TK71145 Просмотр технического описания (PDF) - Toko America Inc

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TK71145

LOW DROPOUT REGULATOR

Toko America Inc  

TK711xx

TERMS AND DEFINITIONS (CONT.)

APPLICATION INFORMATION

The range of usable currents can also be found from the INPUT/OUTPUT DECOUPLING CAPACITOR

graph below.

CONSIDERATIONS

(mW)

3

PD

DPD

6

4

5

25

50

75

150

TA (°C)

Procedure:

1) Find PD

2) PD1 is taken to be PD x (~ 0.8 - 0.9)

3) Plot PD1 against 25 °C

4) Connect PD1 to the point corresponding to the 150 °C

with a straight line.

5) In design, take a vertical line from the maximum

operating temperature (e.g., 75 °C) to the derating

curve.

6) Read off the value of PD against the point at which the

vertical line intersects the derating curve. This is taken

as the maximum power dissipation, DPD.

The maximum operating current is:

IOUT = (DPD / (VIN(MAX) - VOUT)

1000

800

600

MOUNTED

400

200

0

0

50

100

150

TA (°C)

TO-92 POWER DISSIPATION CURVE

Voltage regulators require input and output decoupling

capacitors. The required value of these capacitors vary

with application. Capacitors made by different

manufacturers can have different characteristics,

particularly with regard to high frequencies and Equivalent

Series Resistance (ESR) over temperature. The type of

capacitor is also important. For example, a 4.7 µF aluminum

electrolytic may be required for a certain application. If a

tantalum capacitor is used, a lower value of 2.2 µF would

be adequate. It is important to consider the temperature

characteristics of the decoupling capacitors. While Toko

regulators are designed to operate as low as -40 °C, many

capacitors will not operate properly at this temperature.

The capacitance of aluminum electrolytic capacitors may

decrease to 0 at low temperatures. This may cause

oscillation on the output of the regulator since some

capacitance is required to guarantee stability. Thus, it is

important to consider the characteristics of the capacitor

over temperature when selecting decoupling capacitors.

The ESR is another important parameter. The ESR will

increase with temperature but low ESR capacitors are

often larger and more costly. In general, tantalum capacitors

offer lower ESR than aluminum electrolytic, but new low

ESR aluminum electrolytic capacitors are now available

from several manufacturers. Usually a bench test is

sufficient to determine the minimum capacitance required

for a particular application. After taking thermal

characteristics and tolerance into account, the minimum

capacitance value should be approximately two times this

value. The recommended minimum capacitance for the

TK711xxN is 2.1 µF for a tantalum capacitor or 3.3 µF for

an aluminum electrolytic. Please note that linear regulators

with a low dropout voltage have high internal loop gains

which require care in guarding against oscillation caused

by insufficient decoupling capacitance. The use of high

quality decoupling capacitors suited for your application

will guarantee proper operation of the circuit. Pay attention

to temperature characteristics of the capacitor, especially

the increase of ESR and decrease of capacitance in low

temperatures. Oscillation, reduction of ripple rejection and

increased noise may occur in some cases if the proper

capacitor is not used. An output capacitor more than 1.0 µF

is required to maintain stability. The standard test condition

is 3.3 µF (TA = 25 °C).

July 2000 TOKO, Inc.

Page 15

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