Datasheet

LT8608/LT8608B
13
Rev. D
For more information www.analog.com
APPLICATIONS INFORMATION
Inductor Selection and Maximum Output Current
The LT8608 is designed to minimize solution size by al-
lowing the inductor to be chosen based on the output load
requirements of the application. During overload or short
circuit conditions the LT8608 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
(~0.35V) 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 application. In
addition, the saturation current (typically labeled I
SAT
) rat-
ing of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
I
L(PEAK )
= I
LOAD(M AX )
+
1
2
∆
L
where ∆I
L
is the inductor ripple current as calculated
several paragraphs below and I
LOAD(MAX)
is the maximum
output load for a given application.
As a quick example, an application requiring 0.5A output
should use an inductor with an RMS rating of greater
than 0.5A and an I
SAT
of greater than 0.8A. 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 LT8608 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 2.1A at low
duty cycles and decreases linearly to 1.55A 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(M AX )
= I
LIM
–
∆I
L
2
The peak-to-peak ripple current in the inductor can be
calculated as follows:
∆I
L
=
V
OUT
L • f
SW
1–
V
OUT
V
IN(M AX )
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
where f
SW
is the switching frequency of the LT8608, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8608 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.
For more information about maximum output current and
discontinuous operation, see Analog Device’s 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 LT8608 circuit with a ceramic capaci-
tor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT8608 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 significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT8608 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT8608 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT8608.
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