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XD9268 Datasheet(PDF) 11 Page - Torex Semiconductor

Part # XD9268
Description  36V Operation 600mA Synchronous Step-Down DC/DC Converters
PDF  26 Pages
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Manufacturer  TOREX [Torex Semiconductor]
Direct Link  http://www.torex.co.jp
Logo TOREX - Torex Semiconductor

XD9268 Datasheet(HTML) 11 Page - Torex Semiconductor

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XD9267/XD9268
Series
■TYPICAL APPLICATION CIRCUIT / Parts Selection Method (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, CL=10
μF×2, L=3.3μH
When RFB1
=680kΩ, RFB2=120kΩ, VOUTSET=0.75V× (680kΩ+120kΩ) / 120kΩ=5.0V
And fzfb is set to a target of 19.6 kHz using the above equation,
CFB
=1/ (2×π×19.6 kHz×680kΩ) =11.95pF. A capacitor of E24 series is 12pF.
XD9267B75Dxx/XD9268B75Dxx
VOUTSET
RFB1
RFB2
L
CFB
fzfb
1.2V
120
kΩ
200
kΩ
1.5μH
47pF
29.1kHz
3.3V
510kΩ
150kΩ
2.2μH
13pF
24.0kHz
5.0V
680kΩ
120kΩ
3.3μH
12pF
19.6kHz
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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