LP2952 Datasheet, PDF (14/24 Pages) National Semiconductor (TI) – Adjustable Micropower Low-Dropout Voltage Regulators

Application Hints (Continued)
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FIGURE 1. P_TOTAL = (V_IN − V_OUT) I_L + (V_IN) I_G
Current/Voltage Diagram
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)θ_(J–A) = T_R(max)/P(max)
where: T_J(max) is the maximum allowable junction
temperature
T_A(max) is the maximum ambient temperature
Using the calculated values for T_R(max) and P(max), the
required value for junction-to-ambient thermal resistance,
θ_(J–A), can now be found:
The heatsink 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 given in Table 1.
TABLE 1. Heat Conducting Pins
Part
LP2952IN, LP2952AIN,
LP2952IN-3.3,
LP2952AIN-3.3
LP2953IN, LP2953AIN,
LP2953IN-3.3,
LP2953AIN-3.3
LP2952IM, LP2952AIM,
LP2952IM-3.3,
LP2952AIM-3.3,
LP2953IM, LP2953AIM,
LP2953IM-3.3,
LP2953AIM-3.3
Package
14-Pin DIP
Pins
3, 4, 5,
10, 11, 12
16-Pin DIP 4, 5, 12, 13
16-Pin Surface 1, 8, 9, 16
Mount
Figure 2 shows copper patterns which may be used to
dissipate heat from the LP2952 and LP2953. Table 2 shows
some values of junction-to-ambient thermal resistance (θ_J–A)
for values of L and W for 1 oz. copper.
* For best results, use L = 2H
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** 14-Pin DIP is similar, refer to Table 1 for pins designated for
heatsinking.
FIGURE 2. Copper Heatsink Patterns
TABLE 2. Thermal Resistance for Various Copper
Heatsink Patterns
Package
16-Pin DIP
14-Pin DIP
Surface Mount
L (in.)
1
2
3
4
6
1
2
3
1
2
3
6
4
2
H (in.)
0.5
1
1.5
0.19
0.19
0.5
1
1.5
0.5
1
1.5
0.19
0.19
0.19
θ_J–A (˚C/W)
70
60
58
66
66
65
51
49
83
70
67
69
71
73
HEATSINK REQUIREMENTS (Military Temperature
Range Devices)
The maximum allowable power dissipation for the
LP2953AMJ is limited by the maximum junction temperature
(+150˚C) and the two parameters that determine how quickly
heat flows away from the die: the ambient temperature and
the junction-to-ambient thermal resistance of the part.
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