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LT3470ETS8#TRMPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LT3470
10
3470fd
Table 2. Inductor Vendors
VENDOR URL PART SERIES INDUCTANCE RANGE (µH) SIZE (mm)
Coilcraft www.coilcraft.com DO1605
ME3220
DO3314
10 to 47
10 to 47
10 to 47
1.8 × 5.4 × 4.2
2.0 × 3.2 × 2.5
1.4 × 3.3 × 3.3
Sumida www.sumida.com CR32
CDRH3D16/HP
CDRH3D28
CDRH2D18/HP
10 to 47
10 to 33
10 to 47
10 to 15
3.0 × 3.8 × 4.1
1.8 × 4.0 × 4.0
3.0 × 4.0 × 4.0
2.0 × 3.2 × 3.2
Toko www.tokoam.com DB320C
D52LC
10 to 27
10 to 47
2.0 × 3.8 × 3.8
2.0 × 5.0 × 5.0
Würth Elektronik www.we-online.com WE-PD2 Typ S
WE-TPC Typ S
10 to 47
10 to 22
3.2 × 4.0 × 4.5
1.6 × 3.8 × 3.8
Coiltronics www.cooperet.com SD10 10 to 47
1.0 × 5.0 × 5.0
Murata www.murata.com LQH43C
LQH32C
10 to 47
10 to 15
2.6 × 3.2 × 4.5
1.6 × 2.5 × 3.2
applicaTions inForMaTion
be used. It is important to note that inductor saturation
current is reduced at high temperatures—see inductor
vendors for more information.
Input Capacitor
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the V
IN
pin of the LT3470 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. A 1µF to 2.2µF ceramic
capacitor satisfies these requirements.
If the input source impedance is high, a larger value ca-
pacitor may be required to keep input ripple low. In this
case, an electrolytic of 10µF or more in parallel with a 1µF
ceramic is a good combination. Be aware that the input
capacitor is subject to large surge currents if the LT3470
circuit is connected to a low impedance supply, and that
some electrolytic capacitors (in particular tantalum) must
be specified for such use.
Output Capacitor and Output Ripple
The output capacitor filters the inductor’s ripple current
and stores energy to satisfy the load current when the
LT3470 is quiescent. In order to keep output voltage ripple
low, the impedance of the capacitor must be low at the
LT3470’s switching frequency. The capacitor’s equivalent
series resistance (ESR) determines this impedance. Choose
one with low ESR intended for use in switching regulators.
The contribution to ripple voltage due to the ESR is ap-
proximately I
LIM
• ESR. ESR should be less than ~150mΩ.
The value of the output capacitor must be large enough to
accept the energy stored in the inductor without a large
change in output voltage. Setting this voltage step equal
to 1% of the output voltage, the output capacitor must be:
C
OUT
> 50 • L •
I
LIM
V
OUT
2
where I
LIM
is the top current limit with V
FB
= 0V (see Elec-
trical Characteristics). For example, an LT3470 producing
3.3V with L = 33µH requires 22µF. The calculated value
can be relaxed if small circuit size is more important than
low output ripple.
Sanyo’s POSCAP series in B-case and provides very good
performance in a small package for the LT3470. Similar
performance in traditional tantalum capacitors requires
a larger package (C-case). With a high quality capacitor
filtering the ripple current from the inductor, the output
voltage ripple is determined by the delay in the LT3470’s
feedback comparator. This ripple can be reduced further
by adding a small (typically 22pF) phase lead capacitor
between the output and the feedback pin.
LT3470
11
3470fd
applicaTions inForMaTion
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3470. Not all ceramic capacitors are
suitable. X5R and X7R types are stable over temperature
and applied voltage and give dependable service. Other
types, including Y5V and Z5U have very large temperature
and voltage coefficients of capacitance. In an application
circuit they may have only a small fraction of their nominal
capacitance resulting in much higher output voltage ripple
than expected.
Ceramic capacitors are piezoelectric. The LT3470’s switch-
ing frequency depends on the load current, and at light
loads the LT3470 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3470
operates at a lower current limit during Burst Mode opera-
tion, the noise is typically very quiet to a casual ear. If this
audible noise is unacceptable, use a high performance
electrolytic capacitor at the output. The input capacitor
can be a parallel combination of a 2.2µF ceramic capacitor
and a low cost electrolytic capacitor.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3470. A ceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3470 circuit is plugged into a live supply, the input volt-
age can ring to twice its nominal value, possibly exceeding
the LT3470’s rating. This situation is easily avoided; see
the Hot-Plugging Safely section.
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
Block Diagram) are used to generate a boost voltage that
is higher than the input voltage. In most cases a 0.22µF
capacitor will work well. Figure 2 shows two ways to ar-
range the boost circuit. The BOOST pin must be more than
2.5V above the SW pin for best efficiency. For outputs of
3.3V and above, the standard circuit (Figure 2a) is best.
For outputs between 2.5V and 3V, use a 0.47µF. For lower
output voltages the boost diode can be tied to the input
Figure 2. Two Circuits for Generating the Boost Voltage
Table 3. Capacitor Vendors
VENDOR PHONE URL PART SERIES COMMENTS
Panasonic (714) 373-7366 www.panasonic.com Ceramic,
Polymer,
Tantalum
EEF Series
Kemet (864) 963-6300 www.kemet.com Ceramic,
Tantalum
T494, T495
Sanyo (408) 749-9714 www.sanyovideo.com Ceramic,
Polymer,
Tantalum
POSCAP
Murata (404) 436-1300 www.murata.com Ceramic
AVX www.avxcorp.com Ceramic,
Tantalum
TPS Series
Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic
V
IN
BOOST
LT3470
(2a)
(2b)
SW
C3
0.22µF
V
IN
V
OUT
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
BIAS
GND
V
IN
BOOST
LT3470
SWBIAS
C3
0.22µF
V
IN
V
OUT
3470 F02
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
GND
LT3470
12
3470fd
Figure 3. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
Minimum Input Voltage, V
OUT
= 3.3V
Minimum Input Voltage, V
OUT
= 5V
Figure 4. Diode D1 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output; It Also Protects the Circuit
from a Reversed Input. The LT3470 Runs Only When the Input Is
Present Hot-Plugging Safely
applicaTions inForMaTion
(Figure 2b). The circuit in Figure 2a is more efficient
because the BOOST pin current and BIAS pin quiescent
current comes from a lower voltage source. You must also
be sure that the maximum voltage ratings of the BOOST
and BIAS pins are not exceeded.
The minimum operating voltage of an LT3470 application
is limited by the undervoltage lockout (4V) and by the
maximum duty cycle as outlined in a previous section. For
proper start-up, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3470 is turned on with its SHDN pin when the
output is already in regulation, then the boost capacitor may
not be fully charged. The plots in Figure 3 show minimum
V
IN
to start and to run. At light loads, the inductor current
becomes discontinuous and the effective duty cycle can
be very high. This reduces the minimum input voltage to
approximately 300mV above V
OUT
. At higher load currents,
the inductor current is continuous and the duty cycle is
limited by the maximum duty cycle of the LT3470, requiring
a higher input voltage to maintain regulation.
Shorted Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively at the top switch current limit maximum of 450mA,
an LT3470 buck regulator will tolerate a shorted output
even if V
IN
= 40V. There is another situation to consider
in systems where the output will be held high when the
input to the LT3470 is absent. This may occur in battery
charging applications or in battery backup systems where
a battery or some other supply is diode OR-ed with the
LT3470’s output. If the V
IN
pin is allowed to float and the
SHDN pin is held high (either by a logic signal or because
it is tied to V
IN
), then the LT3470’s internal circuitry will
pull its quiescent current through its SW pin. This is fine
if your system can tolerate a few mA in this state. If you
ground the SHDN pin, the SW pin current will drop to es-
sentially zero. However, if the V
IN
pin is grounded while
the output is held high, then parasitic diodes inside the
LT3470 can pull large currents from the output through
the SW pin and the V
IN
pin. Figure 4 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
LOAD CURRENT (mA)
0
3.0
INPUT VOLTAGE (V)
3.5
4.0
4.5
5.0
5.5
6.0
50 100 150 200
3470 G18
T
A
= 25°C
V
IN
TO START
V
IN
TO RUN
LOAD CURRENT (mA)
0
INPUT VOLTAGE (V)
6
7
200
3470 G19
5
4
50
100
150
8
T
A
= 25°C
V
IN
TO START
V
IN
TO RUN
V
IN
BOOST
LT3470 SOT-23
SWSHDN
3470 F04
V
IN
100k
D1
1M
V
OUT
BACKUP
BIAS
FB
GND
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT3470ETS8#TRMPBF

Mfr. #:
Buy LT3470ETS8#TRMPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 300mA, 40V Micropower Step-Down Reg in ThinSOT
Lifecycle:
New from this manufacturer.
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