RT9167/A
10
DS9167/A-29 April 2011www.richtek.com
Operating Region and Power Dissipation
The maximum power dissipation of RT9167/A depends
on the thermal resistance of the case and circuit board,
the temperature difference between the die junction and
ambient air, and the rate of airflow. The power dissipation
across the device is P = I
OUT
(V
IN
− V
OUT
). The maximum
power dissipation is: PMAX = (T
J
− T
A
) /θ
JA
where T
J
− T
A
is the temperature difference between the
RT9167/A die junction and the surrounding environment,
θ
JA
is the thermal resistance from the junction to the
surrounding environment. The GND pin of the RT9167/A
performs the dual function of providing an electrical
connection to ground and channeling heat away. Connect
the GND pin to ground using a large pad or ground plane.
Current Limit and Thermal Protection
T9167 includes a current limit which monitors and controls
the pass transistor's gate voltage limiting the output current
to 350mA Typ. (700mA Typ. for RT9167A). Thermal-
overload protection limits total power dissipation in the
RT9167/A. When the junction temperature exceeds
T
J
= 155°C, the thermal sensor signals the shutdown logic
turning off the pass transistor and allowing the IC to cool.
The thermal sensor will turn the pass transistor on again
after the IC's junction temperature cools by 10°C, resulting
in a pulsed output during continuous thermal-overload
conditions. Thermal-overloaded protection is designed to
protect the RT9167/A in the event of fault conditions. Do
not exceed the absolute maximum junction-temperature
rating of T
J
= 150°C for continuous operation. The output
can be shorted to ground for an indefinite amount of time
without damaging the part by cooperation of current limit
and thermal protection.
Thermal Considerations
Thermal protection limits power dissipation in RT9167/A.
When the operation junction temperature exceeds 165°C,
the OTP circuit starts the thermal shutdown function and
turns the pass element off. The pass element turn on again
after the junction temperature cools by 30°C.
For continuous operation, do not exceed absolute
maximum operation junction temperature 125°C. The
power dissipation definition in device is :
P
D
= (V
IN
− V
OUT
) x I
OUT
+ V
IN
x I
Q
The maximum power dissipation depends on the thermal
resistance of IC package, PCB layout, the rate of
surroundings airflow and temperature difference
between junction to ambient. The maximum power
dissipation can be calculated by following formula :
P
D(MAX)
= ( T
J(MAX)
− T
A
) / θ
JA
Where T
J(MAX)
is the maximum operation junction
temperature 125°C, T
A
is the ambient temperature and
the θ
JA
is the junction to ambient thermal resistance.
For recommended operating conditions specification of
RT9167/A, where T
J(MAX)
is the maximum junction
temperature of the die (125°C) and T
A
is the operated
ambient temperature. The junction to ambient thermal
resistance θ
JA
is layout dependent. For SOT-23-5 package,
the thermal resistance θ
JA
is 250°C/W on the standard
JEDEC 51-3 single-layer thermal test board. The maximum
power dissipation at T
A
= 25°C can be calculated by
following formula :
P
D(MAX)
= (125°C − 25°C) / 250 = 0.4W for
SOT-23-5 package
P
D(MAX)
= (125°C - 25°C) / 160 = 0.625W for
SOP-8 package
The maximum power dissipation depends on operating
ambient temperature for fixed T
J(MAX)
and thermal
resistance θ
JA
. For RT9167/A packages, the Figure 5 of
derating curves allows the designer to see the effect of
rising ambient temperature on the maximum power
allowed.
Figure 5. Derating Curves for RT9167/A Packages
0
100
200
300
400
500
600
700
0 20406080100120140
Ambient Temperature
Maximum Power Dissipation (mW) 1
SOT-23-5
SOP-8
