LTC1530
APPLICATIO S I FOR ATIO
PMAX should be calculated based primarily on required
efficiency or allowable thermal dissipation. A high effi-
ciency buck converter designed for the Pentium II with 5V
input and a 2.8V, 11.2A output might allow no more than
4% efficiency loss at full load for each MOSFET. Assuming
roughly 90% efficiency at this current level, this gives a
PMAX value of:
(2.8)(11.2A/0.9)(0.04) = 1.39W per FET
and a required RDS(ON) of:
( ) 5V 1.39W
RDS(ON)Q1 = 2.8V11.2A2 = 0.020Ω
( ) 5V 1.39W
( ) RDS(ON)Q2 = 5V − 2.8V 11.2A2 = 0.025Ω
Note that while the required RDS(ON) values suggest large
MOSFETs, the power dissipation numbers are only 1.39W
per device or less — large TO-220 packages and heat
sinks are not necessarily required in high efficiency appli-
cations. Siliconix Si4410DY or International Rectifier
IRF7413 (both in SO-8) or Siliconix SUD50N03 or Motorola
MTD20N03HDL (both in DPAK) are small footprint sur-
face mount devices with RDS(ON) values below 0.03Ω at 5V
of VGS that work well in LTC1530 circuits. With higher
output voltages, the RDS(ON) of Q1 may need to be signifi-
cantly lower than that for Q2. These conditions can often
be met by paralleling two MOSFETs for Q1 and using a
single device for Q2. Using a higher PMAX value in the
RDS(ON) calculations generally decreases the MOSFET
cost and the circuit efficiency and increases the MOSFET
heat sink requirements.
In most LTC1530 applications, RDS(ON) is used as the
current sensing element. MOSFET RDS(ON) has a positive
temperature coefficient. Therefore, the LTC1530 IMAX sink
current is designed with a positive 3300ppm/°C tempera-
ture coefficient. The positive tempco of IMAX provides first
order correction for current limit vs temperature. There-
fore, current limit does not have to be set to an increased
level at room temperature to guarantee a desired output
current at elevated temperatures.
Table 1 highlights a variety of power MOSFETs that are
suitable for use in LTC1530 applications.
Table 1. Recommended MOSFETs for LTC1530 Applications
MANUFACTURER
PART NO.
PACKAGE
RDS(ON)
AT 25°C
(Ω)
RATED CURRENT
(A)
TYPICAL INPUT
CAPACITANCE
Ciss (pF)
θJC
(°C/W)
TJMAX
(°C)
Siliconix
SUD50N03-10
TO-252
0.019
15A at 25°C
3200
10A at 100°C
1.8
175
Siliconix
Si4410DY
SO-8
0.020
10A at 25°C
2700
8A at 75°C
—
150
ON Semiconductor
MTD20N03HDL
DPAK
0.035
20A at 25°C
880
16A at 100°C
1.67
150
Fairchild
FDS6680
SO-8
0.01
11.5A at 25°C
2070
ON Semiconductor MTB75N03HDL*
D2PAK
0.0075
75A at 25°C
4025
59A at 100°C
IR
IRL3103S
D2PAK
0.014
56A at 25°C
1600
40A at 100°C
25
150
1.0
150
1.8
175
IR
IRLZ44
TO-220
0.028
50A at 25°C
3300
1.0
175
36A at 100°C
Fuji
2SK1388
TO-220
0.037
35A at 25°C
1750
2.08
150
Note: Please refer to the manufacturer’s data sheet for testing conditions and detailed information.
*Users must consider the power dissipation and thermal effects in the LTC1530 if driving external MOSFETs with high values of input capacitance.
Refer to the PVCC Supply Current vs GATE Capacitance in the Typical Performance Characteristics section.
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