lt3470a Linear Technology Corporation, lt3470a Datasheet - Page 11

• Manufacturer Part Number: lt3470a
• Description: Micropower Buck Regulator With Integrated Boost And Catch Diodes
• Manufacturer: Linear Technology Corporation

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
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 satisfi es 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 LT3470A
circuit is connected to a low impedance supply, and that
some electrolytic capacitors (in particular tantalum) must
be specifi ed for such use.
Output Capacitor and Output Ripple
The output capacitor fi lters the inductor’s ripple current
and stores energy to satisfy the load current when the
LT3470A is quiescent. In order to keep output voltage
ripple low, the impedance of the capacitor must be low
Table 2. Inductor Vendors
VENDOR
Coilcraft
Sumida
Toko
Würth Elektronik
Coiltronics
Murata
IN
pin of the LT3470A and to force this switching
URL
www.coilcraft.com
www.sumida.com
www.tokoam.com
www.we-online.com
www.cooperet.com
www.murata.com
PART SERIES
DO1605
ME3220
DO3314
CR32
CDRH3D16/HP
CDRH3D28
CDRH2D18/HP
DB320C
D52LC
WE-PD2 Typ S
WE-TPC Typ S
SD10
LQH43C
LQH32C
at the LT3470A’s switching frequency. The capacitor’s
equivalent series resistance (ESR) determines this im-
pedance. Choose one with low ESR intended for use in
switching regulators. The contribution to ripple voltage
due to the ESR is approximately I
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:
Where I
cal Characteristics). For example, an LT3470A 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 LT3470A. Similar
performance in traditional tantalum capacitors requires
a larger package (C-case). With a high quality capacitor
fi ltering the ripple current from the inductor, the output
voltage ripple is determined by the delay in the LT3470A’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.
C
OUT
INDUCTANCE RANGE (μH)
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
LIM
> 50 • L •
is the top current limit with V
V
I
LIM
OUT
2
SIZE (mm)
1.8 × 5.4 × 4.2
2.0 × 3.2 × 2.5
1.4 × 3.3 × 3.3
3.0 × 3.8 × 4.1
1.8 × 4.0 × 4.0
3.0 × 4.0 × 4.0
2.0 × 3.2 × 3.2
2.0 × 3.8 × 3.8
2.0 × 5.0 × 5.0
3.2 × 4.0 × 4.5
1.6 × 3.8 × 3.8
1.0 × 5.0 × 5.0
2.6 × 3.2 × 4.5
1.6 × 2.5 × 3.2
LIM
• ESR. ESR should
FB
LT3470A
= 0V (see Electri-
11
3470af