LT3681
APPLICATIONS INFORMATION
frequency will be necessary to achieve safe operation at
high input voltages.
at least 3.5A at low duty cycles and decreases linearly to
2.5A at DC = 0.8. The maximum output current is a func-
tion of the inductor ripple current:
Iftheoutputisinregulationandnoshort-circuitorstart-up
events are expected, then input voltage transients of up to
36V are acceptable regardless of the switching frequency.
In this mode, the LT3681 may enter pulse skipping opera-
tion where some switching pulses are skipped to maintain
output regulation. In this mode the output voltage ripple
and inductor current ripple will be higher than in normal
operation.
I
= I – ΔI /2
LIM L
OUT(MAX)
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (I ).
OUT(MAX)
The largest inductor ripple current occurs at the highest
V . To guarantee that the ripple current stays below the
IN
specified maximum, the inductor value should be chosen
according to the following equation:
The minimum input voltage is determined by either the
LT3681’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
⎛
⎜
⎞
⎟
⎛
⎞
VOUT + VD
f∆IL
VOUT + VD
L =
1–
⎜
⎟
⎜
⎝
⎟
⎠
V
⎝
⎠
IN MAX
(
)
where V is the voltage drop of the integrated Schottky
D
VOUT + VD
V
=
– VD + VSW
diode (~0.55V), V
is the maximum input voltage,
IN MIN
(
)
IN(MAX)
1– fSWtOFF MIN
(
)
V
OUT
is the output voltage, f is the switching frequency
SW
(set by RT), and L is in the inductor value.
whereV
istheminimuminputvoltage,andt
OFF(MIN)
IN(MIN)
Theinductor’sRMScurrentratingmustbegreaterthanthe
maximumloadcurrentanditssaturationcurrentshouldbe
about 30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 3.5A. 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 vendors
and suitable types.
is the minimum switch off time (150ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current ΔI increases with higher V or V
L
IN
OUT
Table 1. Inductor Vendors
and decreases with higher inductance and faster switch-
ing frequency. A reasonable starting point for selecting
the ripple current is:
VENDOR URL
PART SERIES
TYPE
Murata
TDK
www.murata.com
LQH55D
Open
www.componenttdk.com SLF7045
Shielded
Shielded
ΔI = 0.4(I
)
L
OUT(MAX)
SLF10145
Toko
www.toko.com
D75C
Shielded
Open
where I
is the maximum output load current. To
OUT(MAX)
D75F
guarantee sufficient output current, peak inductor current
FDV0620
CDRH74
CDRH6D38
CR75
Shielded
Shielded
Shielded
Open
mustbelowerthantheLT3681’sswitchcurrentlimit(I ).
The peak inductor current is:
LIM
Sumida
www.sumida.com
I
= I
+ ΔI /2
OUT(MAX) L
L(PEAK)
CDRH8D43
PLC-0745
Shielded
Shielded
where I
is the peak inductor current, I
is
OUT(MAX)
L(PEAK)
NEC
www.nec.tokin.com
the maximum output load current, and ΔI is the inductor
L
ripple current. The LT3681’s switch current limit (I ) is
LIM
3681f
10
Linear [ Linear ]
