LM3501 Datasheet, PDF (14/18 Pages) National Semiconductor (TI) – Synchronous Step-up DC/DC Converter for White LED Applications

Application Information (Continued)
The minimum inductor value required for the LM3501-16 can
be calculated using the following equation:
The typical cycle-by-cycle peak inductor current can be cal-
culated from the following equation:
The minimum inductor value required for the LM3501-21 can
be calculated using the following equation:
where I_OUT is the total load current, F_SW 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:
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)
# LEDs
LED Current
VIN (in
15 20 30 40 50 60
(V) series) mA mA mA mA mA 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
2
64 76 96 116 142 162
3
102 116 148 180 210 246
4.2
4
122 146 186 232 272 318
5
179 206 263 324 388 456
Note 10: C_IN = C_OUT = 1 µF
L = 22 µH, 160 mΩ DCR max. Coilcraft DT1608C-223
2 and 3 LED applications: LM3501-16 or LM3501-21; LED V_F = 3.77V at
20mA; T_A = 25˚C
4 LED applications: LM3501-16 or LM3501-21; LED V_F = 3.41V at 20mA; T_A
= 25˚C
5 LED applications: LM3501-21 only; LED V_F = 3.28V at 20mA; T_A = 25˚C
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
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