Application Information
CAPACITOR SELECTION
(Continued)
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
Q
(V+) is the quiescent power loss of the IC device, and I
L2
R
OUT
is the conversion loss associated with the switch on-
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
1
and C
2
are usually chosen to be the same.
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.
DS100003-36
FIGURE 3. Output Source Resistance vs Oscillator Frequency
TABLE 3. Low ESR Capacitor Manufacturers
Manufacturer
Nichicon Corp.
AVX Corp.
Sprague
Sanyo
Murata
Taiyo Yuden
Tokin
Phone
(708)-843-7500
(803)-448-9411
(207)-324-4140
(619)-661-6835
(800)-831-9172
(800)-348-2496
(408)-432-8020
Capacitor Type
PL, PF series, through-hole aluminum electrolytic
TPS series, surface-mount tantalum
593D, 594D, 595D series, surface-mount tantalum
OS-CON series, through-hole aluminum electrolytic
Ceramic chip capacitors
Ceramic chip capacitors
Ceramic chip capacitors
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