CAPACITOR SELECTION

As discussed in the

Simple Negative Voltage Converter section, the output resistance and ripple voltage are dependent on the

capacitance and ESR values of the external capacitors. The output voltage drop is the load current times the output resistance,

and the power efficiency is

Where I

resistance, the two external capacitors and their ESRs.

Low ESR capacitors (

Table 3) are recommended for both capacitors to maximize efficiency, reduce the output voltage drop and

voltage ripple. For convenience, C

The output resistance varies with the oscillator frequency and the capacitors. In

Figure 3, the output resistance vs. oscillator fre-

quency curves are drawn for four difference capacitor values. At very low frequency range, capacitance plays the most important

role in determining the output resistance. Once the frequency is increased to some point (such as 100 kHz for the 47 µF capaci-

tors), the output resistance is dominated by the ON resistance of the internal switches and the ESRs of the external capacitors.

A low value, smaller size capacitor usually has a higher ESR compared with a bigger size capacitor of the same type. Ceramic

capacitors can be chosen for their lower ESR. As shown in

Figure 3, in higher frequency range, the output resistance using the

10 µF ceramic capacitors is close to these using higher value tantalum capacitors.

TABLE 3. Low ESR Capacitor Manufacturers

Manufacturer

Phone

Capacitor Type

Nichicon Corp.

(708)-843-7500

PL, PF series, through-hole aluminum electrolytic

AVX Corp.

(803)-448-9411

TPS series, surface-mount tantalum

Sprague

(207)-324-4140

593D, 594D, 595D series, surface-mount tantalum

Sanyo

(619)-661-6835

OS-CON series, through-hole aluminum electrolytic

Murata

(800)-831-9172

Ceramic chip capacitors

Taiyo Yuden

(800)-348-2496

Ceramic chip capacitors

Tokin

(408)-432-8020

Ceramic chip capacitors

DS100003-36

FIGURE 3. Output Source Resistance vs Oscillator Frequency

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