SM74503 Datasheet, PDF (8/16 Pages) Texas Instruments – 800mA Low-Dropout Linear Regulator

Application Note
1.0 EXTERNAL CAPACITORS/STABILITY
1.1 Input Bypass Capacitor
An input capacitor is recommended. A 10µF tantalum on the
input is a suitable input bypassing for almost all applications.
1.2 Output Capacitor
The output capacitor is critical in maintaining regulator stabil-
ity, and must meet the required conditions for both minimum
amount of capacitance and ESR (Equivalent Series Resis-
tance). The minimum output capacitance required by the
SM74503 is 10µF, if a tantalum capacitor is used. Any in-
crease of the output capacitance will merely improve the loop
stability and transient response. The ESR of the output ca-
pacitor should range between 0.3Ω - 22Ω.
2.0 LOAD REGULATION
The SM74503 regulates the voltage that appears between its
output and ground pins. In some cases, line resistances can
introduce errors to the voltage across the load. To obtain the
best load regulation, a few precautions are needed.
Figure 1, shows a typical application using a fixed output reg-
ulator. The Rt1 and Rt2 are the line resistances. It is obvious
that the V_LOAD is less than the V_OUT by the sum of the voltage
drops along the line resistances. In this case, the load regu-
lation seen at the R_LOAD would be degraded from the data
sheet specification. To improve this, the load should be tied
directly to the output terminal on the positive side and directly
tied to the ground terminal on the negative side.
30160618
FIGURE 1. Typical Application using Fixed Output
Regulator
3.0 PROTECTION DIODES
Under normal operation, the SM74503 regulators do not need
any protection diode. When a output capacitor is connected
to a regulator and the input is shorted to ground, the output
capacitor will discharge into the output of the regulator. The
discharge current depends on the value of the capacitor, the
output voltage of the regulator, and rate of decrease of V_IN. In
the SM74503 regulators, the internal diode between the out-
put and input pins can withstand microsecond surge currents
of 10A to 20A. With an extremely large output capacitor
(≥1000 µF), and with input instantaneously shorted to ground,
the regulator could be damaged.
In this case, an external diode is recommended between the
output and input pins to protect the regulator, as shown in
Figure 2.
30160615
FIGURE 2. Regulator with Protection Diode
4.0 HEATSINK REQUIREMENTS
When an integrated circuit operates with an appreciable cur-
rent, its junction temperature is elevated. It is important to
quantify its thermal limits in order to achieve acceptable per-
formance and reliability. This limit is determined by summing
the individual parts consisting of a series of temperature rises
from the semiconductor junction to the operating environ-
ment. A one-dimensional steady-state model of conduction
heat transfer is demonstrated in Figure 3. The heat generated
at the device junction flows through the die to the die attach
pad, through the lead frame to the surrounding case material,
to the printed circuit board, and eventually to the ambient en-
vironment. Below is a list of variables that may affect the
thermal resistance and in turn the need for a heatsink.
R^θJC (Component
Variables)
Leadframe Size & Material
No. of Conduction Pins
Die Size
Die Attach Material
Molding Compound Size
and Material
R^θ CA (Application
Variables)
Mounting Pad Size,
Material, & Location
Placement of Mounting Pad
PCB Size & Material
Traces Length & Width
Adjacent Heat Sources
Volume of Air
Ambient Temperatue
Shape of Mounting Pad
30160637
FIGURE 3. Cross-sectional view of Integrated Circuit
Mounted on a printed circuit board. Note that the case
temperature is measured at the point where the leads
contact with the mounting pad surface
The SM74503 regulators have internal thermal shutdown to
protect the device from over-heating. Under all possible op-
erating conditions, the junction temperature of the SM74503
7
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