NCP1117, NCV1117

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9

Protection Diodes

The NCP1117 family has two internal low impedance

diode paths that normally do not require protection when

used in the typical regulator applications. The first path

50

mF, it becomes possible for device damage to occur.

Under these conditions, diode D1 is required to protect the

it can withstand a peak surge current of about 150 mA.

Protection diode D2 is required if the output is shorted or

mF.

Figure 22. Protection Diode Placement

1

2

Output

3

Input

NCP1117

XTA

++

R2

+

R1

1N4001

D1

D2

1N4001

A combination of protection diodes D1 and D2 may be

is greater than 50

mF. The peak current capability stated for

the internal diodes are for a time of 100

ms with a junction

temperature of 25

°C. These values may vary and are to be

used as a general guide.

Load Regulation

The NCP1117 series is capable of providing excellent

load regulation; but since these are three terminal devices,

only partial remote load sensing is possible. There are two

conditions that must be met to achieve the maximum

available load regulation performance. The first is that the

top side of programming resistor R1 should be connected as

close to the regulator case as practicable. This will minimize

the voltage drop caused by wiring resistance RW + from

appearing in series with reference voltage that is across R1.

The second condition is that the ground end of R2 should be

connected directly to the load. This allows true Kelvin

sensing where the regulator compensates for the voltage

drop caused by wiring resistance RW −.

Figure 23. Load Sensing

1

2

Output

3

Input

NCP1117

XTA

+

+

R1

Remote

Load

RW+

RW−

R2

Thermal Considerations

This series contains an internal thermal limiting circuit

that is designed to protect the regulator in the event that the

maximum junction temperature is exceeded. When

activated, typically at 175

°C, the regulator output switches

off and then back on as the die cools. As a result, if the device

is continuously operated in an overheated condition, the

output will appear to be oscillating. This feature provides

protection from a catastrophic device failure due to

accidental overheating. It is not intended to be used as a

substitute for proper heatsinking. The maximum device

power dissipation can be calculated by:

PD +

TJ(max) * TA

R

qJA

The devices are available in surface mount SOT−223 and

DPAK packages. Each package has an exposed metal tab

that is specifically designed to reduce the junction to air

board copper as a heat dissipater. Figures 18 and 19 show

pattern using economical single sided 2.0 ounce copper

board material. The final product thermal limits should be

tested and quantified in order to insure acceptable

considerably from the graphs shown. This will be due to any

changes made in the copper aspect ratio of the final layout,

adjacent heat sources, and air flow.