ADP3170JRU Analog Devices, ADP3170JRU Datasheet - Page 12

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

ADP3170
Power MOSFETs
Two external N-channel power MOSFETs must be selected for
use with the ADP3170, one for the main switch and one for the
synchronous switch. The main selection parameters for the power
MOSFETs are the threshold voltage (V
(R
highly recommended. Only logic-level MOSFETs with V
higher than the absolute maximum value of V
The maximum output current I
requirement for the two power MOSFETs. When the ADP3170
is operating in continuous mode, the simplifying assumption can
be made that one of the two MOSFETs is always conducting
the average load current. For V
maximum duty ratio of the high-side FET is:
The maximum duty ratio of the low-side (synchronous rectifier)
MOSFET is:
The maximum rms current of the high-side MOSFET is:
The maximum rms current of the low-side MOSFET is:
The R
allowable dissipation. If 10% of the maximum output power is
allowed for MOSFET dissipation, the total dissipation will be:
Allocating half of the total dissipation for the high-side MOSFET and
half for the low-side MOSFET, and assuming that the resis-
tive loss of the high-side MOSFET is one-third, and the switching
loss is two-thirds of its portion, the required maximum MOSFET
resistances will be:
Note that there is a trade-off between converter efficiency and
cost. Larger MOSFETs reduce the conduction losses and allow
I
I
DS(ON)
HSF MAX
HSF MAX
I
I
R
LSF MAX
HSF MAX
(
(
D
DS ON HSF
D
D
(
(
LSF MAX
DS(ON)
(
HSF MAX
HSF MAX
R
)
)
), and the gate charge (Q
)
(
)
DS ON LS
)
(
(
D
0 4
(
P
P
.
HSF MAX
D
D FET
D FET
0 6
for each MOSFET can be derived from the
)
.
(
(
LSF MAX
)
(
17 4
)
)
(
1
.
17 4
s
s
3
)
)
)
–
1
1
A
.
2
)
–
– (
D
I
I
P
A
L VALLEY
LSF MAX
I
P
0 1
0 1 1 8
HSF MAX
(
I
D FETS
( .
HSF MAX
183
2
17 4
D FETS
L VALLEY
.
.
f
(
(
MIN
(
(
(
17 4
(
A
V
)
kHz
2
.
.
3
OUT
)
)
)
2
)
28 6
A
2
)
IN
t
I
V
)
OFF
L VALLEY
.
2
3
(
O(MAX)
I
28 6
G
= 5 V and V
60
L VALLEY
A
(
2 18
). Logic-level MOSFETs are
3 3
)
I
.
23
OUT MAX
%
4 1
.
3 14 7
28 6
)
A
.
3
GS(TH)
3
A
.
(
4 1
)
W
I
determines the R
s
L PEAK
.
)
A
28 6
A
(
I
2
L PEAK
.
2
.
W
(
4 1
), the ON-resistance
)
40
14 7
.
)
A
CC
A
OUT
6
.
2
)
W
%
vI
2
m
A
should be used.
L PEAK
18
I
(
= 1.8 V, the
L PEAK
(
6
A
)
2
m
GS
)
2
ratings
DS(ON)
(20)
(22)
(23)
(19)
(21)
(17)
(18)
higher efficiency, but increase the system cost. A Fairchild
FDB7045L (R
good choice for both the low-side and high-side MOSFET.
With this choice, the high-side MOSFET dissipation is:
where the second term represents the turn-off loss of the
MOSFET and the third term represents the turn-on loss due to
the stored charge in the body diode of the low-side MOSFET.
In the second term, Q
the gate for turnoff and I
data sheet, the value of Q
peak gate drive current provided by the ADP3170 is about 1 A. In
the third term, Q
the low-side MOSFET at the valley of the inductor current.
The data sheet of the FDB7045L does not give that informa-
tion, so an estimated value of 100 nC is used. The estimate is
based on information found on the data sheets of similar devices.
The low-side MOSFET dissipation is:
Note that there are no switching losses in the low-side MOSFET.
Surface mount MOSFETs are preferred in CPU core converter
applications due to their ability to be handled by automatic
assembly equipment. The TO-263 package offers the power
handling of a TO-220 in a surface mount package. However,
this package still needs adequate copper area on the PCB to
help move the heat away from the package.
The junction temperature for a given area of two-ounce copper
can be approximated using:
assuming:
For 1 in
ambient temperature of 50 C:
All of the above-calculated junction temperatures are safely
below the 175 C maximum specified junction temperature of
the selected MOSFETs.
V
5
P
P
HSF
IN
HSF
V
100
Q
6
R
RR
2
DS ON HSF
m
T
T
nC
of copper area attached to each transistor and an
(
J
J
HSF
LSF
f
MIN
)
14 7
DS(ON)
183
T
P
P
.
JA
JA
JA
J
LSF
LSF
RR
kHz
A
28
= 45 C/W for 0.5 in
= 36 C/W for 1 in
= 28 C/W for 2 in
28
I
2
= 4.5 m nominal, 6 m worst-case) is a
HSF MAX
is the charge stored in the body diode of
o
o
G
C W
C W
6
J
R
5 28 6
A
(
is the gate charge to be removed from
/
2 04
G
DS ON HSF
m
/
G
.
is the gate turn-off current. From the
(
for the FDB7045L is 50 nC and the
P
)
2
D
.
W
)
1 94
18
2 06
A
V
.
.
IN
T
2 1
A
A
W
50
W
2
2
2
I
I
nC
HSF MAX
L PEAK
A
2
1 94
(
.
50
2
(
50
183
o
W
o
)
C
C
I
G
)
kHz
2
Q
G
104
108
f
o
o
MIN
C
C
(25)
(26)
(24)