Datasheet


SLVS612 − APRIL 2006
23
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DESIGN EXAMPLE
6. Calculate the inductor value
The inductor value is calculated from equation (3).
L +
(
14.4 * 1.25 V
)
1.25 V
14.4 V 3.2 A 170 kHz
+ 2.1 mH
A 2.9-µH Coev DXM1306−2R9 or 2.6-µH Panasonic ETQ−P6F2R9LFA can be used.
7. Setting the switching frequency
The clock frequency is set with a resistor (R
T
) from the RT pin to ground. The value of R
T
can be found from
equation (1), with f
SW
in kHz.
R
T
+ ǒ
1
f
SW
17.82 10
*6
* 23ǓkW + 307 kW N use 309 kW
8. Calculating the output capacitance (C
O
)
In this example the output capacitance is determined by the load response requirement of ∆V = 0.1 V for a 1 A
to 7 A step load. C
O
can be calculated using (9)
C
O
+
2.9 m
ǒ
(
8A
)
2
*
(
1A
)
2
Ǔ
ǒ
(
1.25
)
2
*
(
1.15
)
2
Ǔ
+ 761 mF
Using (4) we can calculate the ESR required to meet the output ripple requirements.
33 mV + 3.2 A
ǒ
ESR )
1
8 761 mF 170 kHz
Ǔ
ESR + 10.3 mW * 1.0 mW + 9.3 mW
For this design example two (2) Panasonic SP EEFUEOD471R capacitors, (2.0 V, 470 µF, 12 mΩ) are used.
9. Calculate the soft-start capacitor (C
SS
)
This design requires a soft−start time (t
START
) of 1 ms. C
SS
can be calculated on (11)
C
SS
+
2.3 mA
0.7 V
1ms+ 3.29 nF + 3300 pF
(54)
(55)
(56)
(57)
(58)
(59)