LTC3622-2_15 Datasheet, PDF (16/24 Pages) Linear Technology – 17V, Dual 1A Synchronous Step-Down Regulator with Ultralow Quiescent Current

LTC3622/
LTC3622-2/LTC3622-23/5
Applications Information
Thermal Conditions
In a majority of applications, the LTC3622 does not dis-
sipate much heat due to its high efficiency. However, in
applications where the LTC3622 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,
all power switches will be turned off until the temperature
drops about 15°C cooler.
To prevent the LTC3622 from exceeding the maximum
junction temperature, the user needs 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 LTC3622 is
used in applications where V_IN = 12V, I_OUT = I_OUT1 = I_OUT2
= 1A, ƒ = 2.25MHz, V_OUT = V_OUT1 = V_OUT2 = 1.8V. The
equivalent power MOSFET resistance R_SW is:
The active current through V_IN at 2.25MHz without load
is about 10mA, which includes switching and internal
biasing current loss, and transition loss. Therefore, the
total power dissipated by the part is:
P_D = 2 • I_OUT2 • R_SW + V_IN • I_IN(Q)
= 2 • 1A² • 183mΩ + 12V • 10mA
= 486mW
For the DFN package, the θ_JA is 40°C/W. Therefore, the
junction temperature of the regulator operating at 25°C
ambient temperature is approximately:
T_J = 486mW • 40°C/W + 25°C = 44.4°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 44.4°C yields a new
junction temperature of 45.4°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.
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
3622fa
16
For more information www.linear.com/LTC3622