LT3470AEDDB#TRMPBF Linear Technology, LT3470AEDDB#TRMPBF Datasheet - Page 9

• Manufacturer Part Number: LT3470AEDDB#TRMPBF
• Description: IC BUCK ADJ .25A 8DFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3470AEDDB#TRMPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 16 V
• Current - Output: 250mA
• Voltage - Input: 4 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-DFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Frequency - Switching: -

APPLICATIONS INFORMATION
Input Voltage Range
The minimum input voltage required to generate a par-
ticular output voltage in an LT3470A application is limited
by either its 4V undervoltage lockout or by its maximum
duty cycle. The duty cycle is the fraction of time that the
internal switch is on and is determined by the input and
output voltages:
where V
(~0.6V) and V
at maximum load (~0.4V). Given DC
to a minimum input voltage of:
This analysis assumes the part has started up such that the
capacitor tied between the BOOST and SW pins is charged
to more than 2V. For proper start-up, the minimum input
voltage is limited by the boost circuit as detailed in the
section BOOST Pin Considerations.
The maximum input voltage is limited by the absolute
maximum V
ficient value is used.
Inductor Selection
The switching action of the LT3470A during continuous
operation produces a square wave at the SW pin that results
in a triangle wave of current in the inductor. The hysteretic
mode control regulates the top and bottom current limits
(see Electrical Characteristics) such that the average induc-
tor current equals the load current. For safe operation, it
must be noted that the LT3470A cannot turn the switch
on for less than ~150ns. If the inductor is small and the
input voltage is high, the current through the switch may
exceed safe operating limit before the LT3470A is able to
turn off. To prevent this from happening, the following
equation provides a minimum inductor value:
DC =
L
V
MIN
IN(MIN)
D
=
V
IN
is the forward voltage drop of the catch diode
V
V
=
IN(MAX)
IN
OUT
– V
SW
⎛
⎜
⎝
rating of 40V, provided an inductor of suf-
V
SW
OUT
DC
+ V
is the voltage drop of the internal switch
+ V
MAX
• t
I
D
MAX
+ V
ON-TIME(MIN)
D
D
⎞
⎟ + V
⎠
SW
– V
MAX
D
= 0.90, this leads
where V
plication, t
allowable increase in switch current during a minimum
switch on-time (150mA). While this equation provides a
safe inductor value, the resulting application circuit may
switch at too high a frequency to yield good efficiency.
It is advised that switching frequency be below 1.2MHz
during normal operation:
where f is the switching frequency, ΔI
in the inductor (~200mA), V
of the catch diode, and V
If the application circuit is intended to operate at high duty
cycles (V
calculated value of the switch off-time:
The calculated t
minimum t
application circuit is capable of delivering full rated output
current. If the full output current of 250mA is not required,
the calculated t
t
See Table 1 for an inductor value selection guide.
Table 1. Recommended Inductors for Loads up to 250mA
Choose an inductor that is intended for power applications.
Table 2 lists several manufacturers and inductor series.
For robust output short-circuit protection at high V
to 40V) use at least a 33μH inductor with a minimum
450mA saturation current. If short-circuit performance is
not required, inductors with I
be used. It is important to note that inductor saturation
current is reduced at high temperatures—see inductor
vendors for more information.
OFF-TIME
f =
t
OFF-TIME
(
V
2.5V
3.3V
12V
1– DC
5V
OUT
IN(MAX)
IN
possibly allowing the use of a smaller inductor.
ON-TIME(MIN)
OFF-TIME
close to V
=
L • ΔI
)
1– DC
(
OFF-TIME
is the maximum input voltage for the ap-
V
OFF-TIME
D
f
L
+ V
(See Electrical Characteristics), so the
V
OUT
OUT
is ~150ns and I
IN
can be made less than minimum
OUT
Up to 16V
should be more than LT3470A’s
10μH
10μH
15μH
33μH
), it is important to look at the
)
D
is the desired output voltage.
SAT
is the forward voltage drop
of 300mA or more may
L
MAX
is the ripple current
LT3470A
V
is the maximum
IN
Up to 40V
33μH
33μH
33μH
47μH
IN
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