NCV8505 Series

http://onsemi.com

10

APPLICATION NOTES

SETTING THE DELAY TIME

The delay time is controlled by the Reset Delay Low

Voltage, Delay Switching Threshold, and the Delay Charge

Current. The delay follows the equation:

tDELAY +

[CDELAY(Vdt * Reset Delay Low Voltage)]

Delay Charge Current

Example:

Assume reset Delay Low Voltage = 0.

5.0 V options).

Use the typical value for Delay Charge Current = 4.2

mA.

tDELAY +

[33 nF(1.8 * 0)]

4.2 mA

+ 14 ms

STABILITY CONSIDERATIONS

The output or compensation capacitor helps determine

three main characteristics of a linear regulator: start−up

delay, load transient response and loop stability.

The capacitor value and type should be based on cost,

availability, size and temperature constraints. A tantalum or

aluminum electrolytic capacitor is best, since a film or

ceramic capacitor with almost zero ESR can cause

instability. The aluminum electrolytic capacitor is the least

expensive solution, but, if the circuit operates at low

temperatures (−25

°C to −40°C), both the value and ESR of

the capacitor will vary considerably. The capacitor

manufacturers data sheet usually provides this information.

should work for most applications, however it is not

necessarily the optimized solution.

Figure 18. Test and Application Circuit Showing

Output Compensation

33 mF

RESET

0.1 mF

NCV8505

filter.

temperature expected.

CALCULATING POWER DISSIPATION IN A

SINGLE OUTPUT LINEAR REGULATOR

The maximum power dissipation for a single output

regulator (Figure 19) is:

PD(max) + [VIN(max) * VOUT(min)]IOUT(max)

(1)

) VIN(max)IQ

where:

application, and

RqJA +

150°C * TA

PD

(2)

package section of the data sheet. Those packages with

the die temperature below 150

°C.

In some cases, none of the packages will be sufficient to

dissipate the heat generated by the IC, and an external

heatsink will be required.

SMART

REGULATOR®

Control

Features

Figure 19. Single Output Regulator with Key

Performance Parameters Labeled

}

HEAT SINKS

A heat sink effectively increases the surface area of the

package to improve the flow of heat away from the IC and

into the surrounding air.

Each material in the heat flow path between the IC and the

outside environment will have a thermal resistance. Like

series electrical resistances, these resistances are summed to

RqJA + RqJC ) RqCS ) RqSA

(3)

where:

functions of the package type, heatsink and the interface

between them. These values appear in heat sink data sheets

of heat sink manufacturers.