XC9248
Series
■TYPICAL
APPLICATION CIRCUIT (Continued)
<V
OUT
setting>
The output voltage can be set by connecting external dividing resistors. The output voltage is determined by the values of R
FB1
and R
FB2
as given in the equation below. The total of R
FB1
and R
FB2
should be less than 150kΩ. Output voltage range can be
set freely from 1.0V to 12V with a 0.8V reference voltage.
V
OUT
=0.8×(R
FB1
+R
FB2
)/R
FB2
Adjust the value of the phase compensation speed-up capacitor C
FB
so that
f
zfp
=1 / (2 ×
π
× C
FB
x R
FB1
)
is about 7kHz.
Adjustments are required from 5kHz to 50kHz depending on the application, value of inductance (L), and value of load
capacitance (C
L
).
【Examples】
R
FB1
=47kΩ, R
FB2
=15kΩ, V
OUT
=0.8V×(47kΩ+15kΩ) /15kΩ =3.3V
C
FB
=470pF, fzfb=1/(2×
π
×470pF×47kΩ)=7.2kHz
< Minimum V
OUT
>
The Minimum V
OUT
is set by MINDUTY. The MINDUTY changes by the external inductance(L).
For the L value, please choose the optimal value – see P.7 <Coil current setting>. The Minimum V
OUT
can be calculated by
using the following equation:
V
OUT
= V
IN
× MINDUTY / 100
【L
vs. MINDUTY】
L[μH]
2.2
3.3
4.7
6.8
10
MINDUTY[%]
18
20
21
21
22
<External soft-start setting>
A capacitor can be connected to the SS pin to set a time longer than the internal soft-start time voluntarily.
By setting the EN pin to the V
ENH
voltage or higher, a current I
SS
=4μA (TYP.) flows to the SS pin and charges the capacitor.
When the SS pin voltage attains the SS threshold voltage V
SSTH
=1.8V (TYP.), the output voltage reaches about 90% of the set voltage.
External soft-start can be calculated by using the following equation:
External soft-start time=V
SSTH
× C
SS
/ I
SS
【Examples】
C
SS
=0.1
μ
F, External soft-start time=1.8V × 0.1
μ
F / 4
μ
A × 1000=45ms
2000ms
External soft-start time
450ms
200ms
45ms
20ms
4.5ms
0.01
μF
0.047
μF
0.1
μF
C
SS
0.47
μF
1
μF
4.7
μF
8/21
TOREX [ TOREX SEMICONDUCTOR ]
