ADP3170JRU Analog Devices, ADP3170JRU Datasheet - Page 10

• Manufacturer Part Number: ADP3170JRU
• Description: VRM 8.5 Compatible Single Phase Core Controller
• Manufacturer: Analog Devices

ADP3170
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.
Coilcraft
[phone]
http://www.coilcraft.com
Coiltronics
[phone]
http://www.coiltronics.com
Sumida Electric Company
[phone]
http://www.sumida.com
R
The value of R
current. The current comparator of the ADP3170 has a minimum
threshold of 69 mV. Note that this minimum value cannot be
used for the maximum specified nominal current, as headroom
is needed for ripple current and transients.
The current comparator threshold sets the peak of the inductor
current yielding a maximum output current, I
equals the peak value less half of the peak-to-peak ripple current.
Solving for R
using the minimum current sense threshold of 69 mV yields:
In this case, 2.5 m
resistors in parallel (for power dissipation reasons). Once R
has been chosen, the output current at the point where current
limit is reached, I
current sense threshold of 87 mV:
At output voltages below 450 mV, the current sense thresh-
old is reduced to 54 mV, and the ripple current is negligible.
Therefore, the worst-case dead short output current is reduced to:
To safely carry the current under maximum load conditions, the
sense resistor must have a power rating of at least:
R
SENSE
SENSE
P
R
SENSE
I
Table III. Power Inductor Manufacturers
I
OUT SC
O MAX
I
I
SENSE
(
OUT CL
OUT CL
V
SENSE
(
I
CS TH
O
OUT(CL)
(
(
(
2
)
)
allowing a 20% margin for overhead and
)
)
is based on the required maximum output
)(
was chosen, assuming two 5 m , 1 W
R
V
R
MIN
I
SENSE
CS SC
RIPPLE
SENSE
V
2 5
87
, can be calculated using the maximum
2
CS TH
.
(
)
R
mV
(
m
)
SENSE
)(
23
MAX
2 5
–
54
23
.
A
5 9
mV
.
2
69
m
A
)
2
–
A
mV
2 5
I
5 9
.
L RIPPLE
.
(
21 6
2
31 6
m
2
A
.
.
O(MAX)
A
A
)
2 66
1 33
.
.
, which
W
m
SENSE
(7)
(8)
(5)
(6)
Output Resistance
Intel’s VRM 8.5 specification requires that the regulator output
voltage measured at the CPU pins drops when the output cur-
rent increases. The specified voltage drop corresponds to a dc
output resistance of:
The required dc output resistance can be achieved by terminating
the g
tion resistance that will yield the correct dc output resistance is:
where n
g
conductance of the g
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
g
ground. The resistive divider introduces an offset to the output
of the g
of the g
allowed maximum at light load. Furthermore, the output of the
g
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 (V
g
The divider resistors (R
can now be calculated assuming that the internal resistance of
the g
Choosing the nearest 1% resistor value gives R
Finally, R
V
V
m
m
m
m
V
5
GNL
GNL
V
amplifier to the PWM comparator and g
IN
amplifier with a divider connected between the REF pin and
amplifier sets the current sense threshold voltage. At no load,
amplifier that commands a current sense threshold of 0 mV:
R
R
1
m
m
R
R
– .
–
B
B
L
OUT
T
amplifier with a resistor. The value of the total termina-
amplifier (R
1 8
V
H
I
m
m
is the division ratio from the output voltage signal of the
V
1
OUT
amplifier that, when reflected back through the gain
stage, accurately positions the output voltage near its
A
V
V
ONL
3
V
n
g
is calculated:
REF
V
I
m
V
8 88
ONL
– .
.
5 9
R
60
R
t
–
.
1 224
D
T
R
I
SENSE
V
L RIPPLE
OUT
ns
k
–
I
OGM
(
A
GNL
O
V
V
m
R
REF
OFL
SENSE
V
amplifier itself.
2 5
) is 130 k :
3 2
A
–
.
.
2
for the upper, and R
)
– .
2 2
g
m
3
.
m
m
2 2
2
1 845
V
R
25 2 5
mmho
.
n
OUT
V
mmho
I
25 1 224
25
V
23
.
–
n
– .
3 2
m
GNL0
I
A
1 771
.
.
45
m
), at the output of the
mV
V
m
V
B
B
is the trans-
for the lower)
= 30.1 k .
8 88
.
3 2
29 7
.
.
k
m
k
(10)
(11)
(12)
(9)