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XC9268 Datasheet(PDF) 11 Page - Torex Semiconductor

Part # XC9268
Description  36V Operation 600mA Synchronous Step-Down DC/DC Converters
PDF  30 Pages
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Manufacturer  TOREX [Torex Semiconductor]
Direct Link  http://www.torex.co.jp
Logo TOREX - Torex Semiconductor

XC9268 Datasheet(HTML) 11 Page - Torex Semiconductor

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XC9268
Series
■TYPICAL APPLICATION CIRCUIT(Continued)
< Output voltage setting >
The output voltage can be set by adding an external dividing resistor.
The output voltage is determined by the equation below based on the values of RFB1 and RFB2.
VOUT=0.75V × (RFB1+RFB2)/RFB2
With RFB2≦2
00kΩ and RFB1+RFB2≦1MΩ
<CFB setting>
Adjust the value of the phase compensation speed-up capacitor CFB using the equation below.
A target value for fzfb of about
is optimum.
【Setting Example】
To set output voltage to 5V with fosc=1.2MHz, CL=10
μF×2, L=6.8μH
When RFB1
=680kΩ, RFB2=120kΩ, VOUTSET=0.75V× (680kΩ+120kΩ) / 120kΩ=5.0V
And fzfb is set to a target of 13.65 kHz using the above equation,
CFB
=1/ (2×π×13.65 kHz×680kΩ) =17.15pF. A capacitor of E24 series is 18pF.
XC9268B75Cxx / fOSC=1.2MHz
XC9268B75Dxx / fOSC=2.2MHz
VOUTSET
RFB1
RFB2
L
CFB
fzfb
VOUTSET
RFB1
RFB2
L
CFB
fzfb
1.2V
120
kΩ
20
0kΩ 3.3μH 68pF 19.6kHz
1.2V
120
kΩ
20
0kΩ 1.5μH 47pF 29.1kHz
3.3V
510kΩ
150kΩ 4.7μH 18pF 16.4kHz
3.3V
510kΩ
150kΩ 2.2μH 12pF 24.0kHz
5.0V
680kΩ
120kΩ 6.8μH 18pF 13.7kHz
5.0V
680kΩ
120kΩ 3.3μH 12pF 19.6kHz
12V
360kΩ
24kΩ
10μH 39pF 11.3kHz
12V
360kΩ
24kΩ
4.7μH 27pF 16.4kHz
<Soft-start Time Setting>
The soft-start time can be adjusted by adding a capacitor and a resistor to the EN/SS pin.
Soft-start time (tSS2) is approximated by the equation below according to values of VEN/SS, RSS, and CSS.
tss2=Css x Rss x ln ( VEN/SS / (VEN/SS-1.45) )
【Setting Example】
When CSS
=0.47μF, RSS=430kΩ and VEN/SS=12V, tSS2=0.47x10-6 x 430 x 103 x (ln (12/ (12-1.45)) =26ms (Approx.)
*The soft-start time is the time from the start of VEN/SS until the output voltage reaches 90% of the set voltage.
If the EN/SS pin voltage rises steeply without connecting CSS and RSS (RSS
=0Ω), Output rises with taking the soft-start time of
tSS1=2.0ms (TYP.) which is fixed internally.
1
2
1
FB
FB
R
fzfb
C
×
×
=
π
L
C
fzfb
L ×
=
π
2
1
VEN/SS
VOUT
90 % of se tting voltage
tSS1
tSS2
EN/SS
CSS
RSS
VEN/SS



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