Pam2800, Application information – Diodes PAM2800 User Manual
Page 4
PAM2800
Document number: DSxxxxx Rev. 1 - 3
4 of 7
www.diodes.com
October 2012
© Diodes Incorporated
PAM2800
A Product Line of
Diodes Incorporated
Application Information
In the typical application (see Figure 1), the LED current will come to constant current level little by little after the device is powered. A 62ΚΩ
resistor is recommended for R1, the value for R2 should be adjusted around 33ΚΩ due to LED forward voltage from lot-to-lot or brand-to-brand.
Power Dissipation and Thermal Consideration
Thermal protection limits power dissipation in the PAM2800. When the operation junction temperature exceeds +150°C, the OTP (Over
Temperature Protection) starts the thermal shutdown and turns the pass transistor off. The pass transistor resumes operation after the junction
temperature drops below +120°C.
For continuous operation, the junction temperature should be maintained below +125°C. The power dissipation is defined as:
(
)
I
*
V
I
*
V
V
P
GND
IN
O
OUT
IN
D
+
−
=
The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surrounding airflow and temperature
difference between junction and ambient. The maximum power dissipation can be calculated by the following formula:
(
)
θ
−
=
JA
A
)
MAX
(
J
)
MAX
(
D
/
T
T
P
Where T
J(MAX)
is the maximum operation junction temperature +125°C. T
A
is the ambient temperature, and is the thermal resistance from the
junction to the ambient.
For example, as θ
JA
is +250°C/W for SOT-23, based on the standard JEDEC 51-3 for a single layer thermal test board, the maximum power
dissipation for SOT-23 package at T
A
= +25°C can be calculated by following formula:
(
)
W
4
.
0
250
/
C
25
C
125
P
)
MAX
(
D
=
°
−
°
=
To calculate the junction temperature of the PAM2800 SOT-23 package, if we use input voltage V
IN
= 4V at an output current I
O
= 300mA and
the case temperature T
A
= 40°C measured by the thermal couple while operating, the power dissipation is defined as:
(
)
mW
360
A
70
*
V
4
mA
300
*
V
8
.
2
V
4
P
D
≈
μ
+
−
=