lt3684emse-trpbf Linear Technology Corporation, lt3684emse-trpbf Datasheet - Page 11

• Manufacturer Part Number: lt3684emse-trpbf
• Description: 36v, 2a, 2.8mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
frequency will be necessary to achieve safe operation at
high input voltages.
If the output is in regulation and no short-circuit or start-up
events are expected, then input voltage transients of up to
36V are acceptable regardless of the switching frequency.
In this mode, the LT3684 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.
The minimum input voltage is determined by either the
LT3684’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 LT3684’s switch current limit (I
The peak inductor current is:
where I
the maximum output load current, and ΔI
ripple current. The LT3684’s switch current limit (I
ΔI
I
V
L(PEAK)
IN MIN
L
(
= 0.4(I
OUT(MAX)
L(PEAK)
IN(MIN)
)
= I
=
OUT(MAX)
1–
OUT(MAX)
is the minimum input voltage, and t
is the peak inductor current, I
V
f
is the maximum output load current. To
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
LIM
OUT
) is
is
).
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:
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
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 3.5A. To keep the
effi ciency 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.
Table 1. Inductor Vendors
VENDOR
Murata
TDK
Toko
Sumida
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
is the voltage drop of the catch diode (~0.4V),
URL
www.murata.com
www.componenttdk.com
www.toko.com
www.sumida.com
f I
∆
is the switching frequency (set by R
= I
+
L
LIM
V
D
⎞
⎠ ⎟
– ΔI
⎛
⎜
⎜
⎝
1–
L
/2
V
V
OUT
IN MAX
(
+
PART SERIES
LQH55D
SLF7045
SLF10145
D62CB
D63CB
D75C
D75F
CR54
CDRH74
CDRH6D38
CR75
V
)
D
⎞
⎟
⎟ ⎟
⎠
OUT(MAX)
OUT
LT3684
is the output
).
TYPE
Open
Shielded
Shielded
Shielded
Shielded
Shielded
Open
Open
Shielded
Shielded
Open
T
), and L
11
3684f