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參數(shù)資料
| 型號： | ADP3170 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Charger front end protection IC with 30V max Vin and 4.5V LDO output 8-WSON 0 to 125 |
| 中文描述： | VRM電源單相兼容8.5核心控制器 |
| 文件頁數(shù)： | 10/16頁 |
| 文件大小： | 183K |
| 代理商： | ADP3170 |
REV. 0
ADP3170
–10–
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.
T able III. Power Inductor Manufacturers
Coilcraft
(847) 639-6400
http://www.coilcraft.com
Coiltronics
(561) 752-5000
http://www.coiltronics.com
Sumida Electric Company
(408) 982-9660
http://www.sumida.com
R
SE NSE
The value of R
SENSE
is based on the required maximum output
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
O(MAX)
, which
equals the peak value less half of the peak-to-peak ripple current.
Solving for
R
SENSE
allowing a 20% margin for overhead and
using the minimum current sense threshold of 69
mV
yields:
R
V
I
I
mV
5 9
A
A
m
SENSE
CS TH
(
MIN
O MAX
(
RIPPLE
2
=
+
=
+
=
)(
)
)
.
.
69
23
2
2 66
(5)
In this case, 2.5
m
was chosen, assuming two 5 m
, 1 W
resistors in parallel (for power dissipation reasons). Once
R
SENSE
has been chosen, the output current at the point where current
limit is reached,
I
OUT(CL)
,
can be calculated using the maximum
current sense threshold of 87 mV:
I
V
R
mV
m
I
I
A
A
OUT CL
CS TH
(
MAX
SENSE
L RIPPLE
(
2
OUT CL
(
)
)(
)
)
(
)
–
.
–
.
.
=
=
=
87
2 5
5 9
2
31 6
(6)
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:
I
V
R
mV
m
A
OUT SC
CS SC
(
SENSE
(
)
)
.
.
=
=
=
54
2 5
21 6
(7)
To safely carry the current under maximum load conditions, the
sense resistor must have a power rating of at least:
P
I
R
A
m
W
R
O
SENSE
SENSE
=
×
=
×
=
2
2
23
2 5
.
1 33
.
(8)
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:
R
V
V
I
V
V
A
m
OUT
ONL
OFL
O
=
=
=
–
.
–1 771
23
.
1 845
3 2
(9)
The required dc output resistance can be achieved by terminating
the g
m
amplifier with a resistor. The value of the total termina-
tion resistance that will yield the correct dc output resistance is:
R
n
R
g
R
m
3 2
.
mmho
m
k
T
I
SENSE
m
OUT
=
×
×
=
×
×
=
25
2 5
.
2 2
.
8 88
.
(10)
where
n
I
is the division ratio from the output voltage signal of the
g
m
amplifier to the PWM comparator and g
m
is the trans-
conductance of the g
m
amplifier itself.
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
m
amplifier with a divider connected between the REF pin and
ground. The resistive divider introduces an offset to the output
of the g
m
amplifier that, when reflected back through the gain
of the g
m
stage, accurately positions the output voltage near its
allowed maximum at light load. Furthermore, the output of the
g
m
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 (V
GNL0
), at the output of the
g
m
amplifier that commands a current sense threshold of 0 mV:
×
(
)
2
V
V
I
R
n
V
V
L
t
R
n
V
V
A
m
V
V
H
ns
m
V
GNL
ONL
L RIPPLE
OUT
I
IN
OUT
D
SENSE
I
GNL
=
+
×
×
×
×
=
+
×
×
×
×
×
=
–
.
.
2
–
–1 8
1
μ
.
.
1
5 9
3 2
25
5
60
2 5
25
1 224
(11)
The divider resistors (
R
A
for the upper, and
R
B
for the lower)
can now be calculated assuming that the internal resistance of
the g
m
amplifier (R
OGM
) is 130 k
:
R
V
V
V
R
g
V
R
V
V
V
k
mmho
mV
k
B
REF
REF
GNL
T
m
B
=
×
+
=
×
=
–
–
–1 224
.
8 88
–2 2
.
3
3
45
29 7
(12)
Choosing the nearest 1% resistor value gives
R
B
=
30.1
k
.
Finally,
R
A
is calculated:
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