NCP5501 Просмотр технического описания (PDF) - ON Semiconductor

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NCP5501

LDO Voltage Regulator 500 mA

ON Semiconductor 

NCP5500, NCV5500, NCP5501, NCV5501

Calculating Resistors for the ADJ Versions

The adjustable version uses feedback resistors to adjust

the output to the desired output voltage. With Vout connected

to ADJ, the adjustable version will regulate at 1.25 V

4.9% (1250  61.25 mV).

Output voltage formula with an external resistor divider:

ǒ ƪ ƫǓ ǒ Ǔ Vout +

1.25 V *

60E−9

@

(R1 @ R2)

(R1 ) R2)

@

(R1 ) R2)

R2

Where

R1 = value of the divider resistor connected between Vout

and ADJ,

R2 = value of the divider resistor connected between ADJ

and GND,

The term “1.25 V” has a tolerance of 4.9%; the term

“60E−9” can vary in the range 15E−9 to 60E−9.

For values of R2 less than 15 KW, the term within brackets

( [ ] ) will evaluate to less than 1 mV and can be ignored. This

simplifies the output voltage formula to:

Vout = 1.25 V * ((R1 + R2) / R2)) with a tolerance of 4.9%,

which is the tolerance of the 1.25 V output when delivering

up to 500 mA of output current.

DEFINITION OF TERMS

Dropout Voltage: The input−to−output voltage differential

at which the circuit ceases to regulate against further

reduction input voltage. Measured when the output voltage

has dropped 2% relative to the value measured at nominal

input voltage. Dropout voltage is dependent upon load

current and junction temperature.

Input Voltage: The DC voltage applied to the input

terminals with respect to ground.

Line Regulation: The change in output voltage for a change

in the input voltage. The measurement is made under

conditions of low dissipation or by using pulse techniques

such that the average chip temperature is not significantly

affected.

Load Regulation: The change in output voltage for a change

in load current at constant chip temperature. Pulse loading

techniques are employed such that the average chip

temperature is not significantly affected.

Quiescent and Ground Current: The quiescent current is

the current which flows through the ground when the LDO

operates without a load on its output: internal IC operation,

bias, etc. When the LDO becomes loaded, this term is called

the Ground current. It is actually the difference between the

input current (measured through the LDO input pin) and the

output current.

Ripple Rejection: The ratio of the peak−to−peak input ripple

voltage to the peak−to−peak output ripple voltage.

Current Limit: Peak current that can be delivered to the

output.

Calculating Power Dissipation

The maximum power dissipation for a single output

regulator (Figure 21) is:

ƪ ƫ (eq. 1)

PD(max) + Vin(max) * Vout(min) Iout(max) ) Vin(max)IGND

Where

Vin(max) is the maximum input voltage,

Vout(min) is the minimum output voltage,

Iout(max) is the maximum output current for the application,

IGND is the ground current at Iout(max).

Once the value of PD(max) is known, the maximum

permissible value of RqJA can be calculated:

RqJA

+

ǒ150°C *

PD

TAǓ

(eq. 2)

The value of RqJA can then be compared with those in the

Thermal Characteristics table. Those packages with RqJA

less than the calculated value in Equation 2 will keep 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 heat

sink will be required.

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

determine the value of RqJA:

RqJA + RqJC ) RqCS ) RqSA

(eq. 3)

where

RqJC is the junction−to−case thermal resistance,

RqCS is the case−to−heatsink thermal resistance,

RqSA is the heatsink−to−ambient thermal resistance.

RqJC appears in the Thermal Characteristics table. Like

RqJA, it too is a function of package type. RqCS and RqSA are

functions of the package type, heat sink and the interface

between them. These values appear in data sheets of heat

sink manufacturers.

Thermal, mounting, and heat sink considerations are

further discussed in ON Semiconductor Application Note

AN1040/D.

http://onsemi.com

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