SC4612HEVB datasheet(13/21 Pages) SEMTECH

13 / 21 page

www.semtech.com

SC4612H

POWER MANAGEMENT

COMPONENT SELECTION:

SWITCHING SECTION

OUTPUT CAPACITORS - Selection begins with the most

critical component. Because of fast transient load current

requirements in modern microprocessor core supplies, the

output capacitors must supply all transient load current

requirements until the current in the output inductor ramps

up to the new level. Output capacitor ESR is therefore one

of the most important criteria. The maximum 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

10mΩ. To meet this kind of ESR level, there are three

available capacitor technologies.

Technology

Each

Capacitor

Qty

Rqd.

Total

(uF)

ESR

Ω)

(uF)

ESR

Ω)

Ceramic

2-10

SP Cap

220

7.0

POS-CAP

680

1360

9.0

Low ESR Aluminum

1500

7500

8.8

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

inductor can be calculated. Too large an inductor will

produce 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 maximum

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 ensuring a good

recovery from transient with no additional excursions. We

must also be concerned with ripple current 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 permitted

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

combination of conduction losses, switching losses and

bottom 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:

100

⋅

or more generally,

)

(

OSC

⋅

c) Body diode recovery losses are more difficult to estimate,

but to a first approximation, it is reasonable to assume

Application Information (Cont.)

SC4612HEVB датащи(PDF) 13 Page - Semtech Corporation

SC4612HEVB датащи(HTML) 13 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

датащи скачать

ссылки URL