LTC4449 datasheet(26/48 Pages) AD

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LTC7872

Rev. 0

For more information www.analog.com

The output ripple current is divided between the various

capacitors connected in parallel at the output voltage.

Although ceramic capacitors are generally known for low

ESR (especially X5R and X7R), these capacitors suffer

from a relatively high voltage coefficient. Therefore, it is

not safe to assume that the entire ripple current flows in

the ceramic capacitor. Aluminum electrolytic capacitors

are generally chosen because of their high bulk capaci-

tance, but they have a relatively high ESR. As a result,

some amount of ripple current will flow in this capacitor.

If the ripple current flowing into a capacitor exceeds its

RMS rating, the capacitor will heat up, reducing its effec-

tive capacitance and adversely affecting its reliability. After

the output capacitor configuration has been determined

using the equations provided, measure the individual

capacitor case temperatures in order to verify good ther-

mal performance.

Setting Output Voltage

The LTC7872 output voltage is set by two external feed-

back resistive dividers carefully placed across VHIGH to

ground and VLOW to ground, as shown in Figure 6. The

regulated output voltage is determined by:

VLOW = 1.2V • 1+

⎛

⎝⎜

⎞

⎠⎟

and VHIGH = 1.2V • 1+

⎛

⎝⎜

⎞

⎠⎟

To improve the frequency response, a feed forward capac-

itor, CFF1/CFF2, may be used. Great care should be taken

to route the feedback line away from noise sources, such

as the inductor or the SW line.

Figure 6.

VHIGH

CFF2

LTC7872

VFBLOW

VLOW

CFF1

7872 F06

VFBHIGH

Setting Output Voltage

ripple will change, depending on the requirements of the

application, and the equations provided below can easily

be modified.

One of the key benefits of multiphase operation is a reduc-

tion in the peak current supplied to the output capacitor

by the boost diodes. As a result, the ESR requirement

of the capacitor is relaxed. For a 1% contribution to the

total ripple voltage, the ESR of the output capacitor can

be determined using the following equation:

ESRCOUT >

0.01• VOUT

ID PEAK

(

)

where:

ID PEAK

(

) =

• 1

+ χ

⎛

⎝

⎞

⎠ •

IO MAX

(

)

1–DMAX

The factor n represents the number of phases and the

factor χ represents the percentage inductor ripple current.

For the bulk capacitance, which we assume contributes

1% to the total output ripple, the minimum required

capacitance is approximately:

COUT ≥

IO MAX

(

)

0.01• n • VOUT • f

For many designs it will be necessary to use one type of

capacitor to obtain the required ESR, and another type to

satisfy the bulk capacitance. For example, using a low ESR

ceramic capacitor can minimize the ESR step, while an

electrolytic capacitor can be used to supply the required

bulk C.

The voltage rating of the output capacitor must be greater

than the maximum output voltage, with sufficient derating

to account for the maximum capacitor temperature.

Because the ripple current in the output capacitor is a

square wave, the ripple current requirements for this

capacitor depend on the duty cycle, the number of phases

and the maximum output current. In order to choose a

ripple current rating for the output capacitor, first establish

the duty cycle range, based on the output voltage and

range of input voltage.

APPLICATIONS INFORMATION

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