NCP1086

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7

Protection Diodes

When large external capacitors are used with a linear

regulator it is sometimes necessary to add protection diodes.

If the input voltage of the regulator gets shorted, the output

capacitor will discharge into the output of the regulator. The

discharge current depends on the value of the capacitor, the

NCP1086 series linear regulator, the discharge path is

through a large junction and protection diodes are not

usually needed. If the regulator is used with large values of

output capacitance and the input voltage is instantaneously

shorted to ground, damage can occur. In this case, a diode

connected as shown in Figure 17 or Figure 18 is

recommended.

Figure 17. Protection Diode Scheme for Large Output

Capacitors (Adjustable Output)

Adj

NCP1086

IN4002 (optional)

Figure 18. Protection Diode Scheme for Large Output

Capacitors (3.3 V Fixed Output)

GND

NCP1086

IN4002 (optional)

Output Voltage Sensing

Since the NCP1086 is a three terminal regulator, it is not

possible to provide true remote load sensing. Load

regulation is limited by the resistance of the conductors

connecting the regulator to the load.

For best results the fixed output regulator should be

connected as shown in Figure 19.

Figure 19. Conductor Parasitic Resistance Effects

Can Be Minimized with the Above Grounding

Scheme for Fixed Output Regulators

NCP1086

Conductor Parasitic

Resistance

For the adjustable regulator, the best load regulation

occurs when R1 is connected directly to the output pin of the

regulator as shown in Figure 20. If R1 is connected to the

resistance between the regulator and the load becomes

RC

R1

) R2

R1

Figure 20. Grounding Scheme for the

Adjustable Output Regulator to Minimize

Parasitic Resistance Effects

Adj

NCP1086

Conductor Parasitic

Resistance

Calculating Power Dissipation and

Heatsink Requirements

The NCP1086 linear regulator includes thermal shutdown

and current limit circuitry to protect the device. High power

regulators such as these usually operate at high junction

temperatures so it is important to calculate the power

dissipation and junction temperatures accurately to ensure

that an adequate heatsink is used.