LT3470
APPLICATIO S I FOR ATIO
Table 2. Inductor Vendors
VENDOR
Coilcraft
URL
www.coilcraft.com
Sumida
www.sumida.com
Toko
W眉rth Elektronik
Coiltronics
Murata
www.tokoam.com
www.we-online.com
www.cooperet.com
www.murata.com
current is reduced at high temperatures鈥攕ee 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 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 1碌F to 2.2碌F 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碌F or more in parallel with a 1碌F
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鈥檚 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
U
PART SERIES
DO1605
ME3220
DO3314
CR32
CDRH3D16/HP
CDRH3D28
CDRH2D18/HP
DB320C
D52LC
WE-PD2 Typ S
WE-TPC Typ S
SD10
LQH43C
LQH32C
INDUCTANCE RANGE (碌H)
10 to 47
10 to 47
10 to 47
10 to 47
10 to 33
10 to 47
10 to 15
10 to 27
10 to 47
10 to 47
10 to 22
10 to 47
10 to 47
10 to 15
SIZE (mm)
1.8
脳
5.4
脳
4.2
2.0
脳
3.2
脳
2.5
1.4
脳
3.3
脳
3.3
3.0
脳
3.8
脳
4.1
1.8
脳
4.0
脳
4.0
3.0
脳
4.0
脳
4.0
2.0
脳
3.2
脳
3.2
2.0
脳
3.8
脳
3.8
2.0
脳
5.0
脳
5.0
3.2
脳
4.0
脳
4.5
1.6
脳
3.8
脳
3.8
1.0
脳
5.0
脳
5.0
2.6
脳
3.2
脳
4.5
1.6
脳
2.5
脳
3.2
W
U U
LT3470鈥檚 switching frequency. The capacitor鈥檚 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 I
LIM
鈥?ESR. ESR should be less than ~150m鈩?
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:
鈳?/div>
I
鈳?/div>
C
OUT
>
50 鈥?L 鈥?/div>
鈳?/div>
LIM
鈳?/div>
鈳?/div>
V
OUT
鈳?/div>
2
Where I
LIM
is the top current limit with V
FB
= 0V (see
Electrical Characteristics). For example, an LT3470 pro-
ducing 3.3V with L = 33碌H requires 22碌F. The calculated
value can be relaxed if small circuit size is more important
than low output ripple.
Sanyo鈥檚 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鈥檚
feedback comparator. This ripple can be reduced further
by adding a small (typically 22pF) phase lead capacitor
between the output and the feedback pin.
3470f
9
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