
degradations in the overall power
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 LTC3621 internal power devices
switch quickly enough that these losses are not sig
-
nificant compared to other sources. These losses plus
other losses, including diode conduction losses during
dead-time
and
inductor core losses, generally account
for less than 2% total additional loss.
Thermal Conditions
In a majority of applications, the LTC3621 does not dis
-
sipate much heat due to its high efficiency and low thermal
resistance of its exposed pad package. However, in ap-
plications where the LTC3621 is running at high ambient
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 LTC3621 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 LTC3621
is used in applications where V
IN
= 12V, I
OUT
= 1A,
f = 2.25MHz, 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
= 370mΩ•
1.8V
12V
+150mΩ • 1–
1.8V
12V
=183mΩ
The V
IN
current during 2.25MHz force continuous opera-
tion with no load is about 5mA, 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
IN(Q)
= 1A
2
• 183mΩ + 12V • 5mA
= 243mW
The DFN 2mm × 3mm package junction-to-ambient thermal
resistance, θ
JA
, is around 64°C/W. Therefore, the junction
temperature of the regulator operating in a 25°C ambient
temperature is approximately:
T
J
= 0.243W • 64°C/W + 25°C = 40.6°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 5% at 40.6°C yields a new
junction temperature of 41.1°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 forced air flow.
