12
Rev. C
For more information
APPLICATIONS INFORMATION
Inductor Selection and Maximum Output Current
The LT8607 is designed to minimize solution size by
allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
short circuit conditions the LT8607 safely tolerates opera-
tion 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
• 2
where f
SW
is the switching frequency in MHz, V
OUT
is
the output voltage, V
SW(BOT)
is the bottom switch drop
(~0.125V) 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 0.25A output
should use an inductor with an RMS rating of greater
than 0.5A and an I
SAT
of greater than 0.7A. 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 LT8607 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 at least 1.2A at low
duty cycles and decreases linearly to at least 0.9A 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:
Δ
I
L
=
V
OUT
L • f
SW
1–
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎞
⎠
⎟
where f
SW
is the switching frequency of the LT8607, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8607 will deliver depends on
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 LT8607 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 Analog Devices 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 LT8607 circuit with a ceramic
capacitor of X7R or X5R type. Y5V types have poor per-
formance over temperature and applied voltage, and
should not be used. A 4.7µF to 10µF ceramic capacitor
is adequate to bypass the LT8607 and will easily handle
the ripple current. Note that larger input capacitance is
required when a lower switching frequency is used. If
the input power source has high impedance, or there is
