Datasheet

LP2952-N, LP2952A, LP2953, LP2953A
www.ti.com SNVS095D –MAY 2004–REVISED SEPTEMBER 2013
APPLICATION HINTS
Heatsink Requirements (Industrial Temperature Range Devices)
The maximum allowable power dissipation for the LP2952/LP2953 is limited by the maximum junction
temperature (+125°C) and the external factors that determine how quickly heat flows away from the part: the
ambient temperature and the junction-to-ambient thermal resistance for the specific application.
The industrial temperature range (−40°C ≤ T
J
≤ +125°C) parts are manufactured in PDIP and surface mount
packages which contain a copper lead frame that allows heat to be effectively conducted away from the die,
through the ground pins of the IC, and into the copper of the PC board. Details on heatsinking using PC board
copper are covered later.
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 specified AC input voltage must be used (since this produces the maximum DC input
voltage to the regulator). Figure 34 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 34:
Figure 34. P
TOTAL
= (V
IN
− V
OUT
) I
L
+ (V
IN
) I
G
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)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 (1)
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 Package Pins
LP2952IN, LP2952AIN, 14-Pin PDIP 3, 4, 5,
LP2952IN-3.3, LP2952AIN-3.3 10, 11, 12
LP2953IN, LP2953AIN, 16-Pin PDIP 4, 5, 12, 13
LP2953IN-3.3, LP2953AIN-3.3
LP2952IM, LP2952AIM, 16-Pin Surface 1, 8, 9, 16
Mount
LP2952IM-3.3, LP2952AIM-3.3,
LP2953IM, LP2953AIM,
LP2953IM-3.3, LP2953AIM-3.3
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Product Folder Links: LP2952-N LP2952A LP2953 LP2953A