SP6120EY datasheet(15/22 Pages) SIPEX

15 / 22 page

Date: 1/21/05

SP6120 Low Voltage, AnyFETTM, Synchronous, Buck Controller

Kemet T510 surface mount capacitors are popu-

lar tantalum capacitors that work well in SP6120

applications. POSCAP from Sanyo is a solid

electrolytic chip capacitor that has low ESR and

high capacitance. For the same ESR value,

POSCAP has lower profile compared with tan-

talum capacitor.

Input Capacitor Selection

The input capacitor should be selected for ripple

current rating, capacitance and voltage rating.

The input capacitor must meet the ripple current

requirement imposed by the switching current.

In continuous conduction mode, the source cur-

rent of the high-side MOSFET is approximately

a square wave of duty cycle VOUT/VIN. Most of

this current is supplied by the input bypass

capacitors. The RMS value of input capacitor

current is determined at the maximum output

current and under the assumption that the peak

to peak inductor ripple current is low, it is given

by:

CIN(rms) = IOUT(max)

√D(1 - D)

The worse case occurs when the duty cycle D is

50% and gives an RMS current value equal to

IOUT /2. Select input capacitors with adequate

ripple current rating to ensure reliable opera-

tion.

The power dissipated in the input capacitor is:

)

(

)

(

CIN

ESR

rms

CIN

This can become a significant part of power

losses in a converter and hurt the overall energy

transfer efficiency.

The input voltage ripple primarily depends on

the input capacitor ESR and capacitance. Ignor-

ing the inductor ripple current, the input voltage

ripple can be determined by:

)

(

)

(

(max)

)

(

OUT

MAX

OUT

CIN

ESR

out

−

∆

The capacitor type suitable for the output ca-

pacitors can also be used for the input capaci-

tors. However, exercise extra caution when tanta-

SP6120 adjusts the inductor current to the new

value. Therefore the capacitance must be large

enough so that the output voltage is held up

while the inductor current ramps up or down to

the value corresponding to the new load current.

Additionally, the ESR in the output capacitor

causes a step in the output voltage equal to the

ESR value multiplied by the change in load

current. Because of the fast transient response

and inherent 100% and 0% duty cycle capabil-

ity provided by the SP6120 when exposed to

output load transient, the output capacitor is

typically chosen for ESR, not for capacitance

value.

The output capacitor’s ESR, combined with the

inductor ripple current, is typically the main

contributor to output voltage ripple. The maxi-

mum allowable ESR required to maintain a

specified output voltage ripple can be calculated

by:

OUT

ESR

∆

≤

where:

∆VOUT = peak to peak output voltage ripple

IPP = peak to peak inductor ripple current

The total output ripple is a combination of the

ESR and the output capacitance value and can

be calculated as follows:

∆VOUT =

(IPP(1–D))

+ (IPPRESR)2

COUTFS

where:

FS = switching frequency

D = duty cycle

COUT = output capacitance value

Recommended capacitors that can be used ef-

fectively in SP6120 applications are: low-ESR

aluminum electrolytic capacitors, OS-CON ca-

pacitors that provide a very high performance/

size ratio for electrolytic capacitors and low-

ESR tantalum capacitors. AVX TPS series and

APPLICATIONS INFORMATION: Continued

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