LTC3605A_15 Datasheet, PDF (14/22 Pages) Linear Technology – 20V, 5A Synchronous Step-Down Regulator

LTC3605A
Operation
Refer to the I_INTVCC vs Frequency curve in the Typical
Performance Characteristics for typical INTV_CC current
at various 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 LTC3605A 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
To avoid the LTC3605A 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 LTC3605A 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
= 40.25mW
In a majority of applications, the LTC3605A 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 LTC3605A 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.
6
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_OUT2 • R_SW + V_IN • I_VIN (No Load)
= 25A² • 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:
5
4
3
2
V_IN = 12V
1 VOUT = 3.3V
f_SW = 1MHz
DC1215 DEMO BOARD
0
20 40 60 80 100 120 140
AMBIENT TEMPERATURE (°C)
3605A F02
Figure 2. Load Current vs Ambient Temperature
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.
3605afc
14
For more information www.linear.com/LTC3605A