lm4510sdx National Semiconductor Corporation, lm4510sdx Datasheet - Page 12

• Manufacturer Part Number: lm4510sdx
• Description: Synchronous Step-up Dc/dc Converter With True Shutdown Isolation
• Manufacturer: National Semiconductor Corporation

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MAXIMUM OUTPUT CURRENT
When the output voltage is set at different level, it is important
to know the maximum load capability. By first order estima-
tion, I
INDUCTOR SELECTION
The larger value inductor makes lower peak inductor current
and reduces stress on internal power NMOS.
On the other hand, the smaller value inductor has smaller
outline, lower DCR and a higher current capacity. Generally
a 4.7 μH to 15 μH inductor is recommended.
I
The average inductor current is given by the following equa-
tion:
Where I
the total driven load and D’ is the off duty cycle of the switching
regulator.
Inductor DC current rating (40°C temperature rise) should be
more than the average inductor current at worst case.
INPUT CAPACITOR SELECTION
Due to the presence of an inductor, the input current wave-
form is continuous and triangular. So the input capacitor is
less critical than output capacitor in boost applications. Typi-
cally, a 4.7 μF to 10 μF ceramic input capacitor is recom-
mended on the VIN pin of the IC.
I
The RMS current in the input capacitor is given by the follow-
ing equation:
The input capacitor should be capable of handling the RMS
current.
L_AVE
CIN_RMS
OUT(MAX)
CHECK
OUT
Output Voltage (V)
Check
DO3316P-472ML
DO3314-472ML
is output current, η is the converter efficiency of
Model
can be estimated by the following equation:
3.3
16
12
5
TABLE 1. Suggested Component Values for Different Output Voltages
TABLE 2. Suggested Inductors and Their Suppliers
R
COILCRAFT
COILCRAFT
F2
20.5
20.5
20.5
20.5
Vendor
(kΩ)
12
R
12.95mm x 9.4mm x 5.4mm
Dimensions LxWxH (mm)
F1
60.4
3.3mm x 3.3mm x 1.4mm
240
174
33
ΔI Define
The inductor ripple current is given by the following equations:
Where D is the on-duty cycle of the switching regulator. A
common choice is to set ΔI
I
The peak inductor current is given by the following equation:
To prevent loss of regulation, ensure that the NMOS power
switch current limit is greater than the worst-case peak in-
ductor current in the target application.
Also make sure that the inductor saturation current is greater
than the peak inductor current under the worst-case load
transient, high ambient temperature and startup conditions.
Refer to Table 2 for suggested inductors.
OUTPUT CAPACITOR SELECTION
The output capacitor in a boost converter provides all the out-
put current when the switch is closed and the inductor is
charging. As a result, it sees very large ripple currents.
A ceramic capacitor of value 4.7 μF to 10 μF is recommended
at the output. If larger amounts of capacitance are desired for
improved line support and transient response, tantalum ca-
pacitors can be used.
I
The RMS current in the output capacitor is given by the fol-
lowing equation:
L_PK
COUT_RMS
(kΩ)
≤
I
CL
Check
Check & I
MIN
R
C
46.4
46.4
46.4
46.4
Define
(kΩ)
L
to about 30% of I
D.C.R (max)
320 mΩ
18 mΩ
C
L_AVE
C1
2.2
2.2
2.2
2.2
(nF)
.