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ADP3155JRU

5-Bit Programmable Triple Power Supply Controller for Pentium® III Processors

Analog Devices 

ADP3155

Transient tolerance (for less than 2 µs) of the supply voltage for

the processor core when the load changes between the mini-

mum and maximum values with a di/dt of 30 A/µs:

∆VOTR+ = 130 mV

∆VOTR– = –130 mV

Input current di/dt when the load changes between the mini-

mum and maximum values: less than 0.1 A/µs

The above requirements correspond to Intel’s published power

supply requirements based on VRM 8.2 guidelines.

CT Selection for Operating Frequency

The ADP3155 uses a constant-off-time architecture with tOFF

determined by an external timing capacitor CT. Each time the

high side N-channel MOSFET switch turns on, the voltage

across CT is reset to approximately 3.3 V. During the off time,

CT is discharged by a constant current of 65 µA. Once CT

reaches 2.3 V, a new on-time cycle is initiated. The value of the

off-time is calculated using the continuous-mode operating

frequency. Assuming a nominal operating frequency of fNOM =

200 kHz at an output voltage of 2.8 V, the corresponding off

time is:

tOFF

=



1

–

VO

VIN





1

fNOM

= 2.2 µs

The timing capacitor can be calculated from the equation:

CT

=

tOFF × 65 µA = 143

1V

pF

The converter operates at the nominal operating frequency only

at the above specified VOUT and at light load. At higher VOUT or

heavy load, the operating frequency decreases due to the para-

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

mum frequency at VOUT = 2.8 V is calculated to be 160 kHz (see

Equation 1), where:

IIN

is the input dc current

(assuming an efficiency of 90%, IIN = 9 A)

RIN

is the resistance of the input filter

(estimated value: 7 mΩ)

RDS(ON)HSF is the resistance of the high side MOSFET

(estimated value: 10 mΩ)

RDS(ON)LSF is the resistance of the low side MOSFET

(estimated value: 10 mΩ)

RSENSE

is the resistance of the sense resistor

(estimated value: 7 mΩ)

RL

is the resistance of the inductor

(estimated value: 6 mΩ)

COUT Selection—Determining the ESR

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

microprocessor. 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.

The total static tolerance of the Pentium II processor is 160 mV.

Taking into account the ±1% setpoint accuracy of the ADP3155,

and assuming a 0.5% (or 14 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 80 mV.

Assuming a step change of ∆I = IOMAX–IOMIN = 13.4 A, and

allocating all of the total allowed static deviation to the contri-

bution of the ESR sets the following limit:

RE(MAX ) = ESRMAX1 = 80 mV = 5.9 mΩ

13.4 A

The output filter capacitor must have an ESR of less than 5.9 mΩ.

One can use, for example, six FA-type capacitors from

Panasonic, with 2700 µF capacitance, 10 V voltage rating, and

34 mΩ ESR. The six capacitors have a total ESR of 5.7 mΩ when

connected in parallel, which gives adequate margin.

Inductor Selection

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

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

LMIN1

= VOtOFF RE(MAX )

VRIPPLE, p − p

=

2.8 V

×

2.2 µs ×

14 mV

5.9

mΩ

=

2.6

µH

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

2.15 A, or 15% of the maximum dc output current IOMAX.

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.

f MIN

=

1

tOFF

×

VIN

VIN – IIN RIN – IOMAX(RDS(ON )HSF + RSENSE + RL ) –VO

=160 kHz

– IIN RIN – IOMAX(RDS(ON )HSF + RSENSE + RL – RDS(ON )LSF )

(1)

–8–

REV. A

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