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LT3475/LT3475-1
APPLICATIONS INFORMATION
The current in the inductor is a triangle wave with an average
value equal to the load current. The peak switch current
is equal to the output current plus half the peak-to-peak
inductor ripple current. The LT3475 limits its switch cur-
rent in order to protect itself and the system from overload
faults. Therefore, the maximum output current that the
LT3475 will deliver depends on the switch current limit,
the inductor value, and the input and output voltages.
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor
Input Capacitor Selection
Bypass the input of the LT3475 circuit with a 4.7μF or
higher ceramic capacitor of X7R or X5R type. A lower
value or a less expensive Y5V type will work if there is
additional bypassing provided by bulk electrolytic capaci-
tors or if the input source impedance is low. The following
paragraphs describe the input capacitor considerations in
more detail.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input ca-
Δ I L =
( 1 – DC ) ( V OUT + V F )
( L ? f )
pacitor is required to reduce the resulting voltage ripple at
the LT3475 input and to force this switching current into a
tight local loop, minimizing EMI. The input capacitor must
I SW ( PK ) = I L ( PK ) = I OUT +
where f is the switching frequency of the LT3475 and L
is the value of the inductor. The peak inductor and switch
current is
Δ I L
2
To maintain output regulation, this peak current must be
have low impedance at the switching frequency to do this
effectively, and it must have an adequate ripple current rat-
ing. With two switchers operating at the same frequency
but with different phases and duty cycles, calculating the
input capacitor RMS current is not simple. However, a
conservative value is the RMS input current for the channel
that is delivering most power (V OUT ? I OUT ):
< OUT
less than the LT3475’s switch current limit I LIM . I LIM is at
least 2.3A at low duty cycles and decreases linearly to 1.8A
at DC = 0.9. The maximum output current is a function of
the chosen inductor value:
C INRMS = I OUT ?
V OUT (V IN – V OUT )
V IN
I
2
I OUT ( MAX ) = I LIM –
Δ I L
2
and is largest when V IN = 2V OUT (50% duty cycle). As the
second, lower power channel draws input current, the
= 2.3A ? ( ) –
1–0.25?DC
Δ I L
2
input capacitor’s RMS current actually decreases as the
out-of-phase current cancels the current drawn by the
higher power channel. Considering that the maximum
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors,
and choose one to meet cost or space goals. Then use
these equations to check that the LT3475 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continu-
ous. Discontinuous operation occurs when I OUT is less
than Δ I L /2.
load current from a single channel is ~1.5A, RMS ripple
current will always be less than 0.75A.
The high frequency of the LT3475 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is less than 10μF. The combination
of small size and low impedance (low equivalent series
resistance or ESR) of ceramic capacitors makes them the
preferred choice. The low ESR results in very low voltage
ripple. Ceramic capacitors can handle larger magnitudes
of ripple current than other capacitor types of the same
value. Use X5R and X7R types.
3475fb
11
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