ADP3163 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADP3163

5-Bit Programmable 2-Phase Synchronous Buck Controller

Analog Devices 

ADP3163

At output voltages below 750 mV, the current sense threshold is

reduced to 108 mV, and the ripple current is negligible. There-

fore, at dead short the output current is reduced to:

IOUT (SC )

=

n

× VCS(SC )

RSENSE

= 3 × 108 mV

5 mΩ

= 65 A

(5)

To safely carry the current under maximum load conditions, the

sense resistor must have a power rating of at least:

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

=

65 A2

3

×

1.5 V

0.85 × 12 V

× 5 mΩ = 1.0 W

Output Resistance

This design requires that the regulator output voltage measured

at the CPU pins drops when the output current increases. The

specified voltage drop corresponds to a dc output resistance of:

ROUT

= VONL − VOFL

IO∆

= 1.475V − 1.377V

65 A

= 1.5 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

n × gm × ROUT

=

12.5 × 5 mΩ

= 6.31 kΩ

3 × 2.2 mmho × 1.5 mΩ

(9)

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

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

ductance of the gm amplifier itself, and n is the number of phases.

Output Offset

Intel’s specification requires that at no load the nominal output

voltage of the regulator be offset to a lower 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 sens-

ing 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 +

I L( RIPPLE ) × RSENSE

2

× nI

− VIN

− VOUT

L

×

n × tD × RSENSE × nI

VGNL

= 1V

+ 10.9

A × 5 mΩ × 12.5

2

− 12 V − 1.5V

600 nH

×

(10)

2 × 60 ns × 5 mΩ × 12.5 = 1.144 V

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 1 MΩ:

RB

=

VREF − VGNL

RT

VREF

− gm × (VONL

− VVID )

RB

=

3V

− 1.144 V

3V

− 2.2 mmho × (1.475V

− 1.5V )

= 8.59 kΩ

6.31 kΩ

(11)

Choosing the nearest 1% resistor value gives RB = 8.66 kΩ.

Finally, RA is calculated:

RA = 1 −

1

1

=

−1

1

1

−1−

1

= 23.8 kΩ

(12)

RT ROGM RB 6.31 kΩ 1 MΩ 8.66 kΩ

Choosing the nearest 1% resistor value gives RA = 23.7 kΩ.

COUT Selection

The required equivalent series resistance (ESR) and capacitance

drive the selection of the type and quantity of the output capaci-

tors. The ESR must be less than or equal to the specified output

resistance (ROUT), in this case 1.5 mΩ. The capacitance must be

large enough that the voltage across the capacitors, which is the

sum of the resistive and capacitive voltage deviations, does not

deviate beyond the initial resistive step while the inductor cur-

rent ramps up or down to the value corresponding to the new

load current.

One can, for example, use nine MBZ-type capacitors from

Rubycon, with 2200 µF capacitance, a 6.3 V voltage rating, and

13 mΩ ESR. The nine capacitors have a maximum total ESR of

1.44 mΩ when connected in parallel.

As long as the capacitance of the output capacitor bank is above

a critical value and the regulating loop is compensated with

Analog Devices’ proprietary compensation technique (ADOPT),

the actual capacitance value has no influence on the peak-to-

peak deviation of the output voltage to a full step change in the

load current. The critical capacitance can be calculated as follows:

COUT (CRIT ) =

IO

ROUT × VOUT

×L

n

=

65 A

× 600 nH = 5.78 mF

(13)

1.5 mΩ × 1.5V

3

–10–

REV. 0

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