lt3470its8-trpbf Linear Technology Corporation, lt3470its8-trpbf Datasheet - Page 9

• Manufacturer Part Number: lt3470its8-trpbf
• Description: Micropower Buck Regulator With Integrated Boost And Catch Diodes
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Input Voltage Range
The minimum input voltage required to generate a particu-
lar output voltage in an LT3470 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
sufficient value is used.
Inductor Selection
The switching action of the LT3470 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 inductor current equals the load current. For safe
operation, it must be noted that the LT3470 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 LT3470 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
–
SW
(
⎛
⎝ ⎜
V
rating of 40V, provided an inductor of
SW
V
is the voltage drop of the internal switch
+
OUT
DC
)
U
V
+
•
I
D
MAX
MAX
t
V
+
ON-TIME(MIN)
D
V
U
D
⎞
⎠ ⎟
+
V
SW
W
MAX
–
V
D
= 0.90, this leads
U
where V
application, t
mum allowable increase in switch current during a mini-
mum 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, ∆I
in the inductor (~150mA), 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 200mA is not required,
the calculated t
t
See Table 1 for an inductor value selection guide.
Table 1. Recommended Inductors for Loads up to 200mA
Choose an inductor that is intended for power applica-
tions. 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
OFF-TIME
f
t
OFF-TIME
=
(
V
2.5V
3.3V
1–
12V
5V
IN(MAX)
OUT
IN
possibly allowing the use of a smaller inductor.
OFF-TIME
DC V
close to V
ON-TIME(MIN)
=
L
)
OFF-TIME
(
1–
•
OFF-TIME
is the maximum input voltage for the
∆
D
f
DC
I
(See Electrical Characteristics), so the
+
L
V
OUT
V
OUT
can be made less than minimum
OUT
IN
should be more than LT3470’s
is ~150ns and I
), it is important to look at the
Up to 16V
10µH
10µH
15µH
33µH
)
D
is the desired output voltage.
SAT
is the forward voltage drop
of 300mA or more may
L
is the ripple current
MAX
V
IN
LT3470
Up to 40V
is the maxi-
33µH
33µH
33µH
47µH
IN
3470fc
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