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RC5042
PRODUCT SPECIFICATION
12
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 I
2
R 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:
,
where
Design Equations:
where
, where
is the gate charge and f is the switching frequency
, where C
RSS
is the reverse transfer capacitance of the high-side MOSFET.
Example:
Efficiency
P
IN
-p
I
′
OUT
+
OUT
V
OUT
P
LOSS
I
=
=
P
LOSS
PD
MOSFET
PD
INDUCTOR
PD
RSENSE
PD
GATE
PD
DIODE
PD
TRAN
PD
CAP
PD
IC
+
+
+
+
+
+
+
=
(1) PD
MOSFET
I
OUT
2
R
DS ON
)
′
DutyCycle
′
=
DutyCycle
V
V
IN
D
V
SW
–
V
D
+
-----------------------+
=
(2) PD
INDUCTOR
I
OUT
2
R
INDUCTOR
′
=
(3) PD
RSENSE
I
OUT
2
R
SENSE
′
=
(4) PD
GATE
q
GATE
f
5V
′
′
=
q
GATE
(5) PD
DIODE
V
f
I
OUT
′
1
Dutycycle
–
(
)
=
(6) PD
TRAN
V
-----------------------------------I
2
C
I
DRIVE
LOAD
f
′
=
(7) PD
CAP
I
RMS
2
ESR
′
=
(8) PD
IC
V
CC
I
CC
′
=
DutyCycle
+
0.5
–
+
5
0.5
0.3
---3.3
0.73
=
=
PD
MOSFET
10
2
0.030
′
0.73
′
2.19W
=
=
PD
INDUCTOR
10
2
0.010
′
1W
=
=
PD
RSENSE
10
2
0.0065
′
0.65W
=
=
PD
GATE
CV
f
′
5V
′
1.75nf
9
1
–
(
)
V
650Khz
5V
′
′
′
0.045W
=
=
=
PD
DIODE
0.5
10 1
0.73
–
)
′
1.35W
=
=
PD
TRAN
2
5
400pf
′
10
′
650khz
′
0.7A
0.010W
~
=
PD
CAP
7.5
2.5
–
(
)
2
0.015
′
0.37W
=
=
PD
IC
0.2W
=