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PRODUCT SPECIFICATION
RC5040
11
Table 5. MOSFET Selection Table
Note:
R
DS(ON
) values at T
J
= 125
°
C for most devices were extrapolated from the typical operating curves supplied by the
manufacturers and are approximations only. Only National Semiconductor offers maximum values at T
J
= 125
°
C.
Manufacturer & Model #
Fuji
2SK1388
Conditions
1
R
DS, ON
(m
W
)
Typ.
25
37
16.5
28
13
Package
TO-220
Thermal
Resistance
F
JA
= 75
Max.
37
—
20
34
15
V
GS
= 4V,
I
D
= 17.5A
T
J
= 25
°
C
T
J
= 125
°
C
T
J
= 25
°
C
T
J
= 125
°
C
T
J
= 25
°
C
Siliconix
SI4410DY
V
GS
= 4.5V,
I
D
= 5A
SO-8
(SMD)
F
JA
= 50
National Semiconductor
NDP706AL
NDP706AEL
National Semiconductor
NDP603AL
National Semiconductor
NDP606AL
Motorola
MTB75N03HDL
Int. Rectifier
IRLZ44
Int. Rectifier
IRL3103S
V
GS
= 5V,
I
D
= 40A
TO-220
F
JA
= 62.5
F
JC
=1.5
T
J
= 125
°
C
T
J
= 25
°
C
T
J
= 125
°
C
T
J
= 25
°
C
T
J
= 125
C
T
J
= 25
°
C
T
J
= 125
°
C
T
J
= 25
°
C
T
J
= 125
°
C
T
J
= 25
°
C
T
J
= 125
°
C
20
31
42
22
33
6
9.3
—
—
—
24
40
54
25
40
9
14
28
46
19
31
V
GS
= 4.5V,
I
D
= 10A
TO-220
F
JA
= 62.5
F
JC
= 2.5
F
JA
= 62.5
F
JC
= 1.5
F
JA
= 62.5
F
JC
= 1.0
F
JA
= 62.5
F
JC
= 1.0
F
JA
= 62.5
F
JC
= 1.0
V
GS
= 5V,
I
D
= 24A
TO-220
V
GS
= 5V,
I
D
= 37.5A
TO-263
(D
2
PAK)
TO-220
V
GS
= 5V,
I
D
= 31A
V
GS
= 4.5V,
I
D
= 28A
TO-220
Two MOSFETs in Parallel
We recommend that two MOSFETs be used in parallel
instead of one single MOSFET. Significant advantages are
realized using two MOSFETs in parallel:
Significant reduction of power dissipation
.
Maximum current of 14A with one MOSFET:
P
MOSFET
= (I
2
R
DS(ON)
)(Duty Cycle) =
(14)
2
(0.050
*
)(3.3+0.4)/(5+0.4-0.35) = 7.2 W
With two MOSFETs in parallel:
P
MOSFET
= (I
2
R
DS(ON)
)(Duty Cycle) = (14/
2)
2
(0.037*)(3.3+0.4)/(5+0.4-0.35) = 1.3W/FET
*Note: R
DS(on)
increases with temperature. Assume R
DS(on)
=25m
W
at
25
°
C. R
DS(on)
can easily increase to 50m
W
at high temperature when
using a single MOSFET. When using two MOSFETs in parallel, the
temperature effects should not cause the R
DS(on)
to rise above the listed
maximum value of 37m
W
.
Less heat sink required.
With power dissipation down to
around one watt and with MOSFETs mounted flat on the
motherboard, there will be considerably less heat sink
required. The junction-to-case thermal resistance for the
MOSFET package (TO-220) is typically at 2
°
C/W and the
motherboard serves as an excellent heat sink.
Higher current capability.
With thermal management
under control, this on-board DC-DC converter is able to
deliver load currents up to 14.5A with no problem at all.
MOSFET Gate Bias
Figure 5 employs a charge pump to provide gate bias. Capac-
itor CP is the charge pump deployed to boost the voltage of
the RC5040 output driver. When the MOSFET switches off,
the source of the MOSFET is at -0.6V. VCCQP is charged
through the Schottky diode to 4.5V. Thus, the capacitor CP is
charged to 5V. When the MOSFET turns on, the source of
the MOSFET voltage is equal to 5V. The capacitor voltage
follows, and hence provides a voltage at VCCQP equal to
10V. The Schottky is required to provide the charge path
when the MOSFET is off. The Schottky reverses bias when
the VCCQP goes to 10V. The charge pump capacitor, CP,
needs to be a high Q and high frequency capacitor. A 1
m
F
ceramic capacitor is recommended here.