SC1109BSTR datasheet(10/19 Pages) SEMTECH

10 / 19 page

 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC1109

COMPONENT SELECTION

SWITCHING SECTION

OUTPUT CAPACITORS - Selection begins with the most

critical component. Because of fast transient load cur-

rent requirements in modern microprocessor core sup-

plies, the output capacitors must supply all transient load

current requirements until the current in the output in-

ductor ramps up to the new level. Output capacitor ESR

is therefore one of the most important criteria. The maxi-

mum ESR can be simply calculated from:

step

current

Transient

excursion

voltage

transient

Maximum

Where

≤

ESR

For example, to meet a 100mV transient limit with a 10A

load step, the output capacitor ESR must be less than

10m

Ω. To meet this kind of ESR level, there are three

available capacitor technologies.

)

(

Ω)

)

(

Ω)

The choice of which to use is simply a cost/performance

issue, with Low ESR Aluminum being the cheapest, but

taking up the most space.

INDUCTOR - Having decided on a suitable type and value

of output capacitor, the maximum allowable value of in-

ductor can be calculated. Too large an inductor will pro-

duce a slow current ramp rate and will cause the output

capacitor to supply more of the transient load current

for longer - leading to an output voltage sag below the

ESR excursion calculated above.

The maximum inductor value may be calculated from:

()

ESR

−

≤

The calculated maximum inductor value assumes 100%

duty cycle, so some allowance must be made. Choosing

an inductor value of 50 to 75% of the calculated maxi-

mum will guarantee that the inductor current will ramp

fast enough to reduce the voltage dropped across the

ESR at a faster rate than the capacitor sags, hence en-

suring a good recovery from transient with no additional

excursions. We must also be concerned with ripple cur-

rent in the output inductor and a general rule of thumb

has been to allow 10% of maximum output current as

ripple current. Note that most of the output voltage ripple

is produced by the inductor ripple current flowing in the

output capacitor ESR. Ripple current can be calculated

from:

OSC

RIPPLE

⋅

Ripple current allowance will define the minimum permit-

ted inductor value.

POWER FETS - The FETs are chosen based on several

criteria with probably the most important being power

dissipation and power handling capability.

TOP FET - The power dissipation in the top FET is a com-

bination of conduction losses, switching losses and bot-

tom FET body diode recovery losses.

a) Conduction losses are simply calculated as:

)

(

COND

≈

⋅

cycle

duty

where

b) Switching losses can be estimated by assuming a

switching time, if we assume 100ns then:

−

⋅

Application Information (Cont.)

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