TDA3681TH datasheet(21/28 Pages) PHILIPS

21 / 28 page

2002 Apr 10

Philips Semiconductors

Product speciﬁcation

Multiple voltage regulator with switch and

ignition buffer

TDA3681

Application information

NOISE

Table 1

Noise ﬁgures

Note

1. Measured at a bandwidth of 30 kHz.

The noise on the supply line depends on the value of the

supply capacitor and is caused by a current noise (the

output noise of the regulators is translated to a current

noise by the output capacitors). The noise is minimal when

a high frequency capacitor of 220 nF in parallel with an

electrolytic capacitor of 100

µF is connected directly to the

supply pins VP1, VP2 and GND.

STABILITY

The regulators are stabilized by the externally connected

output capacitors.

The output capacitors can be selected by using the graphs

given in Figs 12 and 13. When an electrolytic capacitor is

used, its temperature behaviour can cause oscillations at

a low temperature. The two examples below show how an

output capacitor value is selected.

Example 1

Regulators 1, 3 and 4 are stabilized with an electrolytic

output capacitor of 220

µF (ESR = 0.15 Ω). At

Tamb =−30 °C, the capacitor value is decreased to 73 µF

and the ESR is increased to 1.1

Ω. The regulator remains

stable at Tamb = −30 °C (see Fig.12).

Example 2

Regulator 2 is stabilized with a 10

µF electrolytic capacitor

(ESR = 3

Ω). At Tamb = −30 °C, the capacitor value is

decreased to 3

µF and the ESR is increased to 23.1 Ω.

As can be seen from Fig.13, the regulator will be unstable

at Tamb = −30 °C.

Solution

To avoid problems with stability at low temperatures, the

use of tantalum capacitors is recommended. Use a

tantalum capacitor of 10

µF or a larger electrolytic

capacitor.

REGULATOR

NOISE FIGURE (

µV)(1)

Co =10 µFCo =47 µFCo = 100 µF

170

110

440

240

190

120

100

485

handbook, halfpage

0.1

C (

µF)

MGL912

stable region

maximum ESR

100

ESR

(

Ω)

Fig.12 Curve for selecting the value of the output

capacitor for regulators 1, 3 and 4.

handbook, halfpage

−2

0.22

110

C (

µF)

MGL913

stable region

maximum ESR

minimum ESR

100

ESR

(

Ω)

Fig.13 Curve for selecting the value of the output

capacitor for regulator 2.

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