BL8501
Thus, with changing the value of R2 and R4, output voltage can be set in the specified range.
In the DC-DC converter, with the load current and external components such as L and C, phase
might be behind 180 degree. In this case, the phase margin of the system will be less and stability
will be worse. To prevent this, phase margin should be secured with proceeding the phase. A pole
is formed with external components L and C3.
F
pole
~ 1/2
L C
3
A zero (signal back to zero) is formed with R4 and C4.
F
zero
~ 1 /(2
R
4
C
4)
H, C3=47
F, the cut off frequency of the pole is approximately 4.5kHz.
and C4=1000pF. If
For example, if L=27
To make the cut off frequency of the pole as much as 4.5kHz, set R4=33
k
V
OUT
is set at 5.5V, R2=7.3
k
is appropriate.
is appropriate value.
R3 prevents feedback of the noise to
V
FB
pin, about 2.7
k
External Components
1. Inductor
Select an inductor that peak current does not exceed
IL
max
. If larger current than allowable
current flows, magnetic saturation occurs and makes transform efficiency worse.
When the load current is definite, the smaller value of L, the larger the ripple current.
Provided that the allowable current is large in that case and DC current is small, therefore,
for large output current, efficiency is better than using an inductor with a large value of L and
vice versa.
2. Diode
Use a diode with low
V
F
(Schottky type is recommended.) and high switching speed.
Reverse voltage rating should be more than
V
IN
and current rating should be equal or more
than
IL
max
.
3. Capacitors
As for
C
IN
, use a capacitor with low ESR (Equivalent Series Resistance) and a capacity of at
least 10
F for stable operation.
F or more value of tantalum type
C
OUT
can reduce ripple of Output Voltage, therefore 47
capacitor is recommended.
4. Lx Transistor
Pch Power MOSFET is required for this IC.
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Total
10
Pages
8/8/2006
BELLING [ SHANGHAI BELLING CO., LTD. ]
