REV. 0

ADP3156

–8–

The converter operates at the nominal operating frequency only

conditions, the operating frequency decreases due to the para-

sitic voltage drops across the power devices. The actual mini-

and is a function of the finite resistances of various components

in the power converter.

The required ESR and capacitance drive the selection of the

type and quantity of the output capacitors. The ESR must be

small enough that both the resistive voltage deviation due to a

step change in the load current and the output ripple voltage

stay below the values defined in the specification of the supplied

circuitry. The capacitance must be large enough that the output

is held up while the inductor current ramps up or down to the

value corresponding to the new load current.

Taking into account the

±1% setpoint accuracy of the ADP3156,

and assuming a 1% (or 15 mV) peak-to-peak ripple, the allowed

static voltage deviation of the output voltage when the load

changes between the minimum and maximum values is:

(2

× 5% × 1.5 V) – (2 × 1% × 1.5 V) – (1% × 1.5 V) = 105 mV

This sets the maximum ESR at 105 mV/7 A = 15 m

Ω. Four

parallel capacitors of 470

µF with a maximum ESR of 60 mΩ

will achieve the 15 m

Ω maximum net ESR. Whether or not the

capacitance is sufficient must be determined after the inductor

is selected.

Inductor Selection

The minimum inductor value can be calculated from ESR, off-

time, dc output voltage and allowed peak-to-peak ripple voltage

using the following equation:

L

Vt

R

V

sm

mV

H

MIN

O OFF

E MAX

RIPPLE p p

1

15

32

7

15

224

==

××

=µ

−

()

..

.

µΩ

The minimum inductance gives a peak-to-peak ripple current of

inductor peak current in normal operation is:

The inductor valley current is:

The inductor for this application should have an inductance of

not less than 2.24

µH at full load current and should not satu-

rate at the worst-case overload or short circuit current at the

maximum specified ambient temperature. For this example, it is

assumed the inductance might drop as much as 33% due to

load current, so its initial value might be as high as 3.36

µH.

Tips for Selecting the Inductor Core

Ferrite designs have very low core loss, so the design should

focus on copper loss and on preventing saturation. Molypermalloy,

or MPP, is a low loss core material for toroids, and it yields the

smallest size inductor, but MPP cores are more expensive than

ferrite cores or the Kool M

µ® cores from Magnetics, Inc. The

lowest cost core is made of powdered iron, for example the #52

material from Micrometals, Inc., but yields a larger size inductor.

The minimum capacitance of the output capacitor is determined

from the requirement that the output be held up while the in-

ductor current ramps up (or down) to the new value. The mini-

mum capacitance should produce an initial dv/dt which is equal

(but opposite in sign) to the dv/dt obtained by multiplying the

di/dt in the inductor and the ESR of the capacitor.

C

II

Rdi dt

C

AA

mV

H

F

MIN

OMAX

OMIN

E

MIN

=

=

−

()

=

–

(/

)

./ .

70

15

18

336

871

Ωµ

µ

In the above equation the value of di/dt is calculated as the

smaller voltage across the inductor (i.e., the smaller of

(3.36

µH) of the inductor. The four parallel-connected

470

µF capacitors have a total capacitance of 1880 µF, so the

minimum capacitance requirement is met with ample margin.

The current comparator of the ADP3156 has a threshold range

that extends from 0 mV to 125 mV (minimum). Note that the

full 125 mV range cannot be used for the maximum specified

nominal current, as headroom is needed for current ripple and

transients.

The current comparator threshold sets the peak of the inductor

the peak value less half of the peak-to-peak ripple current. Solv-

using the minimum current sense threshold of 125 mV yields:

Once

The actual short-circuit current is less than the above-calculated

output voltage drops below 1 V. The relationship between the

off-time and the output voltage is:

t

CV

V

k

A

OFF

T

O

≈

×

+

1

360

2

Ω

µ

With a short circuit across the output, the off-time will be about

104

µs. During that time the inductor current gradually decays.

The amount of decay depends on the L/R time constant in the

output circuit. With an inductance of 2.24

µH and total resis-

tance of 40 m

Ω (the inductor’s series resistance plus the sense

resistor), the time constant will be 56

µs. This yields a valley

f

t

VI

R

I

R

R

R

V

VI

R

I

R

R

R

R

MIN

OFF

IN

IN

IN

OMAX

DS ON HSF

SENSE

L

O

IN

IN

IN

OMAX

DS ON HSF

SENSE

L

DS ON LSF

=×

++

++

1

––

(

) –

––

(

–

)

()

()

()

(1)