LP2954IS Datasheet, PDF (11/24 Pages) Texas Instruments – LP2954/LP2954A 5V and Adjustable Micropower Low-Dropout Voltage Regulators

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LP2954, LP2954A
SNVS096D – JUNE 1999 – REVISED MARCH 2013
*See EXTERNAL CAPACITORS
P_Total = (V_IN −5) I_L+ (V_IN) I_G
Figure 23. 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_(J-A), can now be found:
θ_(J-A) = 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 can be calculated using the formula:
θ_(H-A) = θ_(J-A) − θ_(J-C) − θ_(C-H)
where
• θ_(J-C) is the junction-to-case thermal resistance, which is specified as 3° C/W maximum for the LP2954
• θ_(C-H) is the case-to-heatsink thermal resistance, which is dependent on the interfacing material (if used). For details
and typical values⁽²⁾
• θ_(H-A) 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 θ_(H-A) calculated from the above listed formula
(3)
PROGRAMMING THE OUTPUT VOLTAGE
The regulator may be pin-strapped for 5V operation using its internal resistive divider by tying the Output and
Sense pins together and also tying the Feedback and 5V Tap pins together.
Alternatively, it may be programmed for any voltage between the 1.23V reference and the 30V maximum rating
using an external pair of resistors (see Figure 24). The complete equation for the output voltage is:
(4)
(2) The maximum allowable power dissipation is a function of the maximum junction temperature, T_J (MAX), the junction-to-ambient thermal
resistance, θ_J-A, and the ambient temperature, T_A. The maximum allowable power dissipation at any ambient temperature is calculated
using:
. Exceeding the maximum allowable power dissipation will result in excessive die temperature, and the regulator
will go into thermal shutdown. The junction-to-ambient thermal resistance of the TO-220 (without heatsink) is 60°C/W, 73°C/W for the
DDPAK/TO-263, and 160°C/W for the SOIC-8. If the DDPAK/TO-263 package is used, the thermal resistance can be reduced by
increasing the P.C. board copper area thermally connected to the package: Using 0.5 square inches of copper area, θ_JA is 50°C/W; with
1 square inch of copper area, θ_JA is 37°C/W; and with 1.6 or more square inches of copper area, θ_JA is 32°C/W. The junction-to-case
thermal resistance is 3°C/W. If an external heatsink is used, the effective junction-to-ambient thermal resistance is the sum of the
junction-to-case resistance (3°C/W), the specified thermal resistance of the heatsink selected, and the thermal resistance of the interface
between the heatsink and the LP2954. Some typical values are listed for interface materials used with TO-220:
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