EUP6514
Application Information
Inductor Selection
The selection of output inductor is based on the
considerations of efficiency, output power and
operating frequency. Low inductance value has smaller
size, but results in low efficiency, large ripple current
and high output ripple voltage. Generally, an inductor
that limits the ripple current (∆I
L
) between 20% and
50% of output current is appropriate. Figure 3 shows
the typical topology of synchronous step-down
converter and its related waveforms.
According to Figure 1 the ripple current of inductor
can be calculated as follows :
V
IN
−
V
OUT
=
L
D
V
∆
I
L
;
∆
t
=
; D
=
OUT
∆
t
fs
V
IN
V
OUT
V
IN
×
fs
× ∆
I
L
L
=
(
V
IN
−
V
OUT
)
×
Where:
V
IN
= Maximum input voltage
V
OUT
= Output Voltage
∆t
= S1 turn in time
∆IL
= Inductor current ripple
fs = Switching frequency
D = Duty Cycle
r
C
= Equivalent series resistor of output capacitor
Output Capacitor
The selection of output capacitor depends on the output
ripple voltage requirement. Practically, the output
ripple voltage is a function of both capacitance value
and the equivalent series resistance (ESR) r
C
. Figure 4
shows the related waveforms of output capacitor.
Figure 3.The waveforms of synchronous
step-down converter
DS6514
Ver 1.1
Aug. 2008
Figure 4.The related waveforms of output
capacitor
9
EUTECH [ EUTECH MICROELECTRONICS INC ]
