LP2957AIS National Semiconductor, LP2957AIS Datasheet - Page 8

• Manufacturer Part Number: LP2957AIS
• Description: 5V Low-Dropout Regulator for P Applications
• Manufacturer: National Semiconductor

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Application Hints
P
*See EXTERNAL CAPACITORS
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T
by using the formula:
T
where: T
Using the calculated values for T
quired value for junction-to-ambient thermal resistance,
(JA)
If the calculated value is 60˚C/W or higher , the regulator
may be operated without an external heatsink. If the calcu-
lated value is below 60˚C/W, an external heatsink is re-
quired. The required thermal resistance for this heatsink,
where:
specified as 3˚C/W for the LP2957.
pendent on the interfacing material (see Table 1 and Table
2 ).
TOTAL
(JA)
(HA)
(HA)
(JC)
(CH)
R
(Max) = T
, can now be found:
, can be calculated using the formula:
= T
is the case-to-heatsink thermal resistance, which is de-
=
is the junction-to-case thermal resistance, which is
= (V
perature
T
R
(JA)
FIGURE 1. Basic 5V Regulator Circuit
IN
J
A
(Max)/P(Max)
(Max) is the maximum allowable junction tem-
(Max) is the maximum ambient temperature
− 5)I
J
−
(Max) − T
L
(JC)
+ (V
IN
−
)I
G
A
(CH)
(Max)
(Continued)
R
(Max) and P(Max), the re-
R
(Max). This is calculated
DS011340-7
8
Typical TO-220 Case-To-Heatsink
Thermal Resistance in ˚C/W
specification (listed on the heatsink manufacturers data
sheet) which defines the effectiveness of the heatsink. The
heatsink selected must have a thermal resistance which is
equal to or lower than the value of
above listed formula.
ERROR COMPARATOR
This comparator produces a logic “LOW” whenever the out-
put falls out of regulation by more than about 5%. This figure
results from the comparator’s built-in offset of 60 mV divided
by the 1.23V reference. An out-of-regulation condition can
result from low input voltage, current limiting, or thermal lim-
iting.
Figure 2 gives a timing diagram showing the relationship be-
tween the output voltage, the ERROR output, and input volt-
age as the input voltage is ramped up and down to the regu-
lator without snap-on/snap-off output. The ERROR signal
becomes low at about 1.3V input. It goes high at about 5V in-
put, where the output equals 4.75V. Since the dropout volt-
age is load dependent, the input voltage trip points will vary
with load current. The output voltage trip point does not
vary.
The comparator has an open-collector output which requires
an external pull-up resistor. This resistor may be connected
to the regulator output or some other supply voltage. Using
the regulator output prevents an invalid “HIGH” on the com-
parator output which occurs if it is pulled up to an external
voltage while the regulator input voltage is reduced below
1.3V. In selecting a value for the pull-up resistor, note that
while the output can sink 400 µA, this current adds to battery
drain. Suggested values range from 100k to 1 M . The re-
sistor is not required if the output is unused.
(HA)
is the heatsink-to-ambient thermal resistance. It is this
TABLE 2. (From Thermalloy)
Thermalfilm (0.002)
TABLE 1. (From AAVID)
Mica with Grease
Silicone Grease
Dry Interface
Thermasil III
with Grease
Thermasil II
(HA)
1.0
1.3
1.4
1.3
1.5
2.2
calculated from the