LTC3355_15 Datasheet, PDF (13/20 Pages) Linear Technology – 20V 1A Buck DC/DC with Integrated SCAP Charger and Backup Regulator

LTC3355
APPLICATIONS INFORMATION
Buck Inductor L1 Selection and
Maximum Output Current
A good starting point for the inductor value is:
( ) ^{L =}
V_OUT + V_D
• ^1.8
f_SW
where f_SW is the switching frequency in MHz, V_OUT is the
buck output voltage, V_D is the catch diode drop (~0.5V)
and L is the inductor value in µH.
The inductor’s RMS current rating must be greater than
the maximum load current and its saturation current
should be 30% higher. To keep the efficiency high, the
series resistance (DCR) should be less than 0.1Ω, and
the core material should be intended for high frequency
applications. Table 1 lists several inductor vendors.
For robust operation and fault conditions (start-up or
short-circuit) and high input voltage (>15V), the saturation
current should be chosen high enough to ensure that the
inductor peak current does not exceed 2.2A.
The current in the inductor is a triangle wave with an av-
erage value equal to the load current. The peak inductor
and switch current is:
I_SW(PEAK)
=
I_L(PEAK)
=
I_OUT(MAX)
+
ΔI_L
2
where I_L(PEAK) is the peak inductor current, I_OUT(MAX) is the
maximum output load current and ∆I_L is the inductor ripple
current. The LTC3355 limits the switch current in order to
protect the part. Therefore, the maximum output current
that the buck will deliver depends on the switch current
limit, the inductor value, 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_D
L • f_SW
where f_SW is the switching frequency of the buck, DC is
the duty cycle and L is the value of the inductor.
To maintain output regulation, the inductor peak current
must be less than the buck switch current limit. The
maximum output current is:
I_OUT(MAX)
=
I_LIM
–
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.
Table 1. Inductor Vendors
VENDOR URL
Murata
www.murata.com
TDK
www.tdk.com
Toko
www.toko.com
Coilcraft www.coilcraft.com
Sumida
www.sumida.com
Viashay www.vishay.com
PART SERIES
TYPE
LQH5BPB
Shielded
LTF5022T
Shielded
FDS50xx
Shielded
XAL40xx, LPS40xx Shielded
DCRH5D, CDRH6D Shielded
IHLP2020
Shielded
One approach to choosing the inductor is to start with
the simple rule above, look at the available inductors, and
choose one to meet cost or space goals. Then use the
equations to check that the buck will be able to deliver the
required output current. These equations assume that the
inductor current is continuous. Discontinuous operation
occurs when I_OUT is less than ∆I_L/2.
For more information www.linear.com/LTC3355
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