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參數(shù)資料
| 型號： | LT3470IDDB#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.2 A SWITCHING REGULATOR, PDSO8 |
| 封裝： | 3 X 2 MM, 0.75MM INCH HEIGHT, LEAD FREE, MO-229WECD-1, PLASTIC, DFN-8 |
| 文件頁數(shù)： | 2/20頁 |
| 文件大小： | 256K |
| 代理商： | LT3470IDDB#PBF |
10
LT3470
3470fc
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 supply
in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the VIN 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 1
F to 2.2F ceramic
capacitor satisfies these requirements.
If the input source impedance is high, a larger value
capacitor may be required to keep input ripple low. In this
case, an electrolytic of 10
F or more in parallel with a 1F
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
approximately ILIMESR.ESRshouldbelessthan~150m.
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:
CL
I
V
OUT
LIM
OUT
>
50
2
Where ILIM is the top current limit with VFB = 0V (see
Electrical Characteristics). For example, an LT3470 pro-
ducing 3.3V with L = 33
H requires 22F. 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.
APPLICATIO S I FOR ATIO
WU
UU
Table 2. Inductor Vendors
VENDOR
URL
PART SERIES
INDUCTANCE RANGE (
H)
SIZE (mm)
Coilcraft
www.coilcraft.com
DO1605
10 to 47
1.8
× 5.4 × 4.2
ME3220
10 to 47
2.0
× 3.2 × 2.5
DO3314
10 to 47
1.4
× 3.3 × 3.3
Sumida
www.sumida.com
CR32
10 to 47
3.0
× 3.8 × 4.1
CDRH3D16/HP
10 to 33
1.8
× 4.0 × 4.0
CDRH3D28
10 to 47
3.0
× 4.0 × 4.0
CDRH2D18/HP
10 to 15
2.0
× 3.2 × 3.2
Toko
www.tokoam.com
DB320C
10 to 27
2.0
× 3.8 × 3.8
D52LC
10 to 47
2.0
× 5.0 × 5.0
Würth Elektronik
www.we-online.com
WE-PD2 Typ S
10 to 47
3.2
× 4.0 × 4.5
WE-TPC Typ S
10 to 22
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
10 to 47
2.6
× 3.2 × 4.5
LQH32C
10 to 15
1.6
× 2.5 × 3.2
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