EUP7917
On/Off Input Operation
The EUP7917 is turned off by pulling the V
EN
pin low,
and turned on by pulling it high. If this feature is not
used, the V
EN
pin should be tied to V
IN
to keep the
regulator output on at all time. To assure proper
operation, the signal source used to drive the V
EN
input must be able to swing above and below the
specified turn-on/off voltage thresholds listed in the
Electrical Characteristics section under V
IL
and V
IH
.
Fast Start up
The EUP7917 output starts up after Vref voltage
reaches its final value (1.2V nomial). The start-up time
is determined by the time constant of the bypass
capacitor. The smaller the capacitor value, the shorter
the start-up time, but less noise gets reduced. As a
result, start-up time and noise reduction need to be
taken into design consideration when choosing the
value of the bypass capacitor.
Thermal Considerations
Thermal protection limits power dissipation in
EUP7917.When the operation junction temperature
exceeds 155°C, the OTP circuit starts the thermal
shutdown function turn the pass element off. The pass
element turn on again after the junction temperature
cools by 15°C. For continue operation, do not exceed
absolute maximum operation junction temperature
125°C. The power dissipation definition in device is :
P
D
=
(
V
IN
- V
OUT
)
×
I
OUT
+
V
IN
×
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
EUP7917, where T
J(MAX)
is the maximum junction
temperature of the die (125°C) and T
A
is the maximum
ambient temperature. The junction to ambient thermal
resistance (θ
JA
is layout dependent) only for SC-70
package is 330°C/W on standard 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
)
/ 330
=
300 mW (SC
−
70)
The maximum power dissipation depends on operating
ambient temperature for fixed T
J(MAX)
and thermal
resistance
θ
JA
.
DS7917
Ver1.2
Mar. 2008
10
EUTECH [ EUTECH MICROELECTRONICS INC ]
