With an extremely large output capacitor (≥1000 µF), and
with input instantaneously shorted to ground, the regulator
could be damaged.
Application Note (Continued)
In this case, an external diode is recommended between the
output and input pins to protect the regulator, as shown in
Figure 4.
10091918
FIGURE 2. Typical Application using Fixed Output
Regulator
When the adjustable regulator is used (Figure 3), the best
performance is obtained with the positive side of the resistor
R1 tied directly to the output terminal of the regulator rather
than near the load. This eliminates line drops from appearing
effectively in series with the reference and degrading regu-
lation. For example, a 5V regulator with 0.05Ω resistance
between the regulator and load will have a load regulation
due to line resistance of 0.05Ω x IL. If R1 (=125Ω) is con-
nected near the load, the effective line resistance will be
0.05Ω (1+R2/R1) or in this case, it is 4 times worse. In
addition, the ground side of the resistor R2 can be returned
near the ground of the load to provide remote ground sens-
ing and improve load regulation.
10091915
FIGURE 4. Regulator with Protection Diode
5.0 Heatsink Requirements
When an integrated circuit operates with an appreciable
current, its junction temperature is elevated. It is important to
quantify its thermal limits in order to achieve acceptable
performance and reliability. This limit is determined by sum-
ming the individual parts consisting of a series of tempera-
ture rises from the semiconductor junction to the operating
environment. A one-dimensional steady-state model of con-
duction heat transfer is demonstrated in Figure 5. 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 environment. Below is a list of variables that
may affect the thermal resistance and in turn the need for a
heatsink.
RθJC (Component
Variables)
Rθ CA (Application
Variables)
Leadframe Size & Material Mounting Pad Size,
Material, & Location
10091919
No. of Conduction Pins
Placement of Mounting
Pad
FIGURE 3. Best Load Regulation using Adjustable
Output Regulator
Die Size
PCB Size & Material
Traces Length & Width
Adjacent Heat Sources
4.0 Protection Diodes
Die Attach Material
Molding Compound Size
and Material
Under normal operation, the LM1117 regulators do not need
any protection diode. With the adjustable device, the internal
resistance between the adjust and output terminals limits the
current. No diode is needed to divert the current around the
regulator even with capacitor on the adjust terminal. The
adjust pin can take a transient signal of 25V with respect to
the output voltage without damaging the device.
Volume of Air
Ambient Temperatue
Shape of Mounting Pad
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 de-
pends on the value of the capacitor, the output voltage of the
regulator, and rate of decrease of VIN. In the LM1117 regu-
lators, the internal diode between the output and input pins
can withstand microsecond surge currents of 10A to 20A.
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