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LM3501 Fiches technique(PDF) 14 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
No de pièce LM3501
Description  Synchronous Step-up DC/DC Converter for White LED Applications
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3501 Fiches technique(HTML) 14 Page - National Semiconductor (TI)

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Application Information (Continued)
The minimum inductor value required for the LM3501-16 can
be calculated using the following equation:
The minimum inductor value required for the LM3501-21 can
be calculated using the following equation:
For both equations above, L is in µH, V
IN is the input supply
of the chip in Volts, R
DSON is the ON resistance of the NMOS
power switch found in the Typical Performance Characteris-
tics section in ohms and D is the duty cycle of the switching
regulator. The above equation is only valid for D greater than
or equal to 0.5. For applications where the minimum duty
cycle is less than 0.5, a 22 µH inductor is the typical recom-
mendation for use with most applications. Bench-level veri-
fication of circuit performance is required in these special
cases, however. The duty cycle, D, is given by the following
equation:
where V
OUT is the voltage at pin C1.
Typical Peak Inductor Current (mA)(Note 10)
VIN
(V)
# LEDs
(in
series)
LED Current
15
mA
20
mA
30
mA
40
mA
50
mA
60
mA
2.7
2
82
100
134
160
204
234
3
118
138
190
244
294
352
4
142
174
244
322
X
X
5
191
232
319
413
X
X
3.3
2
76
90
116
136
172
198
3
110
126
168
210
250
290
4
132
158
212
270
320
X
5
183
216
288
365
446
X
4.2
2
64
76
96
116
142
162
3
102
116
148
180
210
246
4
122
146
186
232
272
318
5
179
206
263
324
388
456
Note 10: CIN =COUT =1µF
L = 22 µH, 160 m
Ω DCR max. Coilcraft DT1608C-223
2 and 3 LED applications: LM3501-16 or LM3501-21; LED VF = 3.77V at
20mA; TA = 25˚C
4 LED applications: LM3501-16 or LM3501-21; LED VF = 3.41V at 20mA; TA
= 25˚C
5 LED applications: LM3501-21 only; LED VF = 3.28V at 20mA; TA = 25˚C
The typical cycle-by-cycle peak inductor current can be cal-
culated from the following equation:
where I
OUT is the total load current, FSW is the switching
frequency, L is the inductance and
η is the converter effi-
ciency of the total driven load. A good typical number to use
for
η is 0.8. The value of η can vary with load and duty cycle.
The average inductor current, which is also the average V
SW
pin current, is given by the following equation:
The maximum output current capability of the LM3501 can
be estimated with the following equation:
where I
CL is the current limit. Some recommended inductors
include but are not limited to:
Coilcraft DT1608C series
Coilcraft DO1608C series
TDK VLP4612 series
TDK VLP5610 series
TDK VLF4012A series
CAPACITOR SELECTION
Choose low ESR ceramic capacitors for the output to mini-
mize output voltage ripple. Multilayer X7R or X5R type ce-
ramic capacitors are the best choice. For most applications,
a 1 µF ceramic output capacitor is sufficient.
Local bypassing for the input is needed on the LM3501.
Multilayer X7R or X5R ceramic capacitors with low ESR are
a good choice for this as well. A 1 µF ceramic capacitor is
sufficient for most applications. However, for some applica-
tions at least a 4.7 µF ceramic capacitor may be required for
proper startup of the LM3501. Using capacitors with low
ESR decreases input voltage ripple. For additional bypass-
ing, a 100 nF ceramic capacitor can be used to shunt high
frequency ripple on the input. Some recommended capaci-
tors include but are not limited to:
TDK C2012X7R1C105K
Taiyo-Yuden EMK212BJ105 G
LAYOUT CONSIDERATIONS
The input bypass capacitor C
IN, as shown in Figure 2, must
be placed close to the device and connect between the V
IN
and GND pins. This will reduce copper trace resistance
which effects the input voltage ripple of the IC. For additional
input voltage filtering, a 100 nF bypass capacitor can be
placed in parallel with C
IN to shunt any high frequency noise
to ground. The output capacitor, C
OUT, should also be placed
close to the LM3501 and connected directly between the
V
OUT and GND pins. Any copper trace connections for the
C
OUT capacitor can increase the series resistance, which
directly effects output voltage ripple and efficiency. The cur-
rent setting resistor, R
LED, should be kept close to the FB pin
www.national.com
14


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