Datasheet

Data Sheet ADP1621
Rev. B | Page 21 of 32
EFFICIENCY CONSIDERATIONS
The efficiency, η, of a dc/dc converter is given by
%100×=
IN
OUT
P
P
η
(38)
where P
OUT
is the output power, and P
IN
is the input power to the
converter. While switching regulators are ideally lossless converters
of power, the nonideal characteristics of regulator components
degrade the efficiency of the regulator.
The primary sources of power dissipation in the regulator include
• The power dissipation in the external power MOSFET due
to conduction and switching losses.
SWC
MOSFET
PPP +=
(39)
+






+×××






−
= )1(
1
KRD
D
I
DSON
LOAD












×+×
−
×+
2
)(
1
)(
SW
FR
LOAD
D
OUT
f
tt
D
I
VV
• The power dissipation in the external current-sense
resistor if lossless current sensing is not used.
CS
LOAD
CS
RD
D
I
P ××






−
=
2
1
(40)
• The power dissipation in the external diode.
LOAD
D
DIODE
IVP ×=
(41)
• The power dissipation in the winding resistance of the
power stage inductor.
W
LOAD
WL
R
D
I
P ×






−
=
2
,
1
(42)
• The supply current to the ADP1621 IC, which includes the
quiescent current and the gate driver charging current. The
power dissipation due to gate charging loss is approximated by
SWG
PIN
G
fQVP ××=
(43)
where P
G
is the gate charging power loss, V
PIN
is the voltage at
the PIN pin, Q
G
is the MOSFET total gate charge, and f
SW
is
the converter switching frequency. Therefore, the total power
dissipation in the IC itself is given by
( )
Q
IN
GIC
IVPP ×+=
(44)
( )
( )
Q
IN
SWG
PIN
IVfQV ×+××=
where P
IC
is the total power dissipated in the IC, I
Q
is the
quiescent current, and V
IN
is the voltage at the IN pin.
The secondary sources of power dissipation in the regulator include
• The power dissipation in the ESR of the input and output
capacitors.
• Inductor core losses due to hysteresis and eddy currents.