ADP3153 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADP3153

5-Bit Programmable Dual Power Supply Controller for Pentium II Processor

Analog Devices 

ADP3153

The inductor peak current in normal operation is:

ILPEAK = IOMAX + IRPP/2 = 15.3 A

The inductor valley current is:

ILVALLEY = ILPEAK – IRPP = 13 A

The inductor for this application should have an inductance

of 2.6 µH at full load current and should not saturate at the

worst-case overload or short circuit current at the maximum

specified ambient temperature. A suitable inductor is the

CTX12-13855 from Coiltronics, which is 4.4 µH at 1 A and

about 2.5 µH at 14.2 A.

Tips for Selecting 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

cores or the Kool Mµ® 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.

CO 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 which is equal

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

dt in the inductor and the ESR of the capacitor.

CMIN = IOMAX – IOMIN =

14.2 – 0.8

= 4.5 mF

RE (di/dt ) 5.9 m (2.2 / 4. 4 µH )

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

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

divided by the maximum inductance (4.4 µH) of the CTX12-

13855 inductor from Coiltronics. The parallel-connected six

2700 µF/10 V FA series capacitors from Panasonic have a total

capacitance of 16,200 µF, so the minimum capacitance is met

with ample margin.

RSENSE

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

The current comparator of the ADP3153 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, tran-

sients and inductor core saturation.

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 and allowing a margin for tolerances inside the

ADP3153 and in the external component values yields:

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

A practical solution is to use three 20 mΩ resistors in parallel,

with an effective resistance of about 6.7 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:

tOFF ≈

CT × 1V

VO + 2 µA

360 kΩ

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

70 µs. During that off time the inductor current gradually de-

cays. 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 mΩ, the time constant will be 108 µs, which 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 A2 × 6.8 mΩ =

1.8 W.

Current Transformer Option

An alternative to using 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. If we use a transformer

with one primary and 50 secondary turns, the worst-case resistor

dissipation is reduced to a fraction of a mW. Another advantage

of using this option is the separation of the current and voltage

sensing, which makes the voltage sensing more accurate.

Power MOSFET

Two external N-channel power MOSFETs must be selected for

use with the ADP3153, one for the main switch, and an identi-

cal one for the synchronous switch. The main selection param-

eters 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 ADP3153

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 VO = 2.8 V, the maximum duty ratio of the

high side FET is:

DMAXHF = (1 – fMIN × tOFF) =(1 – 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:

IRMSLS = [DMAXHF (ILVALLEY2 + ILPEAK2 + ILVALLEYILPEAK)/3]0.5

= 11.5 Arms

–8–

REV. 0

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