Application Information: continued
Calculate Inductor Value:
than ±165mV. Repeating step 2a, we select four (4)
1200µF/10V Sanyo GX output capacitors.
(VIN - VOUT) tTR
(5V-2V) ´ 6µs
3V ´ 6µs
L=
=
=
=1.2µH.
DI
15A
15A
Step 3: Duty Cycle, Switching Frequency, TON & TOFF
Duty Cycle » VOUT / VIN.
Step 4a: Select 2% Ripple on 2V Output
DVOUT = 2% ´ 2V = 40mV
D = 2.0V / 5V = 40% for 2V output.
D = 3.3V / 5V = 66% for 3.3V output.
Select 200kHz Switching Frequency (FSW).
The maximum allowable Inductor Ripple Current for a 2%
ripple on the 2V output is:
DVOUT
40mV
DIL=
=
= 7.3A,
Step 3a: Calculate On-Time for 2V Output
Total ESR
5.5m½
0.40
D
TON
=
=
= 2µs
which corresponds to the following maximum Inductor
Peak and Valley currents:
FSW
200kHz
Calculate Off-Time:
TOFF =
DIL
2
7.3A
2
IL(PEAK) = IOUT
+
= 16A +
= 19.6A,
= 12.4A.
(
)
(
)
1
- TON
= 5µs - 2µs = 3µs.
FSW
Select the COFF1 capacitor in order to set the Off-Time:
7.3A
2
DIL
2
IL(VALLEY) = IOUT
-
= 16A -
(
)
(
)
Period ´ (1-D)
The selected 1.2µH inductor yields the following ripple
current:
5µs ´ 0.6
COFF1
=
=
= 750pF.
3980
3980
(VIN - VOUT) ´ D
(5V - 2V) ´ 0.4
A standard COFF1 capacitance value of 680pF can be used.
The 3980 factor is a characteristic of the CS5132.
DIL=
=
= 5A.
FSW ´ L
200kHz ´ 1.2µH
The maximum inductor peak current becomes:
5A
Step 3b: Calculate On-Time for 3.3V Output
IL(PEAK) = 16A +
= 16A + 2.5A = 18.5A.
0.66
D
FSW
2
TON
=
=
= 3.3µs
200kHz
The inductor valley current becomes:
5A
Calculate Off-Time:
IL(VALLEY) = 16A -
= 16A - 2.5A = 13.5A.
2
1
FSW
TOFF
=
- TON = 5µs Ð 3.3µs = 1.7µs.
The above values are well within the maximum allowable
inductor peak and valley currents for a 2% output voltage
ripple.
Select Toroid Powdered Iron Core, low cost, low core loss-
es at 200kHz, low EMI.
Select COFF2 to be 390pF.
Step 4: Output Inductor
Select XFMRS Inc, XF0016-VO4 1.2µH inductor with RDC
0.003½ typical, 0.008½ maximum.
=
The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the induc-
tor value will decrease output voltage ripple, but degrade
transient response. There are many factors to consider in
selecting the inductor including: cost, efficiency, EMI and
ease of manufacture. The inductor must be able to handle
the peak current at the switching frequency without satu-
rating, and the copper resistance in the winding should be
kept as low as possible to minimize resistive power loss.
There are a variety of materials and types of magnetic
cores that could be used for this application. Among them
are: ferrites, molypermalloy cores (MPP), amorphous and
powdered iron cores. We will use a powdered iron core.
Iron powdered cores are very suitable due to their high sat-
uration flux density and have low loss at high frequencies,
a distributed gap and exhibit very low EMI.
Step 4b: Select 2% Ripple on 3.3V Output
Repeating Step 4a for the 3.3V output, we find 3.5µH is a
suitable value for this output.
Step 5: Input Capacitors
These components must be selected and placed carefully to
yield optimal results. Capacitors should be chosen to pro-
vide acceptable ripple on the input supply lines. Key speci-
fications for input capacitors are their ripple rating.
Step 5a: VCC(CORE) Buck Regulator Input Capacitors
The input capacitor CIN should also be able to handle the
12
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