–8–

REV. 0

ADP3161

The cumulative errors in the output voltage regulations cut into

load regulation this relates directly to the ESR. When consider-

ing dc load regulation, this relates directly to the programmed

output resistance of the power converter.

Some error sources, such as initial voltage accuracy and ripple

voltage, can be directly deducted from the available regulation

window, while other error sources scale proportionally to the

amount of voltage positioning used, which, for an optimal design,

should utilize the maximum that the regulation window will allow.

The error determination is a closed-loop calculation, but it can

be closely approximated. To maintain a conservative design while

avoiding an impractical design, various error sources should be

considered and summed statistically.

The output ripple voltage can be factored into the calculation by

summing the output ripple current with the maximum output

current to determine an effective maximum dynamic current

change. The remaining errors are summed separately according

to the formula:

VV

V

k

I

II

k

k

kk

mV

WIN

VID

VID

O

OO

RCS

CSF

RT

EA

=×

×

+

+ 







++

















=

(– (

))

–.

∆

∆

2

1

2

83 5

2

2

22

(3)

8% accounts for the IC current loop gain tolerance including

The remaining window is then divided by the maximum output

current plus the ripple to determine the maximum allowed ESR

and output resistance:

RR

V

II

mV

AA

m

E MAX

OUT MAX

WIN

OO

()

()

.

.

.

==

+

=

+

=

∆

Ω

83 5

26

2 6

29

(4)

The output filter capacitor bank must have an ESR of less than

2.9 m

Ω. One can, for example, use four SP-Type OS-CON

capacitors from Sanyo, with 1.2 mF capacitance, a 2.5 V voltage

rating, and 11 m

Ω ESR. The four capacitors have a maximum

total ESR of 2.75 m

Ω when connected in parallel. Another

possibility is the ZA series from Rubycon. The trade-off is

size versus cost. Nine 1 mF capacitors would give an ESR of

2.67 m

Ω. These capacitors take up more space than four

OS-CON capacitors, but are significantly less expensive.

As long as the capacitance of the output capacitor is above a

critical value and the regulating loop is compensated with ADOPT,

the actual value has no influence on the peak-to-peak deviation

of the output voltage to a full step change in the load current.

The critical capacitance can be calculated as follows:

C

I

RV

L

A

m

H

mF

OUT CRIT

O

E

OUT

()

..

.

=

×

×

=

Ω×

× µ

=

2

26

275

1 8

1

2

26

(5)

The critical capacitance for the four OS-CON capacitors with

an equivalent ESR of 2.75 m

Ω is 2.6 mF, while the equivalent

capacitance of those capacitors is 4.8 mF. The critical capacitance

for the nine ZA series Rubycon capacitors is 2.8 mF while the

equivalent capacitance is 9 mF. With both choices, the capaci-

tance is safely above the critical value.

current. The current comparator of the ADP3161 has a mini-

mum current limit threshold of 69 mV. Note that the 69 mV

value cannot be used for the maximum specified nominal cur-

rent, as headroom is needed for ripple current and tolerances.

The current comparator threshold sets the peak of the inductor

twice the peak inductor current value less half of the peak-to-

peak inductor ripple current. From this the maximum value of

R

V

I

I

mV

AA

m

SENSE

CS CL

MIN

O

L RIPPLE

≤

+

=

+

=Ω

()(

)

()

.

.

22

69

13

2 85

44

(6)

In this case, 4 m

Ω was chosen as the closest standard value.

the maximum current sense threshold of 89 mV:

I

V

R

I

mV

m

AA

OUT CL

CS CL

MAX

SENSE

L RIPPLE

()

()(

)

()

–

–.

.

=×

=×

Ω

=

2

2

89

4

5 7

38 8

(7)

At output voltages below 425 mV, the current sense threshold is

reduced to 58 mV, and the ripple current is negligible. There-

fore, at dead short the output current is reduced to:

I

mV

mV

A

OUT SC

()

.

=×

=

2

58

4

29 0

(8)

To safely carry the current under maximum load conditions, the

sense resistor must have a power rating of at least:

PI

R

R

SENSE RMS

SENSE

SENSE

=×

()

2

(9)

where:

I

I

n

V

V

SENSE RMS

O

OUT

IN

()

2

2

=×

×

η

(10)

In this formula, n is the number of phases, and

η is the con-

verter efficiency, in this case assumed to be 85%. Combining

Equations 9 and 10 yields:

P

AV

V

mW

×

=

26

2

18

085

5

573

2

.

.

Power MOSFETs

In the standard two-phase application, two pairs of N-channel

power MOSFETs must be used with the ADP3161 and ADP3412,

one pair as the main (control) switches, and the other pair as

the synchronous rectifier switches. The main selection parameters