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MAX660

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SNOS405B – NOVEMBER 1999 – REVISED MAY 2017

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where

•

RSW is the sum of the ON resistance of the internal MOS switches shown in Figure 13.

(1)

High-value, low-ESR capacitors reduce the output resistance. Instead of increasing the capacitance, the

oscillator frequency can be increased to reduce the 2/(ƒOSCc × C1) term. Once this term is trivial compared with

RSW and ESRs, further increase to oscillator frequency and capacitance become ineffective. The peak-to-peak

output voltage ripple is determined by the oscillator frequency, and the capacitance and ESR of the output

capacitor C2:

(2)

Again, using a low-ESR capacitor results in lower ripple.

10.2.1.2.1

Capacitor Selection

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

shown in Equation 3:

where

•

IQ(V+) is the quiescent power loss of the device

•

IL

2

ROUT is the conversion loss associated with the switch on-resistance, the two external capacitors and their

ESRs

(3)

Because the switching current charging and discharging C1 is approximately twice that of the output current, the

effect of the ESR of the pumping capacitor C1 is multiplied by four in the output resistance. The output capacitor

C2 is charging and discharging at a current approximately equal to the output current; therefore, its ESR only

counts once in the output resistance. However, the ESR of C2 directly affects the output voltage ripple.

Therefore, TI recommends low-ESR capacitors (Table 3) for both capacitors to maximize efficiency, reduce the

output voltage drop and voltage ripple. For convenience, C1 and C2 are usually chosen to be the same. The

output resistance varies with the oscillator frequency and the capacitors. In Figure 15, the output resistance vs

oscillator frequency curves are drawn for three different tantalum capacitors. 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 20 kHz for the 150-μF capacitors), 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 larger size capacitor of the same type. For lower ESR, use ceramic

capacitors.

Figure 15. Output Source Resistance vs Oscillator Frequency