LM3551 Datasheet, PDF (11/15 Pages) National Semiconductor (TI) – 1A White LED Driver with Flash Timeout Protection

Application Information (Continued)
one resistor while Flash-mode is set by utilizes the currents
though both current set resistors. The following equations
are used to set the LED currents.
R_TORCH = (1.265V / I_TORCH) - R_DSON-T
R_FLASH = (1.265V / (I_FLASH- I_TORCH)) - R_DSON-F
R_DSON-T= 0.98Ω and R_DSON-F = 0.36Ω
Note: Flash LEDs from different manufacturers can have very different
continuous and pulse current ratings. See the manufacturers
datasheets to ensure that the proper current levels are used to avoid
damaging the flash LED.
INDUCTOR SELECTION
Special care must be taken when selecting an inductor for
use in LM3551/2 applications. The inductor should have a
current saturation rating that is larger than the worst case
peak inductor current of the application to ensure proper
operation. Using an inductor with a lower saturation current
rating than is required can cause a dramatic drop in the
inductance and can derate the maximum output current
levels severely. It is worth noting that the output voltage
ripple is also affected by the total ripple current in the induc-
tor. The following equations can help give a good approxi-
mation as to what the peak inductor current will be for a
given application at room temperature (T_A = +25˚C).
I_L(average) = [I_LED x V_OUT-MAX] ÷ [V_IN-MIN x Eff.]
∆I_L = [V_IN x D] ÷ [L x F_SW]
I_L(peak) = I_L(ave) + [∆I_L ÷ 2]
V_OUT-MAX Maximum Output Voltage. Maximum output volt-
age over temperature with OVP used is 11V
(12.4V typically).
V_IN-MIN
Minimum Input Voltage. Recommended minimum
input voltage is 3.0V. The LM3551/2 will work
down to 2.7V however, use at lower input volt-
ages will required an inductor with a higher satu-
ration current rating.
Eff.
Converter Efficiency (approx. 85% over input volt-
age range).
D
Duty Cycle = 1 - [V_IN / V_OUT]
L
Inductance. Recommended inductance value
is 4.7µH.
F_SW
Switching Frequency = 1.25MHz
DIODE SELECTION
The output diode for a boost regulator must be chosen
correctly depending on the output voltage and output cur-
rent. The output diode must have a reverse voltage rating
equal to or greater than the output voltage used. The aver-
age current rating must be greater than the maximum load
current expected, and the peak current rating must be
greater than the peak inductor current. Using Schottky di-
odes with lower forward voltage drop will decrease power
dissipation and increase efficiency.
CAPACITOR SELECTION
Input Capacitor
An input capacitor is required to reduce the input ripple and
noise for proper operation of the regulator. The size used is
dependant on the application and board layout. If the regu-
lator will be loaded uniformly, with very little load changes,
and at lower current outputs, the input capacitor size can
often be reduced. The size can also be reduced if the input
of the regulator is very close to the source output. The size
will generally need to be larger for applications where the
regulator is supplying nearly the maximum rated output or if
large load steps are expected. A minimum value of 10µF
should be used under normal operating condtions while a
10-22µF capacitor may be required for higher power and
dynamic loads. Larger values and/or lower ESR may be
needed if the application requires very low ripple on the input
source voltage.
Output Capacitor
A minimum output capacitor value of 4.7µF (Sharp LED) and
10µF (Lumiled) is recommended and may be increased to a
larger value. The ESR of the output capacitor is important
because it determines the peak to peak output voltage ripple
according to the approximate equation:
∆V_OUT ) 2 x ∆I_Lx R_ESR (in Volts)
After choosing the output capacitor you can determine a
pole-zero pair introduced into the control loop by the follow-
ing equations:
The zero created by the ESR of the output capacitor is
generally at a very high frequency if the ESR is small. If low
ESR capacitors are used it can be neglected. The output
capacitor pole information is useful in selecting the proper
compensation components and is discussed in the COM-
PENSATION COMPONENTS section of the datasheet.
Capacitor Properties
Surface-mount multi-layer ceramic capacitors are recom-
mended for both the input and output capacitors. These
capacitors are small, inexpensive and have very low equiva-
lent series resistance (ESR <20mΩ typ.). Tantalum capaci-
tors, OS-CON capacitors, and aluminum electrolytic capaci-
tors are not recommended for use with the LM3551/2 due to
their high ESR, as compared to ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM3551/2. These capacitors have tight capacitance toler-
ance (as good as ±10%) and hold their value over tempera-
ture (X7R: ±15% over -55˚C to 125˚C; X5R: ±15% over
-55˚C to 85˚C).
Capacitors with Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM3551/2.
Capacitors with these temperature characteristics typically
have wide capacitance tolerance (+80%, -20%) and vary
significantly over temperature (Y5V: +22%, -82% over -30˚C
to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C
range). Under some conditions, a nominal 1µF Y5V or Z5U
capacitor could have a capacitance of only 0.1µF. Such
deviation is likely to cause Y5V and Z5U capacitors to fail to
meet the minimum capacitance requirements of the
LM3551/2.
The minimum voltage rating acceptable for the input capaci-
tor is 6.3V (10V recommended) and 16V for the output
capacitor. In applications that have DC operating points near
the maximum voltage rating of the ceramic capacitor, larger
capacitor values may be required to compensate for capaci-
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