Datasheet

LT8609/LT8609A/LT8609B
15
Rev. H
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APPLICATIONS INFORMATION
Note that higher switching frequency will increase the
minimum input voltage below which cycles will be
dropped to achieve higher duty cycle.
Inductor Selection and Maximum Output Current
The LT8609/LT8609A/LT8609B is designed to minimize solu-
tion size by allowing the inductor to be chosen based on the
output load requirements of the application. During overload
or short circuit conditions the LT8609/LT8609A/LT8609B
safely tolerates operation with a saturated inductor through
the use of a high speed peak-current mode architecture.
A good first choice for the inductor value is:
L =
V
OUT
+
V
SW(BOT)
f
SW
where f
SW
is the switching frequency in MHz, V
OUT
is
the output voltage, V
SW(BOT)
is the bottom switch drop
(~0.25V) and L is the inductor value in μH.
To avoid overheating and poor efficiency, an inductor
must be chosen with an RMS current rating that is greater
than the maximum expected output load of the applica-
tion. In addition, the saturation current (typically labeled
I
SAT
) rating of the inductor must be higher than the load
current plus 1/2 of in inductor ripple current:
I
L(PEAK)
=I
LOAD(MAX)
+
1
2
Δ
L
where ∆I
L
is the inductor ripple current as calculated sev-
eral paragraphs below and I
LOAD(MAX)
is the maximum
output load for a given application.
As a quick example, an application requiring 1A output
should use an inductor with an RMS rating of greater
than 1A and an I
SAT
of greater than 1.3A. To keep the
efficiency high, the series resistance (DCR) should be less
than 0.04Ω, and the core material should be intended for
high frequency applications.
The LT8609/LT8609A/LT8609B limits the peak switch current
in order to protect the switches and the system from overload
faults. The top switch current limit (I
LIM
) is typically 4.75A at
low duty cycles and decreases linearly to 4.0A at D = 0.8. The
inductor value must then be sufficient to supply the desired
maximum output current (I
OUT(MAX)
), which is a function of
the switch current limit (I
LIM
) and the ripple current:
I
OUT(MAX)
=I
LIM
–
Δ
I
L
2
The peak-to-peak ripple current in the inductor can be
calculated as follows:
ΔI
L
=
V
OUT
L • f
SW
1–
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
where f
SW
is the switching frequency of the LT8609/
LT8609A/LT8609B, and L is the value of the inductor.
Therefore, the maximum output current that the LT8609/
LT8609A/LT8609B will deliver depends on minimum the
switch current limit, the inductor value, and the input
and output voltages. The inductor value may have to be
increased if the inductor ripple current does not allow
sufficient maximum output current (I
OUT(MAX)
) given the
switching frequency, and maximum input voltage used in
the desired application.
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger value
inductor provides a higher maximum load current and
reduces the output voltage ripple. For applications requir-
ing smaller load currents, the value of the inductor may
be lower and the LT8609/LT8609A/LT8609B may operate
with higher ripple current. This allows use of a physically
smaller inductor, or one with a lower DCR resulting in
higher efficiency. Be aware that low inductance may result
in discontinuous mode operation, which further reduces
maximum load current.
For more information about maximum output current and
discontinuous operation, see Application Note 44.
Finally, for duty cycles greater than 50% (V
OUT
/V
IN
> 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
Input Capacitor
Bypass the input of the LT8609/LT8609A/LT8609B circuit
with a ceramic capacitor of X7R or X5R type. Y5V types have
poor performance over temperature and applied voltage,
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