ADP3155 datasheet(9/14 Pages) AD

9 / 14 page

REV. A

ADP3155

–9–

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 the largest size

inductor.

COUT Selection—Determining the Capacitance

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 that 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:

Rdi dt

MIN

OMAX

OMIN

−

()

–

)

/ .

14 2

0 8

Ωµ

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

smaller voltage across the inductor (i.e., VIN–VOUT rather than

VOUT) divided by the maximum inductance (4.4 µH) of the

Coiltronics CTX12-13855 inductor. The six parallel-connected

2700

µF capacitors have a total capacitance of 16,200 µF, so the

minimum capacitance requirement is met with ample margin.

RSENSE

The value of RSENSE is based on the required output current.

The current comparator of the ADP3155 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

current yielding a maximum output current, IOMAX, which equals

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

ing for RSENSE allowing a 20% margin for overhead, and using

the minimum current sense threshold of 125 mV yields:

RSENSE = (125 mV)/[1.2(IOMAX + IRPP /2)] = 6.8 mΩ

Once RSENSE has been chosen, the peak short-circuit current

ISC(PK) can be predicted from the following equation:

ISC(PK) = (145 mV)/RSENSE = (145 mV)/(6.7 mΩ) = 21.5 A

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

ISC(PK) value because the off-time rapidly increases when the

output voltage drops below 1 V. The relationship between the

off-time and the output voltage is:

OFF

≈

360

Ω

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

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 2.5

µH and total resis-

tance of 23 m

Ω, the time constant will be 108 µs. This yields a

valley current of 11.3 A and an average short-circuit current of

about 16.3 A. To safely carry the short-circuit current, the sense

resistor must have a power rating of at least 16.3 A

× 6.8 mΩ =

1.8 W.

Current Transformer Option

An alternative to using a low value and high power current sense

resistor is to reduce the sensed current by using a low cost cur-

rent transformer and a diode. The current can then be sensed

with a small-size, low cost SMT resistor. Using a transformer

with one primary and 50 secondary turns reduces the worst-case

resistor dissipation to a few mW. Another advantage of using

this option is the separation of the current and voltage sensing,

which makes the voltage sensing more accurate.

Power MOSFETs

Two external N-channel power MOSFETs must be selected for

use with the ADP3155, one for the main switch and an identical

one for the synchronous switch. The main selection parameters

for the power MOSFETs are the threshold voltage VGS(TH) and

the on resistance RDS(ON).

The minimum input voltage dictates whether standard threshold

or logic-level threshold MOSFETs must be used. For VIN > 8 V,

standard threshold MOSFETs (VGS(TH) < 4 V) may be used. If

VIN is expected to drop below 8 V, logic-level threshold MOSFETs

(VGS(TH) < 2.5 V) are strongly recommended. Only logic-level

MOSFETs with VGS ratings higher than the absolute maximum

of VCC should be used.

The maximum output current IOMAX determines the RDS(ON)

requirement for the two power MOSFETs. When the ADP3155

is operating in continuous mode, the simplifying assumption can

be made that one of the two MOSFETs is always conducting

the average load current. For VIN = 5 V and VOUT = 2.8 V, the

maximum duty ratio of the high side FET is:

DMAXHF = (1 – fMIN × tOFF) = (1 kHz–160 kHz × 2.2 µs) = 65%

The maximum duty ratio of the low side (synchronous rectifier)

FET is:

DMAXLF = 1 – DMAXHF = 35%

The maximum rms current of the high side FET is:

IRMSHS = [DMAXHF (ILVALLEY2 + ILPEAK2 + ILVALLEYILPEAK)/3]

0.5

= 13.1 A rms

The maximum rms current of the low side FET is:

IRMSLS = [DMAXLF (ILVALLEY2 + ILPEAK2 + ILVALLEYILPEAK)/3]

0.5

= 8.41 A rms

The RDS(ON) for each FET can be derived from the allowable

dissipation. If 5% of the maximum output power is allowed for

FET dissipation, the total dissipation will be:

PFETALL = 0.05 VOIOMAX = 2 W

Allocating half of the total dissipation for the high side FET and

half for the low side FET, the required minimum FET resis-

tances will be:

RDS(ON)HSF(MIN) = 1.33 W/(11.5 A)

2 = 10 m

Ω

RDS(ON)LSF(MIN) = 0.67 W/(8.41 A)

2 = 9.5 m

Ω

Note that there is a trade-off between converter efficiency and

cost. Larger FETs reduce the conduction losses and allow

higher efficiency, but increase the system cost. If efficiency is

not a major concern, the International Rectifier IRL3103 is an

economical choice for both the high side and low side positions.

Those devices have an RDS(ON) of 14 mΩ at VGS = 10 V and at

+25

°C. The low side FET is turned on with at least 10 V. The

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