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LT3474EFE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LT3474/LT3474-1
13
3474fd
APPLICATIONS INFORMATION
You can estimate output ripple with the following
equation:
V
RIPPLE
=
ΔI
L
8•f•C
OUT
()
for ceramic capacitors
where ΔI
L
is the peak-to-peak ripple current in the inductor.
The RMS content of this ripple is very low so the RMS
current rating of the output capacitor is usually not of
concern. It can be estimated with the formula:
I
CRMS
()
=
ΔI
L
12
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3474 applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coeffi cients. In particular, Y5V
and Z5U types lose a large fraction of their capacitance
with applied voltage and at temperature extremes.
Because loop stability and transient response depend on
the value of C
OUT
, this loss may be unacceptable. Use X7R
and X5R types. Table 3 lists several capacitor vendors.
Table 3. Low-ESR Surface Mount Capacitors
VENDOR TYPE SERIES
Taiyo-Yuden Ceramic X5R, X7R
AVX Ceramic X5R, X7R
TDK Ceramic X5R, X7R
A fi nal caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit. If power is applied quickly (for example by plugging
the circuit into a live power source), this tank can ring,
doubling the input voltage and damaging the LT3474. The
solution is to either clamp the input voltage or dampen the
tank circuit by adding a lossy capacitor in parallel with the
ceramic capacitor. For details, see Application Note 88.
Output Capacitor Selection
For most LEDs, a 2.2μF 6.3V ceramic capacitor (X5R or
X7R) at the output results in very low output voltage ripple
and good transient response. Other types and values will
also work; the following discusses tradeoffs in output
ripple and transient performance.
The output capacitor fi lters the inductor current to generate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and stabilizes the LT3474’s
control loop. Because the LT3474 operates at a high
frequency, minimal output capacitance is necessary. In
addition, the control loop operates well with or without
the presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
and small circuit size, are therefore an option.
LT3474/LT3474-1
14
3474fd
APPLICATIONS INFORMATION
Diode Selection
The catch diode (D1 from Figure 1) conducts current only
during switch off time. Average forward current in normal
operation can be calculated from:
I
IVV
V
DAVG
OUT IN OUT
IN
()
=
()
–
The only reason to consider a diode with a larger current
rating than necessary for nominal operation is for the
worst-case condition of shorted output. The diode cur-
rent will then increase to one half the typical peak switch
current.
Peak reverse voltage is equal to the regulator input voltage.
Use a diode with a reverse voltage rating greater than the
input voltage.
If using the PWM mode of the LT3474, select a diode with
low reverse leakage.
Table 4 lists several Schottky diodes and their
manufacturers.
Table 4. Schottky Diodes
PART NUMBER
V
R
(V)
I
AVE
(A)
V
F
at 0.5A
(mV)
V
F
at 1A
(mV)
On Semiconductor
MBR0520L
20 0.5 385
MBR0540 40 0.5 510 620
MBRM120E 20 1 530
MBRM140 40 1 550
Diodes Inc.
B0530W
30 0.5 430
B120 20 1 500
B130 30 1 500
B140 HB 40 1 530
International Rectifi er
10BQ030
30 1 420
LT3474/LT3474-1
15
3474fd
APPLICATIONS INFORMATION
BOOST and BIAS Pin Considerations
The capacitor and internal diode tied to the BOOST pin
generate a voltage that is higher than the input voltage.
In most cases, a 0.22μF capacitor will work well. Figure 4
shows three ways to arrange the boost circuit. The BOOST
pin must be more than 2.5V above the SW pin for full ef-
fi ciency. For outputs of 2.8V or higher, the standard circuit
(Figure 4a) is best. For lower output voltages, the BIAS pin
can be tied to the input (Figure 4b). The circuit in Figure
4a is more effi cient because the BOOST pin current comes
from a lower voltage source. The BIAS pin can be tied to
another source that is at least 3V (Figure 4c). For example,
if a 3.3V source is on whenever the LED is on, the BIAS
pin can be connected to the 3.3V output. For LT3474-1
applications with higher output voltages, an additional
Zener diode may be necessary (Figure 4d) to maintain the
BOOST pin voltage below the absolute maximum. In any
case, be sure that the maximum voltage at the BOOST pin
is both less than 51V and the voltage difference between
the BOOST and SW pins is less than 25V.
Programming LED Current
The LED current can be set by adjusting the voltage on
the V
ADJ
pin. For a 1A LED current, either tie V
ADJ
to REF
or to a 1.25V source. For lower output currents, program
the V
ADJ
using the following formula:
I
LED
=
1A • V
ADJ
1.25V
Voltages less than 1.25V can be generated with a voltage
divider from the REF pin, as shown in Figure 5.
Figure 4. Generating the Boost Voltage
REF
V
ADJ
GND
LT3474
3474 F04
R1
R2
Figure 5. Setting V
ADJ
with a Resistor Divider
In order to have accurate LED current, precision resistors
are preferred (1% or better is recommended). Note that
the V
ADJ
pin sources a small amount of bias current, so
use the following formula to choose resistors:
R
V
VV
R
nA
ADJ
ADJ
2
125
1
50
=
+
.–
BIAS
V
IN
3474 F04c
BOOST
V
IN
V
IN2
> 3V
SW
GND
LT3474
(4c)
V
BOOST
– V
SW
≈ V
IN2
MAX V
BOOST
≈ V
IN2
+ V
IN
MINIMUM VALUE FOR V
IN2
= 3V
V
OUT
C3
BIAS
V
IN
3474 F04b
BOOST
V
IN
SW
GND
LT3474
(4b)
V
BOOST
– V
SW
≈ V
IN
MAX V
BOOST
≈ 2V
IN
V
OUT
C3
BIAS
V
IN
3474 F04a
BOOST
V
IN
SW
GND
LT3474
(4a)
V
BOOST
– V
SW
≈ V
OUT
MAX V
BOOST
≈ V
IN
+ V
OUT
V
OUT
C3
BIAS
V
IN
3474 F04d
BOOST
V
IN
SW
GND
LT3474
(4d)
V
BOOST
– V
SW
≈ V
OUT
–V
Z
MAX V
BOOST
≈ V
IN
+ V
OUT
–V
Z
V
OUT
C3
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT3474EFE#PBF

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Buy LT3474EFE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LED Lighting Drivers 1A Step Down LED Driver in TSSOP-16
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