ADP3164 Analog Devices, ADP3164 Datasheet - Page 12

• Manufacturer Part Number: ADP3164
• Description: 5-Bit Programmable 4-Phase Synchronous Buck Controller
• Manufacturer: Analog Devices

ADP3164
The maximum output current I
ment for the power MOSFETs. When the ADP3164 is operating
in continuous mode, the simplifying assumption can be made
that in each phase one of the two MOSFETs is always conducting
the average inductor current. For V
the duty ratio of the high-side MOSFET is:
The duty ratio of the low-side (synchronous rectifier) MOSFET is:
The maximum rms current of the high-side MOSFET during
normal operation is:
The maximum rms current of the low-side MOSFET during
normal operation is:
The R
dissipation. If 10% of the maximum output power is allowed for
MOSFET dissipation, the total dissipation in the eight MOSFETs
of the 4-phase converter will be:
Allocating half of the total dissipation for the four high-side
MOSFETs and half for the four low-side MOSFETs, and
assuming that the resistive and switching losses of the high-side
MOSFETs are equal, the required maximum MOSFET resis-
tances will be:
and:
P
P
R
R
FET TOTAL
FET TOTAL
DS ON HSF
DS ON HSF
7 02
D
D
I
I
R
R
80
HSF MAX
LSF MAX
DS(ON)
.
HSF
LSF MAX
(
(
DS ON LSF
DS ON LSF
4
(
(
A
(
(
(
(
(
)
)
=
A
×
V
V
)
)
×
for each MOSFET can be derived from the allowable
OUT
)
)
IN
)
)
)
0 123
=
=
=
=
=
= −
=
.
0 877
0 123
0 1
0 1 1 3845
=
=
=
1
4
4 4 7 02
.
.
I
I
.
.
n
HFS M AX
1 475
× ×
× ×
O
2
2 4 18 75
.
11 08
12
×
×
D
×
P
n I
× ×
× ×
×
(
V
FET TOTAL
HSF MAX
P



n I
11 08
V
.
=
.
1
MIN
FET TOTAL
V
D
18 75
+
(
(
.
HSF MAX
.
W
=
HSF
)
(
3 80
.
LSF MAX
10 8
×
12 3
×
.
×
W
(
V
I
A
)
O
. %
.
O
(
×
=
)
2
A
determines the R
D
×
D

 

A
87 7
A
)
=
IN
1
80
HSF
)
A
LSF
2
2
2
+
14
. %
2
)
=12 V and V
2
=

 =

I
A
L RIPPLE
m
3 94
=
3
(
=
.
Ω
×
7 02
11 08
.
I
O
m
.
2
Ω
)
2
A
W

  =

DS(ON)
OUT
= 1.475 V,
require-
(20)
(21)
(22)
(16)
(17)
(18)
(19)
Note that there is a trade-off between converter efficiency and
cost. Larger MOSFETs reduce the conduction losses and allow
higher efficiency, but increase the system cost. A Fairchild
FDB7030L (R
the high-side and a Fairchild FDB8030L (R
nominal, 5.6 mΩ worst-case) for the low-side are good choices.
The high-side MOSFET dissipation is:
Where the first term is the conduction loss of the MOSFET, 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 sec-
ond term, Q
turn-off and I
for the FDB7030L the value of Q
gate drive current provided by the ADP3414 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 FDB8030L does not give that information, so
an estimated value of 150 nC is used. This estimate is based on
information found on data sheets of similar devices. In both
terms, f
or 200 kHz. I
The worst-case low-side MOSFET dissipation is:
Note that there are no switching losses in the low-side MOSFET.
C
In continuous inductor-current mode, the source current of the
high-side MOSFET is approximately a square wave with a duty
ratio equal to V
maximum output current. To prevent large voltage transients, a
low ESR input capacitor sized for the maximum rms current
must be used. The maximum rms capacitor current is given by:
Note that the capacitor manufacturer’s ripple current ratings are
often based on only 2000 hours of life. This makes it advisable
to further derate the capacitor, or to choose a capacitor rated at
a higher temperature than required. Several capacitors may be
placed in parallel to meet size or height requirements in the
design. In this example, the input capacitor bank is formed by
three 270 µF, 16 V OS-CON capacitors with a ripple current
rating of 4.4 A each.
IN
V
P
P
+
P
P
I
I
12
C RMS
C RMS
HSF
HSF
Selection and Input Current di/dt Reduction
IN
LSF
LSF
(
(
×
V
=
=
SW
=
=
I
10
×
L PK
)
)
R
5 6
R
(
150
is the actual switching frequency of the MOSFETs,
2
=
=
DS ON HSF
.
DS ON LSF
m
G
×
80
(
)
I
L(PK)
G
n
m
Ω ×
(
O
×
I
is the gate charge to be removed from the gate for
DS(ON)
nC
4
G
is the gate turn-off current. From the data sheet,
Ω ×
Q
)
OUT
×
A
G
RR
)
7 02
×
is the peak current in the inductor, or 26 A.
×
.
×
, is the charge stored in the body diode of
18 75
200
/V
n D
×
= 7 mΩ nominal, 10 mΩ worst-case) for
f
×
I
SW
×
IN
.
4 0 123
HSF MAX
A
I
kHz
×
2
and an amplitude of one-half of the
LSF MAX
+
HSF
(
+
A
V
.
12
(
2
IN
=
– (
=
)
2
V
×
1 95
1 97
+
.
n D
Q
−
.
)
×
G
2
(
RR
×
26
4 0 123
is about 35 nC and the peak
W
W
×
×
A
HSF
f
SW
2 1
×
.
×
)
35
2
DS(ON)
)
nC
A
2
=
×
10
200
= 3.1 mΩ
A
kHz
(23)
(24)
(25)