lt3680 Linear Technology Corporation, lt3680 Datasheet - Page 10

• Manufacturer Part Number: lt3680
• Description: 36v, 3.5a, 2.4mhz Step-down Switching Regulator With 75?a Quiescent Current
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3680
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve safe operation at
high input voltages.
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to 36V are acceptable regardless of the
switching frequency. In this mode, the LT3680 may enter
pulse skipping operation 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.
The minimum input voltage is determined by either the
LT3680’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:
where V
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
and decreases with higher inductance and faster switch-
ing frequency. A reasonable starting point for selecting
the ripple current is:
where I
guarantee suffi cient output current, peak inductor current
must be lower than the LT3680’s switch current limit (I
The peak inductor current is:
where I
the maximum output load current, and ΔI
10
ΔI
I
V
L(PEAK)
IN MIN
L
(
= 0.4(I
OUT(MAX)
L(PEAK)
IN(MIN)
)
= I
=
OUT(MAX)
OUT(MAX)
1–
is the minimum input voltage, and t
is the peak inductor current, I
V
is the maximum output load current. To
f
SW OFF MIN
OUT
t
L
+
)
increases with higher V
+ ΔI
V
(
D
L
/2
)
–
V
D
+
V
SW
L
is the inductor
OUT(MAX)
IN
OFF(MIN)
or V
LIM
OUT
is
).
ripple current. The LT3680’s switch current limit (I
5.5A at low duty cycles and decreases linearly to 4.5A at
DC = 0.8. The maximum output current is a function of
the inductor ripple current:
Be sure to pick an inductor ripple current that provides
suffi cient maximum output current (I
The largest inductor ripple current occurs at the highest
V
specifi ed maximum, the inductor value should be chosen
according to the following equation:
where V
V
voltage, f
L is in the inductor value.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should be
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 5A. To keep the
effi ciency high, the series resistance (DCR) should be less
than 0.1 , and the core material should be intended for
high frequency applications. Table 1 lists several vendors
and suitable types.
Table 1. Inductor Vendors
VENDOR
Murata
TDK
Toko
Sumida
NEC
IN
IN(MAX)
I
L
. To guarantee that the ripple current stays below the
OUT(MAX)
=
⎛
⎜
⎝
D
V
is the maximum input voltage, V
SW
OUT
f
is the voltage drop of the catch diode (~0.4V),
SW
URL
www.murata.com
www.componenttdk.com
www.toko.com
www.sumida.com
www.nec.com
= I
is the switching frequency (set by RT), and
Δ
+
I
LIM
L
V
D
⎞
⎟
⎠
– ΔI
⎛
⎜
⎝
1–
L
/2
V
V
OUT
IN MAX
(
+
PART SERIES
LQH55D
SLF10145
D75C
D75F
CDRH74
CR75
CDRH8D43
MPLC073
MPBI0755
V
)
D
⎞
⎟ ⎟
⎠
OUT(MAX)
OUT
is the output
).
TYPE
Open
Shielded
Shielded
Open
Shielded
Open
Shielded
Shielded
Shielded
LIM
3680f
) is