LP2957 Datasheet, PDF (8/14 Pages) National Semiconductor (TI) – 5V Low-Dropout Regulator for P Applications

Application Hints (Continued)
DS011340-7
P_TOTAL = (V_IN − 5)I _L + (V_IN)I_G
*See EXTERNAL CAPACITORS
FIGURE 1. Basic 5V Regulator Circuit
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T_R(Max). This is calculated
by using the formula:
T_R(Max) = T_J(Max) − T _A(Max)
where: T_J(Max) is the maximum allowable junction tem-
perature
T_A(Max) is the maximum ambient temperature
Using the calculated values for T_R(Max) and P(Max), the re-
quired value for junction-to-ambient thermal resistance, θ
(JA), can now be found:
θ_(JA) = T_R(Max)/P(Max)
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,
θ_(HA), can be calculated using the formula:
θ_(HA) = θ_(JA) − θ _(JC) − θ_(CH)
where:
θ_(JC) is the junction-to-case thermal resistance, which is
specified as 3˚C/W for the LP2957.
θ_(CH) is the case-to-heatsink thermal resistance, which is de-
pendent on the interfacing material (see Table 1 and Table
2).
Typical TO-220 Case-To-Heatsink
Thermal Resistance in ˚C/W
TABLE 1. (From AAVID)
Silicone Grease 1.0
Dry Interface
1.3
Mica with Grease 1.4
TABLE 2. (From Thermalloy)
Thermasil III
1.3
Thermasil II
1.5
Thermalfilm (0.002) 2.2
with Grease
θ_(HA) is the heatsink-to-ambient thermal resistance. It is this
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 θ_(HA)calculated from the
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 MΩ. The re-
sistor is not required if the output is unused.
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