Thermal Performance and Power
Dissipation Information
Although the device is guaranteed for T
A
= -40°C to
125°C, care must be taken when using heavy loads
(e.g., I
FWD
above 500mA to 1A) where the forward
current across the ideal diode is large. The forward voltage
drop across the VDD and OUT pins increases linearly
with forward current. The device’s power dissipation is
directly proportional to the voltage drop across the device.
The power dissipation is going to be the differential
voltage (V
FWD
) multiplied by the current passed
by the device (I
FWD
). The quiescent current of the
device is negligible for these calculations. The ambient
temperature is essentially the PCB temperature, since
this is where all the heat is sunk to. Therefore, the
parts temperature rise is [V
FWD
x I
FWD
x θ
JA
] + T
A
,
where T
A
is the temperature of the board or ambient
temperature. From this exercise, we observe that the
internal temperature from power dissipation will be higher
than the ambient temperature. The device has an internal
thermal shutdown temperature of about +154°C and,
typically, 12°C hysteresis.
For example:
WLP:
At 1A I
FWD
, T
A =
85°C. V
FWD
= 110mV. Therefore,
P
DIS
= 110mW.
Package Derate Calculation:
For 85°C: Maximum Power Dissipation from the data
sheet: 766mW – [(85 - 70) x 9.58] = 622mW. The device is
within specification. Therefore, the junction temperature:
85°C + (104.41°C/W x 0.110W) = 85°C + 11.5°C = 96.5°C
SOT-23 (Small Outline Transistor Package):
At 1A I
FWD
, T
A =
85°C. V
FWD
= 250mV. Hence,
P
DIS
= 250mW.
Package Derate Calculation:
For 85°C: Maximum Power Dissipation from the data
sheet: 312.6mW – [(85 - 70)°C x 3.9mW/°C] = 254.1mW.
The device is very close to the power dissipation ratings
provided in the absolute maximum specification.
Hence the device’s junction temperature: 85°C +
(255.90°C/W x 0.2541W) = 85°C + 65.02°C = 150.02°C
As the above example shows, the thermal performance of
the WLP exceeds the SOT package.
When the device’s junction temperature rises to 154°C
thermal trip is triggered, the thermal cycle for the WLP
and SOT packages are shown in Figure 5 and Figure 6.
Figure 4. Typical Application Circuit
SUB
CIRCUIT-N
EN
MAX40200
SUB
CIRCUIT 1
SUB
CIRCUIT 2
BATTERY
MAX40200 Ultra-Tiny Micropower, 1A Ideal Diode
with Ultra-Low Voltage Drop
www.maximintegrated.com
Maxim Integrated
│
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