BL8530C
BL8530C has a drive pin (EXT) for external transistor. So it is possible to load a large output
current with a power transistor and a low saturation voltage. At very light load condition, the switch
current and quiescent current of chip will effect efficiency certainly. So in very light load condition, the
efficiency will drop. Therefore, it is recommended that user use BL8530C in the condition of load
current as large as several tens of mA to several hundreds of mA.
SELECTION OF THE EXTERNAL COMPONENTS
Thus it can be seen, the inductor, schottky diode and external NMOSFET or NPN transistor.
affect the conversion efficiency greatly. The inductor and the capacitor also have great influence on
the output voltage ripple of the converter. So it is necessary to choose a suitable inductor, a
capacitor, an external NMOSFET or NPN transistor and a right schottky diode, to obtain high
efficiency and low ripple.
Before discussion，we define
D
≡
Vout
−
Vin
.
Vout
（1）Inductor
Selection
Above all, we should define the minimum value of the inductor that can ensure the boost
DC-DC to operate in the continuous current-mode condition.
D
(1
−
D
)
2
R
L
L
min
≥
2
f
The above expression is got under conditions of continuous current mode, neglect Schottky
diode’s voltage, ESR of both inductor and capacitor. The actual value is greater that it. If inductor’s
value is less than
Lmin，
efficiency of DC-DC converter will drop greatly, and the DC-DC circuit will
the
not be stable.
Secondly, consider the ripple of the output voltage,
∆
I
=
D
•
Vin
Lf
Vin
DVin
+
Im
ax
=
2
Lf
(1
−
D
)
2
R
L
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 effect efficiency greatly. Suppose ESR value of inductor is r
L
，R
load
is
load resistor，then the energy can be calculated by following expression
：
∆
η
≈
r
L
R
load
(1
−
D
)
2
For example: input 1.5V, output is 3.0V, R
load
=20Ω, r
L
=0.5Ω, The energy loss is 10%.
Consider all above，inductor value of 47uH、ESR<0.5Ω is recommended in most applications.
Large value is recommended in high efficiency applications and smaller value is recommended
（2）Output
Capacitor Selection
Ignore ESR of capacitor，the ripple of output voltage is:
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Total 13 Pages
9/18/2006
BELLING [ SHANGHAI BELLING CO., LTD. ]
