XC9248 Series
■TYPICAL APPLICATION CIRCUIT (Continued)
<VOUT setting>
The output voltage can be set by connecting external dividing resistors. The output voltage is determined by the values of RFB1
and RFB2 as given in the equation below. The total of RFB1 and RFB2 should be less than 150kΩ. Output voltage range can be
set freely from 1.0V to 12V with a 0.8V reference voltage.
VOUT=0.8×(RFB1+RFB2)/RFB2
Adjust the value of the phase compensation speed-up capacitor CFB so that fzfp=1 / (2 × π × CFB x RFB1) is about 7kHz.
Adjustments are required from 5kHz to 50kHz depending on the application, value of inductance (L), and value of load
capacitance (CL).
【Examples】
RFB1=47kΩ, RFB2=15kΩ, VOUT=0.8V×(47kΩ+15kΩ) /15kΩ =3.3V
CFB=470pF, fzfb=1/(2×π×470pF×47kΩ)=7.2kHz
< Minimum VOUT
>
The Minimum VOUT 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 VOUT can be calculated by
using the following equation:
VOUT= VIN × MINDUTY / 100
【L vs. MINDUTY】
L[μH]
MINDUTY[%]
2.2
3.3
4.7
6.8
10
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 VENH voltage or higher, a current ISS=4μA (TYP.) flows to the SS pin and charges the capacitor.
When the SS pin voltage attains the SS threshold voltage VSSTH=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=VSSTH × CSS / ISS
【Examples】
CSS=0.1μF, External soft-start time=1.8V × 0.1μF / 4μA × 1000=45ms
2000ms
450ms
200ms
45ms
20ms
4.5ms
0.01
0.047
0.1
0.47
1
4.7
μF
μF
μF
μF
μF
μF
CSS
8/21
TOREX [ Torex Semiconductor ]
