LP2956A Datasheet, PDF (12/20 Pages) National Semiconductor (TI) – Dual Micropower Low-Dropout Voltage Regulators

Application Hints
HEATSINK REQUIREMENTS
A heatsink may be required with the LP2956 depending on
the maximum power dissipation and maximum ambient tem-
perature of the application. Under all expected operating
conditions, the junction temperature must be within the range
specified under Absolute Maximum Ratings.
To determine if a heatsink is required, the maximum power
dissipated by the regulator, P(max), must be calculated. It is
important to remember that if the regulator is powered from a
transformer connected to the AC line, the maximum speci-
fied AC input voltage must be used (since this produces the
maximum DC input voltage to the regulator). Figure 1 shows
the voltages and currents which are present in the circuit. The
formula for calculating the power dissipated in the regulator
is also shown in Figure 1 (the currents and power due to ex-
ternal resistive dividers are not included, and are typically
negligible).
1133909
FIGURE 1. Current/Voltage Diagram
The next parameter which must be calculated is the maximum
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 temper-
ature
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 _(J-
A)^{, can now be found:}
θ_(J-A) = T_R(max)/P(max)
The heatsink for the LP2956 is made using the PC board
copper. The heat is conducted from the die, through the lead
frame (inside the part), and out the pins which are soldered
to the PC board. The pins used for heat conduction are shown
in Table 1.
Part
LP2956IN
LP2956AIN
LP2956IM
LP2956AIM
TABLE 1.
Package
16-Pin Plastic DIP
16-Pin Plastic DIP
16-Pin Surface Mt.
16-Pin Surface Mt.
Pins
4, 5, 12, 13
4, 5, 12, 13
1, 8, 9, 16
1, 8, 9, 16
Figure 2 shows copper patterns which may be used to dissi-
pate heat from the LP2956:
*For best results, use L = 2H
1133910
FIGURE 2. Copper Heatsink Patterns
Table 2 shows some typical values of junction-to-ambient
thermal resistance (θ _J-A) for values of L and W (1 oz. copper).
TABLE 2.
Package
16-Pin
Plastic
DIP
L (In.)
1
2
H (In.)
0.5
1
θ_J-A (°C/W)
70
60
3
1.5
58
4
0.19
66
6
0.19
66
16-Pin
1
0.5
83
Surface
2
1
70
Mount
3
1.5
67
6
0.19
69
4
0.19
71
2
0.19
73
EXTERNAL CAPACITORS
A 2.2 μF (or greater) capacitor is required between the main
output pin and ground to assure stability. The auxiliary output
requires 10 μF to ground. Without these capacitors, the part
may oscillate. Most types of tantalum or aluminum electrolyt-
ics will work here. Film types will work, but are more expen-
sive. Many aluminum electrolytics contain electrolytes which
freeze at −30°C, which requires the use of solid tantalums
below −25°C. The important characteristic of the capacitors
is an ESR of 5Ω (or less) on the main regulator output and an
ESR of 1Ω (or less) on the auxiliary regulator output (the ESR
may increase by a factor of 20 or 30 as the temperature is
reduced from +25°C to −30°C). The value of these capacitors
may be increased without limit.
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11339 Version 8 Revision 4 Print Date/Time: 2009/12/07 17:53:15