SC1458
10
Applications Information (continued)
until the fault is removed and the load current returns to
a speci ed level.
Component Selection
A capacitance of 1F or larger on each output is
recommended to ensure stability. Ceramic capacitors
of type X5R or X7R should be used because of their
low ESR and stable temperature coe cients. It is also
recommended that the input be bypassed with a 2.2F,
low ESR X5R or X7R capacitor to minimize noise and
improve transient response. Note: Tantalum and Y5V
capacitors are not recommended.
A bypass capacitor (minimum of 22nF) should be
connected between the BYP and GND pins to meet all
noise-sensitive requirements. Increasing the capacitance
to 100nF will further improve PSRR and output noise
(SC1458B only).
Thermal Considerations
Although each of the two LDOs in the SC1458 can
provide 300mA of output current, the maximum power
dissipation in the device is restricted by the miniature
package size. The graphs in Figure 2 and Figure 3 can
be used as a guideline to determine whether the input
voltage, output voltages, output currents, and ambient
temperature of the system result in power dissipation
within the operating limits or if further thermal relief is
required.
0
0.1
0.2
0.3
0.4
0.5
0.6
3 3.5 4 4.5 5 5.5
Input Voltage (V)
Maximum Total Output Current (A)
______
T
A
=+25°C, P
D(MAX)
= 1.9W
- - - - T
A
=+85°C, P
D(MAX)
= 1.0W
Maximum Recommended
Input Voltage
Figure 2 — Safe Operating Limit
0
0.3
0.6
0.9
1.2
1.5
1.8
-40 -20 0 20 40 60 80 100
Ambient Temperature (
o
C)
Maximum Power Dissipation (W)
T
J
(Max)=150°C
T
J
(Max)=125°C
Figure 3 — Maximum P
D
vs. T
A
The following procedure can be followed to determine if
the thermal design of the system is adequate. The junction
temperature of the SC1458 can be determined in known
operating conditions using the following equation:
T
J
= T
A
+(P
D
x θ
JA
)
where
T
J
= Junction Temperature (°C)
T
A
= Ambient Temperature (°C)
P
D
= Power Dissipation (W)
θ
JA
= Thermal Resistance Junction to Ambient (°C/W)
Example
An SC1458A is used to provide outputs of 2.5V, 150mA
from LDOA and 1.8V, 200mA from LDOB. The input
voltage is 4.2V, and the ambient temperature of the
system is 60°C.
P
D
= 0.15(4.2 – 2.5) + 0.2(4.2 – 1.8)
= 0.74W
and
T
J
= 60 + (0.74 x 50) = 97°C
Figure 3 shows that the power dissipation is within limits
at T
A
= 60°C and calculation of T
J
shows that it is within the
speci ed limit of 150°C.
This means that operation of the SC1458 under these
conditions is within the speci ed limits and the device
would not require further thermal relief measures.
