BL8534
∆
I
=
DV
IN
Lf
V
IN
DV
IN
+
(1
−
D
)
2
R
L
2
Lf
CAPACITOR SELECTION
Ignore ESR of capacitor，the ripple of output
voltage is:
I
MAX
=
r
=
If inductor value is too small, the current
ripple through it will be great. Then the
current through diode and power switch will
be great. Because the power switch on chip
is not ideal switch, the energy of switch will
improve. The efficiency will fall.
Thirdly，in general, smaller inductor values
supply more output current while larger
values start up with lower input voltage and
acquire high efficiency.
An inductor value of 3uH to 1mH works well
in most applications. If DC-DC converter
delivers large output current (for example:
output current is great than 50mA), large
inductor value is recommended in order to
improve efficiency. If DC-DC must output very
large current at low input supply voltage,
small inductor value is recommended.
The ESR of inductor will affect efficiency
greatly. Suppose ESR value of inductor is
r
L
，R
load
is load resistor，then the energy can
be calculated by following expression
：
∆
V
OUT
D
=
V
OUT
R
LOAD
Cf
So large value capacitor is needed to reduce
ripple. But too large capacitor value will slow
down system reaction and cost will improve.
So 100uF capacitor is recommended. Larger
capacitor value will be used in large output
current system. If output current is small
(<10mA), small value is needed.
Consider ESR of capacitor
，
ripple will
increase：
r
'
=
r
+
I
MAX
R
ESR
V
OUT
When current is large, ripple caused by ESR
will be main factor. It may be greater than
100mV。The ESR will affect efficiency and
increase energy loss. So low-ESR capacitor
(for example: tantalum capacitor) is
recommend or connects two or more filter
capacitors in parallel.
DIODE SELECTION
Rectifier diode will affects efficiency greatly，
Though a common diode (such as 1N4148)
will work well for light load
，
it will reduce
about 5%~10% efficiency for heavy load
，
For optimum performance, a Schottky diode
(such as 1N5817
、
1N5819
、
1N5822) is
recommended.
∆
η
≈
r
L
R
load
(1
−
D
)
2
For example: input 1.5V, output is 3.0V,
R
load
=20 , r
L
=0.5 , the energy loss is 10%.
Consider all above，inductor value of 47uH、
ESR<0.5
is recommended in most
applications. Large value is recommended in
high efficiency applications and smaller value
is recommended
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