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SC4612HMLTRT

40V Synchronous Buck Controller

Semtech Corporation 

SC4612H

Applications Information (continued)

0dB at a rate of -20dB/dec.

4. Cancel wESR with compensation pole wP1 (wP1=wESR).

5. Place a high frequency compensation pole wP2 at

half the switching frequency to get the maximum

attenuation of the switching ripple and the high

frequency noise with adequate phase lag at wC.

Technology

Ceramic

Each

Qty

Capacitor

Rqd.

C

ESR

(mF) (mW)

22 2-10

1

Total

C

ESR

(mF) (mW)

22 2-10

SP Cap

220

7

POS-CAP

680

18

Low ESR Aluminum 1500 44

1

220 7.0

2

1360 9.0

5

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.

Fig2. Power stage and compensated loop gain.

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.

RESR

≤

Vt

It

where,

. . . . . . . . . (4)

Vt = Maximum transient voltage excursion

It = Transient current step

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

10A load step, the output capacitor ESR must be less

than 10mW. To meet this kind of ESR level, there are

four available capacitor technologies.

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:

( ) L ≤ RESR ⋅ C

It

VIN − VO

. . . . . . . . . (5)

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%-20% 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:

ILRIPPLE

=

VIN

4 ⋅ L ⋅ fOSC

. . . . . . . . . (6)

Ripple current allowance will define the minimum

permitted inductor value.

11

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