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LT3475IFE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LT3475/LT3475-1
10
3475fb
current have signifi cantly increased. Exceeding V
IN(MAX)
is safe if the external components have adequate ratings
to handle the peak conditions and if the peak inductor
current does not exceed 3.2A. A saturating inductor may
further reduce performance.
Table 2. Inductors
PART NUMBER
VALUE
(μH)
I
RMS
(A)
DCR
()
HEIGHT
(mm)
Sumida
CR43-3R3 3.3 1.44 0.086 3.5
CR43-4R7 4.7 1.15 0.109 3.5
CDRH4D16-3R3 3.3 1.10 0.063 1.8
CDRH4D28-3R3 3.3 1.57 0.049 3.0
CDRH4D28-4R7 4.7 1.32 0.072 3.0
CDRH6D26-5R0 5.0 2.20 0.032 2.8
CDRH6D26-5R6 5.6 2.0 0.036 2.8
CDRH5D28-100 10 1.30 0.048 3.0
CDRH5D28-150 15 1.10 0.076 3.0
CDRH73-100 10 1.68 0.072 3.4
CDRH73-150 15 1.33 0.130 3.4
CDRH104R-150 15 3.1 0.050 4.0
Coilcraft
DO1606T-332 3.3 1.30 0.100 2.0
DO1606T-472 4.7 1.10 0.120 2.0
DO1608C-332 3.3 2.00 0.080 2.9
DO1608C-472 4.7 1.50 0.090 2.9
MOS6020-332 3.3 1.80 0.046 2.0
MOS6020-472 10 1.50 0.050 2.0
DO3316P-103 10 3.9 0.038 5.2
DO3316P-153 15 3.1 0.046 5.2
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current, and
reduces the output voltage ripple. If your load is lower than
the maximum load current, then you can relax the value of the
inductor and operate with higher ripple current. This allows
you to use a physically smaller inductor, or one with a lower
DCR resulting in higher effi ciency. In addition, low inductance
may result in discontinuous mode operation, which further
reduces maximum load current. For details of maximum
output current and discontinuous mode operation, see Linear
Technology’s Application Note 44. Finally, for duty cycles
greater than 50% (V
OUT
/V
IN
> 0.5), a minimum inductance
is required to avoid sub-harmonic oscillations:
L
MIN
= (V
OUT
+ V
F
)•
800kHz
f
V
SW
20V/DIV
I
L
1A/DIV
V
OUT
500mV/DIV
(AC COUPLED)
3475 F04
Figure 4. Operation above V
IN(MAX)
. Output
Ripple and Peak Inductor Current Increases
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
L = (V
OUT
+ V
F
)•
1.2MHz
f
where V
F
is the voltage drop of the catch diode (~0.4V),
f is the switching frequency and L is in μH. With this value
the maximum load current will be above 1.6A at all duty
cycles. The inductor’s RMS current rating must be greater
than the maximum load current and its saturation current
should be at least 30% higher. For highest effi ciency,
the series resistance (DCR) should be less than 0.15Ω.
Table 2 lists several vendors and types that are suitable.
For robust operation at full load and high input voltages
(V
IN
> 30V), use an inductor with a saturation current
higher than 3.2A.
APPLICATIONS INFORMATION
LT3475/LT3475-1
11
3475fb
The current in the inductor is a triangle wave with an average
value equal to the load current. The peak switch current
is equal to the output current plus half the peak-to-peak
inductor ripple current. The LT3475 limits its switch cur-
rent in order to protect itself and the system from overload
faults. Therefore, the maximum output current that the
LT3475 will deliver depends on the switch current limit,
the inductor value, and the input and output voltages.
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor
ΔI
L
=
1–DC
()
V
OUT
+ V
F
()
L•f
()
where f is the switching frequency of the LT3475 and L
is the value of the inductor. The peak inductor and switch
current is
I
SW PK
()
= I
LPK
()
= I
OUT
+
ΔI
L
2
To maintain output regulation, this peak current must be
less than the LT3475’s switch current limit I
LIM
. I
LIM
is at
least 2.3A at low duty cycles and decreases linearly to 1.8A
at DC = 0.9. The maximum output current is a function of
the chosen inductor value:
I
OUT MAX
()
= I
LIM
–
ΔI
L
2
= 2.3A• 1–0.25•DC
()
–
ΔI
L
2
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors,
and choose one to meet cost or space goals. Then use
these equations to check that the LT3475 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continu-
ous. Discontinuous operation occurs when I
OUT
is less
than ΔI
L
/2.
Input Capacitor Selection
Bypass the input of the LT3475 circuit with a 4.7μF or
higher ceramic capacitor of X7R or X5R type. A lower
value or a less expensive Y5V type will work if there is
additional bypassing provided by bulk electrolytic capaci-
tors or if the input source impedance is low. The following
paragraphs describe the input capacitor considerations in
more detail.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input ca-
pacitor is required to reduce the resulting voltage ripple at
the LT3475 input and to force this switching current into a
tight local loop, minimizing EMI. The input capacitor must
have low impedance at the switching frequency to do this
effectively, and it must have an adequate ripple current rat-
ing. With two switchers operating at the same frequency
but with different phases and duty cycles, calculating the
input capacitor RMS current is not simple. However, a
conservative value is the RMS input current for the channel
that is delivering most power (V
OUT
• I
OUT
):
C
INRMS
= I
OUT
•
V
OUT
(V
IN
–V
OUT
)
V
IN
<
I
OUT
2
and is largest when V
IN
= 2V
OUT
(50% duty cycle). As the
second, lower power channel draws input current, the
input capacitor’s RMS current actually decreases as the
out-of-phase current cancels the current drawn by the
higher power channel. Considering that the maximum
load current from a single channel is ~1.5A, RMS ripple
current will always be less than 0.75A.
The high frequency of the LT3475 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is less than 10μF. The combination
of small size and low impedance (low equivalent series
resistance or ESR) of ceramic capacitors makes them the
preferred choice. The low ESR results in very low voltage
ripple. Ceramic capacitors can handle larger magnitudes
of ripple current than other capacitor types of the same
value. Use X5R and X7R types.
APPLICATIONS INFORMATION
LT3475/LT3475-1
12
3475fb
An alternative to a high value ceramic capacitor is a
lower value ceramic along with a larger electrolytic
capacitor. The electrolytic capacitor likely needs to be greater
than 10μF in order to meet the ESR and ripple current
requirements. The input capacitor is likely to see high
surge currents when the input source is applied. Tanta-
lum capacitors can fail due to an over-surge of current.
Only use tantalum capacitors with the appropriate surge
current rating. The manufacturer may also recommend
operation below the rated voltage of the capacitor.
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 plug-
ging the circuit into a live power source) this tank can ring,
doubling the input voltage and damaging the LT3475. 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
LT3475’s control loop. Because the LT3475 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.
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
C(RMS)
= ΔI
L
/12
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3475 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
Diode Selection
The catch diode (D3 from the Block Diagram) conducts
current only during switch off time. Average forward cur-
rent in normal operation can be calculated from:
I
D(AVG)
= I
OUT
(V
IN
– V
OUT
)/V
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 limit.
Peak reverse voltage is equal to the regulator input
voltage. Use a diode with a reverse voltage rating greater
than the input voltage. Table 4 lists several Schottky
diodes and their manufacturers.
Diode reverse leakage can discharge the output capacitor
during LED off times while PWM dimming. If operating at
high ambient temperatures, use a low leakage Schottky
for the widest PWM dimming range.
APPLICATIONS INFORMATION
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT3475IFE#TRPBF

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Description:
LED Lighting Drivers Dual Step-Down 1.5A LED Driver
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