LP2957_15 Datasheet, PDF (10/24 Pages) Texas Instruments – 5V Low-Dropout Regulator for μP Applications

LP2957, LP2957A
SNVS102C – JUNE 1998 – REVISED APRIL 2013
www.ti.com
HEATSINK REQUIREMENTS
A heatsink may be required with the LP2957 depending on the maximum power dissipation and maximum
ambient temperature of the application. Under all possible 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 specified AC input voltage must be used (since this produces the maximum DC input
voltage to the regulator), and the maximum load current must also be used. Figure 30 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 30.
P_TOTAL = (V_IN − 5)I _L + (V_IN)I_G
*See EXTERNAL CAPACITORS
Figure 30. Basic 5V Regulator Circuit
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 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 _(JA), can now be found:
θ_(JA) = T_R(Max)/P(Max)
(2)
If the calculated value is 60°C/W or higher , the regulator may be operated without an external heatsink. If the
calculated value is below 60°C/W, an external heatsink is required. 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 dependent on the interfacing material
(see Table 1 and Table 2)
(3)
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
10
Submit Documentation Feedback
Copyright © 1998–2013, Texas Instruments Incorporated
Product Folder Links: LP2957 LP2957A