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ADP3162

5-Bit Programmable 2-Phase Synchronous Buck Controller

Analog Devices 

ADP3162

Selecting a Standard Inductor

The companies listed in Table III can provide design consul-

tation and deliver power inductors optimized for high power

applications upon request.

Table III. Power Inductor Manufacturers

Coilcraft

(847) 639-6400; (800) 322-2645

http://www.coilcraft.com

Coiltronics

(561) 752-5000

http://www.coiltronics.com

Sumida Electric Company

(408) 982-9660

http://www.sumida.com

RSENSE

The value of RSENSE is based on the required maximum output

current. The current comparator of the ADP3162 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

current yielding a maximum output current, IO, which equals

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

peak inductor ripple current. From this the maximum value of

RSENSE is calculated as:

RSENSE

≤

VCS(CL )( MIN )

IO + IL(RIPPLE )

= 69 mV = 4.08 mΩ

14 A + 2 A

(3)

2

2

In this design example, 4 mΩ was chosen as the closest standard

value.

Once RSENSE has been chosen, the maximum output current can

be calculated at the point where current limit is reached, using

the maximum current sense threshold of 89 mV:

IOUT (CL )

= 2 × VCS(CL )( MAX )

RSENSE

− IL(RIPPLE )

= 2 × 89 mV

4 mΩ

− 5.8

A = 38.7

A

(4)

At output voltages below 375 mV, the current sense threshold is

reduced to 58 mV maximum, and the ripple current is negli-

gible. Therefore, at dead short the maximum output current is

reduced to:

IOUT (SC )

= 2 × 58 mV

4 mΩ

= 29 A

(5)

The capability of the resistor’s power rating should be checked

at maximum load current:

P = I × R RSENSE

2

SENSE (RMS )

SENSE

(6)

where:

I2

SENSE (RMS )

=

IO 2

n

×

VOUT

η ×VIN

(7)

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 6 and 7 yields:

PRSENSE

=

28 A 2

2

× 1.8V

0.85 × 5V

× 4 mΩ = 664 mW

Output Resistance

Intel’s VRM 8.5 specification requires that the regulator

output voltage measured at the CPU pins drops when the

output current increases. The specified voltage drop corre-

sponds to a dc output resistance of:

ROUT

= VONL −VOFL

∆IO

= 1.845V − 1.755V

28 A

= 3.2 mΩ

(8)

The required dc output resistance can be achieved by terminating

the gm amplifier with a resistor. The value of the total termina-

tion resistance that will yield the correct dc output resistance:

RT

=

nI × RSENSE

gm × ROUT × 2

=

25 × 4 mΩ

2.2 mmho × 3.2 mΩ × 2

= 7.1 kΩ

(9)

where nI is the division ratio from the output voltage signal of

the gm amplifier to the PWM comparator CMP1, gm is the

transconductance of the gm amplifier itself, and the factor of 2

is the result of the two-phase configuration.

Output Offset

Intel’s VRM 8.5 specification requires that at no load the output

voltage of the regulator module be offset to a higher value than

the nominal voltage corresponding to the VID code. The offset

is introduced by realizing the total termination resistance of the

gm amplifier with a divider connected between the REF pin and

ground. The resistive divider introduces an offset to the output

of the gm amplifier that, when reflected back through the gain

of the gm stage, accurately positions the output voltage near its

allowed maximum at light load. Furthermore, the output of the

gm amplifier sets the current sense threshold voltage. At no

load, the current sense threshold is increased by the peak of the

ripple current in the inductor and reduced by the delay between

sensing when the current threshold has been reached and when

the high side MOSFET actually turns off. These two factors are

combined with the inherent voltage (VGNL0), at the output of the

gm amplifier that commands a current sense threshold of 0 mV:

VGNL

= VGNL0

+

IL(RIPPLE )

× ROUT

2

× nI

− VIN

−VOUT

L

×

tD × nφ × RSENSE × nI

VGNL

= 1V

+

5.8

A × 3.2 mΩ × 25

2

−

5V − 1.8V

1 µH

×

(10)

60 ns × 2 × 4 mΩ × 25 = 1.194V

The divider resistors (RA for the upper, and RB for the lower)

can now be calculated assuming that the internal resistance of

the gm amplifier (ROGM) is 200 kΩ:

–8–

REV. A

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