PROGRAMMABLE SYNCHRONOUS DC/DC

CONVERTER WITH LOW DROPOUT

REGULATOR CONTROLLER

SC1172/3

© 1999 SEMTECH CORP.

October 25, 1999

652 MITCHELL ROAD NEWBURY PARK CA 91320

10

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

I

excursion

voltage

transient

Maximum

V

Where

I

V

R

t

t

t

t

ESR

=

=

≤

Each Capacitor

Total

Technology

C

(µF)

ESR

(m

Ω)

Qty.

Rqd.

C

(µF)

ESR

(m

Ω)

Low ESR Tantalum

330

60

6

2000

10

OS-CON

330

25

3

990

8.3

Low ESR Aluminum

1500

44

5

7500

8.8

()

O

IN

t

ESR

V

V

I

C

R

L

−

≤

OSC

IN

L

f

L

4

V

I

RIPPLE

⋅

⋅

=

IN

O

)

on

(

DS

2

O

COND

V

V

cycle

duty

=

where

R

I

P

≈

δ

δ

⋅

⋅

=

2

IN

O

SW

10

V

I

P

−

⋅

⋅

=

4

f

)

t

t

(

V

I

P

OSC

f

r

IN

O

SW

⋅

+

⋅

⋅

=

OSC

IN

RR

RR

f

V

Q

P

⋅

⋅

=

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:

The choice of which to use is simply a cost /perfor-

mance 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 in-

ductor will produce a slow current ramp rate and will

cause the output capacitor to supply more of the tran-

sient load current for longer - leading to an output volt-

age sag below the ESR excursion calculated above.

The maximum inductor value may be calculated from:

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 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 capac-

itor ESR. Ripple current can be calculated from:

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

combination of conduction losses, switching losses and

bottom FET body diode recovery losses.

a) Conduction losses are simply calculated as:

b) Switching losses can be estimated by assuming a

switching time, if we assume 100ns then:

or more generally,

c) Body diode recovery losses are more difficult to esti-

mate, but to a first approximation, it is reasonable to as-

sume that the stored charge on the bottom FET body

diode will be moved through the top FET as it starts to

turn on. The resulting power dissipation in the top FET

will be:

To a first order approximation, it is convenient to only

consider conduction losses to determine FET suitability.

For a 5V in; 2.8V out at 14.2A requirement, typical FET

losses would be: