LP2954IT datasheet(11/24 Pages) TI

Part #	LP2954IT

Description	LP2954/LP2954A 5V and Adjustable Micropower Low-Dropout Voltage Regulators

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Manufacturer	TI [Texas Instruments]

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. Exceeding the maximum allowable power dissipation will result in excessive die temperature, and the regulator

LP2954, LP2954A

www.ti.com

SNVS096D – JUNE 1999 – REVISED MARCH 2013

*See EXTERNAL CAPACITORS

PTotal = (VIN −5) IL+ (VIN) IG

Figure 23. Basic 5V Regulator Circuit

The next parameter which must be calculated is the maximum allowable temperature rise, TR(max). This is

calculated by using the formula:

TR(max) = TJ(max) − TA(max)

where

•

TJ(max) is the maximum allowable junction temperature

•

TA(max) is the maximum ambient temperature

(1)

Using the calculated values for TR(max) and P(max), the required value for junction-to-ambient thermal

resistance,

θ(J-A), can now be found:

θ(J-A) = TR(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, TJ (MAX), the junction-to-ambient thermal

resistance,

θJ-A, and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated

using:

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,

θJA is 50°C/W; with

1 square inch of copper area,

θJA is 37°C/W; and with 1.6 or more square inches of copper area, θJA 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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