ADP3088

–6–

REV. PrK

PRELIMINARY TECHNICAL DATA

equal, this is generally better accomplished with a better

quality or proportionally larger output capacitor instead,

since a larger inductor degrades the large-signal transient

performance capability.

A conservative nominal design target value for the inductor

of a typical application circuit is that which creates a peak-

to-peak ripple current,

that is approximately a third of the nominal 500mA rating

of the ADP3088. The reason for not suggesting to base the

ripple current on the maximum load current is for concern

of overload protection. Scaling of the ripple currents with

lower load currents would yield higher inductor values that

might give satisfactory operation, but in order for overload

operation up to the current limit level of the ADP3088 to

be satisfactory, it would be necessary to choose an inductor

rated up to that higher current, which would likely yield an

unsatisfactory inductor size and cost. In any case, having

chosen a target level for

∆I

value is given by:

()

()

1

OF

SW

L

DV

V

L

fI

−×

+

=

×∆

(4)

where D is the duty ratio - the suffix indicating continuous

inductor current - and is given by:

OF

IN

F

SW

VV

D

VV

V

+

=

+−

(5)

switching frequency of the ADP3088. The formula sug-

gests the calculation of L using a nominal input voltage, and

to optimize the design. In applications where load tran-

sients are not severe, this conservative design for L is rec-

ommended. A more aggressive minimization of L is

outlined below, but a few restrictions are noted.

As inductance becomes smaller the ripple current becomes

larger. If the ripple becomes particularly large or, as an

additional factor, if the load is particularly dynamic, then

there is an increasing possibility that the peak inductor cur-

rent will undesirably reach the current limit shutdown

minimum inductor value to keep the ripple current moder-

ated. An alternate way to prevent excessive dynamic over-

shoot of inductor current during a load transient is to

reduce the DC gain of the error amplifier by adding resis-

tive feedback; this idea is discussed below.

Another important restriction of the minimum inductor

value may apply. The design should ensure against possible

subharmonic oscillation that can occur in all fixed-fre-

quency current-controlled switching power supplies when

switching at high duty ratios. The subharmonic oscillation

phenomenon will not be explained here - there are plenty

of papers written on the subject - except to say that it is

characterized by alternating high and low duty ratios - i.e.,

every other cycle - that produces additional ripple on the

output. To prevent subharmonic oscillation the following

restriction for the minimum inductor value is recom-

mended:

()

()

OF

OF

IN MIN

VV

H

LV

V

VV

2

0.35

+

µ

>

×+×

−

(6)

input voltage for which normal high performance operation

must be assured. Note the value returned for L may be

negative in which case the restriction does not apply. If the

preceding formula yields a lower inductor value than the

conservative recommendation given previously, as is likely

for most applications, then it is time to consider further

limitations to see how low the value can be minimized.

For a given inductor selection, the earlier formula is rear-

ranged for convenience and skewed to the worst case input

voltage to determine the maximum inductor ripple current,

()

()

()

O

OSW

IN MAX

F

LMAX

FSW

SW

IN MAX

VV

V

VV

I

VV

V

f

L

−−

+

∆=

×

+−

×

(7)

Performance degradation of the inductor - consisting of

some loss of inductance or excessive power loss - may be

encountered at higher ripple currents, so the ripple current

figure, together with knowledge of the expected DC cur-

rent should be checked against specifications of the induc-

tor.

If the ESR of the output capacitor is substantial - as it is

likely to be if a MLC capacitor is not used - then the ripple

voltage on the output, dominated by the ESR, may be

substantial and of concern for regulation specifications.

The resistive component of the output voltage ripple is

simply the ripple current times the ESR, and if it is more

than a few millivolts it will dominate the output capacitance

in contributing to output ripple voltage.

The boundary condition of the inductor reaching the bor-

()

OF

IN

O

SW

OBL

SW

IN

F

SW

VV

V

V

V

I

fL

V

V

V

2

+−

−

=×

+−

(8)

Below this output current level, the inductor current will be

discontinuous and the duty ratio will be modulated to lower

values, by factors substantially more than thus the losses

that cause only a small amount of modulation in continuous

inductor current operation. PSM is initiated automatically

by a proprietary technique comprising a duty ratio amplifier

with an internal time constant. As load current drops well

into the low current region and the duty ratio passes below

vated. The corresponding level of output current is given

by:

()

IN

F

SW

IN

SW

PSM

OPSM

OF

SW

VV

V

V

V

V

ID

VV

f

L

2

0

1

2

+−

−

−

=×

×

+×

(9)

It can be seen in the formula that this current threshold is

inversely proportional to inductance, so although it is usu-

ally not a relevant concern, it is noted that an aggressively