RC5042 View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
Manufacturer
RC5042 Datasheet PDF : 20 Pages
| |||
RC5042
PRODUCT SPECIFICATION
Converter Efficiency
Losses due to parasitic resistance in the switches, coil,
and sense resistor dominate at high load-current level.
The major loss mechanisms under heavy loads, in usual
order of importance, are:
• MOSFET I2R Losses
• Inductor Coil Losses
• Sense Resistor Losses
• gate-charge losses
• diode-conduction losses
• transition losses
• Input Capacitor losses
• losses due to the operating supply current of the
IC
Efficiency of the converter under heavy loads can be calculated as follows:
Efficiency
=
P----O----U---T--
pIN
=
I---O---U----T--I--O-´---U-V--T---O--´--U--V-T----O-+---U-P--T--L---O----S---S-
,
where PLOSS = PDMOSFET + PDINDUCTOR + PDRSENSE + PDGATE + PDDIODE + PDTRAN + PDCAP + PDIC
Design Equations:
(1) PDMOSFET = IOUT2 ´ RDS(ON) ´ DutyCycle
where DutyCycle = -------V----O----U----T----+-----V-----D--------
VIN + VD – VSW
(2)
PDINDUCTOR
=
IO
U
2
T
´
RINDUCTOR
(3)
PDRSENSE
=
IO
U
2
T
´
RSENSE
(4) PDGATE = qGATE ´ f ´ 5V , where qGATE is the gate charge and f is the switching frequency
(5) PDDIODE = Vf ´ IOUT(1 – Dutycycle)
(6)
PDTRAN
=
V-----I--N--2----´-----C----R---S---S----´-----I--L----O---A----D-----´-----f-
IDRIVE
,
where
CRSS
is
the
reverse
transfer
capacitance
of
the
high-side
MOSFET.
(7)
PDCAP
=
IR
2
MS
´
ESR
(8) PDIC = VCC ´ ICC
Example:
DutyCycle = 5-----+3---.-3-0---.-+5-----–0---.-05---.--3- = 0.73
PDMOSFET = 10 2 ´ 0.030 ´ 0.73 = 2.19W
PDINDUCTOR = 10 2 ´ 0.010 = 1W
PDRSENSE = 102 ´ 0.0065 = 0.65W
PDGATE = CV ´ f ´ 5V = 1.75nf ´ (9 – 1)V ´ 650Khz ´ 5V = 0.045W
PDDIODE = 0.5 ´ 10(1 – 0.73) = 1.35W
PDTRAN
=
5----2----´-----4---0---0---p---f----´-----1---0----´-----6---5---0----k---h---z--
0.7A
~
0.010W
PDCAP = (7.5 – 2.5)2 ´ 0.015 = 0.37W
PDIC = 0.2W
12
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