LP2957 datasheet(8/14 Pages) NSC

8 / 14 page

Application Hints (Continued)

The next parameter which must be calculated is the maxi-

mum allowable temperature rise, T

R(Max). This is calculated

by using the formula:

R(Max) = TJ(Max) − T A(Max)

where: T

J(Max) is the maximum allowable junction tem-

perature

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,

(JA)

, can now be found:

(JA) = TR(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,

(HA), can be calculated using the formula:

(HA) = θ(JA) − θ (JC) − θ(CH)

where:

(JC) is the junction-to-case thermal resistance, which is

specified as 3˚C/W for the LP2957.

(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

(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

(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 M

Ω. The re-

sistor is not required if the output is unused.

DS011340-7

PTOTAL = (VIN − 5)I L +(VIN)IG

*See EXTERNAL CAPACITORS

FIGURE 1. Basic 5V Regulator Circuit

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