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LTC3261

3261f

APPLICATIONS INFORMATION

Effective Open-Loop Output Resistance

The effective open-loop output resistance (ROL) of a charge

pump is a very important parameter which determines the

strength of the charge pump. The value of this parameter

depends on many factors such as the oscillator frequency

(fOSC), value of the flying capacitor (CFLY), the nonoverlap

time, the internal switch resistances (RS) and the ESR of

the external capacitors.

Typical ROL values as a function of temperature are shown

in Figure 2

Figure 2. Typical ROL vs Temperature

VRIPPLE(P-P) ≈

IOUT

COUT

•

fOSC

–tON

⎛

⎝

⎜

⎞

⎠

⎟

where fOSC is the oscillator frequency tON is the on-time

of the oscillator (1μs) typical and COUT is the value of the

output capacitor.

Just as the value of COUT controls the amount of output

ripple, the value of CINcontrolstheamountofripplepresent

at the input (VIN) pin. The amount of bypass capacitance

required at the input depends on the source impedance

driving VIN. For best results it is recommended that VIN

be bypassed with at least 2μF of low ESR capacitance. A

high ESR capacitor such as tantalum or aluminum will

have higher input noise than a low ESR ceramic capacitor.

Therefore, a ceramic capacitor is recommended as the

main bypass capacitance with a tantalum or aluminum

capacitor used in parallel if desired.

Flying Capacitor Selection

The flying capacitor controls the strength of the charge

pump. A 1μF or greater ceramic capacitor is suggested

for the flying capacitor for applications requiring the full

rated output current of the charge pump.

For very light load applications, the flying capacitor may

be reduced to save space or cost. For example, a 0.2μF

capacitor might be sufficient for load currents up to 20mA.

A smaller flying capacitor leads to a larger effective open-

loop resistance (ROL) and thus limits the maximum load

current that can be delivered by the charge pump.

Ceramic Capacitors

Ceramic capacitors of different materials lose their capaci-

tance with higher temperature and voltage at different rates.

For example, a capacitor made of X5R or X7R material will

retain most of its capacitance from –40°C to 85°C whereas a

Z5UorY5Vstylecapacitorwillloseconsiderablecapacitance

over that range. Z5U and Y5V capacitors may also have a

poorvoltagecoefficientcausingthemtolose60%ormoreof

theircapacitancewhentheratedvoltageisapplied.Therefore

when comparing different capacitors, it is often more ap-

propriate to compare the amount of achievable capacitance

for a given case size rather than discussing the specified

capacitance value. The capacitor manufacture’s data sheet

Input/Output Capacitor Selection

The style and value of capacitors used with the LTC3261

determine several important parameters such as regulator

control loop stability, output ripple, charge pump strength

and minimum turn-on time. To reduce noise and ripple, it is

recommended that low ESR ceramic capacitors be used for

the charge pump output. The charge pump output capacitor

should retain at least 2μF of capacitance over operating

temperature and bias voltage. Tantalum and aluminum

capacitors can be used in parallel with a ceramic capacitor

to increase the total capacitance but should not be used

alone because of their high ESR. In constant frequency

mode, the value of COUT directly controls the amount of

output ripple for a given load current. Increasing the size of

COUT will reduce the output ripple at the expense of higher

minimum turn-on time. The peak-to-peak output ripple at

the VOUT pin is approximately given by the expression:

TEMPERATURE (°C)

–50

100

–25

150

125

3261 F02

VIN = 12V

VIN = 25V

VIN = 32V

fOSC = 500kHz

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