ADP3164 Ver la hoja de datos (PDF) - Analog Devices
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ADP3164 Datasheet PDF : 16 Pages
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ADP3164
RSENSE
The value of RSENSE is based on the maximum required output
current. The current comparator of the ADP3164 has a mini-
mum current limit threshold of 143 mV. Note that the 143 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
≤
VCSCL( MIN )
IO + I L( RIPPLE )
=
143 mV
80 A + 10.8 A
= 5.6 mΩ
(3)
n
2
4
2
In this case, 5 mΩ was chosen as the closest standard value.
Once RSENSE has been chosen, the output current at the point
where current limit is reached, IOUT(CL), can be calculated using
the maximum current sense threshold of 173 mV:
IOUT (CL )
=
n
× VCSCL( MAX )
RSENSE
−
n
×
I L( RIPPLE )
2
IOUT (CL )
= 4 × 173 mV
5 mΩ
−
4 ×10.8
2
A
= 116.8
A
(4)
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
= 4 × 108 mV
5 mΩ
= 86.4 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
=
80 A2
4
×
1.475 V
0.85 ×12 V
× 5 mΩ = 1.2 W
Output Resistance
This design requires that the regulator output voltage measured
at the CPU drop when the output current increases. The speci-
fied voltage drop corresponds to a dc output resistance of:
ROUT
= VONL − VOFL
IO∆
= 1.4605V −1.3845V
80 A
= 0.95 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
= 7.48 kΩ
4 × 2.2 mmho × 0.95 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
transconductance of the gm amplifier itself, and n is the number
of phases.
Output Offset
Intel’s VRM 9.1 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 to make sure that
circuit tolerances never cause the output voltage to exceed the
VID value. The offset is introduced by realizing the total termina-
tion 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
+
I L( RIPPLE )
× RSENSE
2
× nI
− VIN
− VOUT
L
×
n × tD × RSENSE × nI
VGNL
= 1V
+ 10.8
A × 5 mΩ ×12.5
2
− 12 V −1.475V
600 nH
×
(10)
4 × 60 ns × 5 mΩ ×12.5 = 1.074 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
−1.074 V
3V
− 2.2 mmho × (1.4605V
−1.475V )
= 10.37 kΩ
7.48 kΩ
(11)
Choosing the nearest 1% resistor value gives RB = 10.5 kΩ.
Finally, RA is calculated:
RA = 1 −
1
1
=
−1
1
1
−1− 1
= 26.7 kΩ
(12)
RT ROGM RB 7.48 kΩ 1 MΩ 10.5 kΩ
Choosing the nearest 1% resistor value gives RA = 26.7 kΩ.
–10–
REV. 0
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