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LM3500TL-16 Datasheet(Fiches technique) 14 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Numéro de pièce LM3500TL-16
Description  Synchronous Step-up DC/DC Converter for White LED
Télécharger  18 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3500TL-16 Datasheet(HTML) 14 Page - National Semiconductor (TI)

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Application Information (Continued)
Typical Peak Inductor Currents (mA)
V
IN
(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
CIN =COUT =1µF
L = 22 µH, 160 m
Ω DCR max. Coilcraft DT1608C-223
2 and 3 LED applications: LM3500-16 or LM3500-21; LED VF = 3.77V at
20mA; TA = 25˚C
4 LED applications: LM3500-16 or LM3500-21; LED VF = 3.41V at 20mA; TA
= 25˚C
5 LED applications: LM3500-21 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 LM3500 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 1µF ceramic output capacitor is sufficient.
Local bypassing for the input is needed on the LM3500.
Multilayer X7R or X5R ceramic capacitors with low ESR are
a good choice for this as well. A 1µF ceramic capacitor is
sufficient for most applications. However, for some applica-
tions at least a 4.7µF ceramic capacitor may be required for
proper startup of the LM3500. Using capacitors with low
ESR decreases input voltage ripple. For additional bypass-
ing, a 100nF 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 1, 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 100nF 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 LM3500 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
to minimize copper trace connections that can inject noise
into the system. The ground connection for the current set-
ting resistor should connect directly to the GND pin. The
AGND pin should connect directly to the GND pin. Not
connecting the AGND pin directly, as close to the chip as
possible, may affect the performance of the LM3500 and
limit its current driving capability. Trace connections made to
the inductor should be minimized to reduce power dissipa-
tion, EMI radiation and increase overall efficiency. It is good
practice to keep the V
SW routing away from sensitive pins
such as the FB pin. Failure to do so may inject noise into the
FB pin and affect the regulation of the device. See Figure 2
and Figure 3 for an example of a good layout as used for the
LM3500 evaluation board.
www.national.com
14


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