LT3474/LT3474-1
APPLICATIONS INFORMATION
at the LT3474 input and to force this switching current into
a tight local loop, minnimizing EMI. The input capacitor
must have low impedance at the switching frequency to
do this effectively, and it must have an adequate ripple
current rating. The RMS input is:
To maintain output regulation, this peak current must be
less than the LT3474’s switch current limit I . For SW1,
LIM
I
is at least 1.6A (1.5A at 125°C) at low duty cycles and
LIM
decreases linearly to 1.15A (1.08A at 125°C) at DC = 0.8.
The maximum output current is a function of the chosen
inductor value:
VOUT V – V
(
)
<
IN
OUT
IOUT
2
ΔIL
2
CINRMS = IOUT
•
IOUT MAX = ILIM
–
V
(
)
IN
ΔIL
2
and is largest when V = 2V
sidering that the maximum load current is 1A, RMS ripple
current will always be less than 0.5A
(50% duty cycle). Con-
=1.6A • 1– 0.35•DC –
IN
OUT
(
)
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
The high switching frequency of the LT3474 reduces the
energystoragerequirementsoftheinputcapacitor, sothat
thecapacitancerequiredislessthan10μF.Thecombination
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.
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 LT3474 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continuous.
Discontinuous operation occurs when I
is less than
OUT
An alternative to a high value ceramic capacitor is a lower
value ceramic along with a larger electrolytic capaci-
tor. The electrolytic capacitor likely needs to be greater
than 10μF in order to meet the ESR and ripple current
requirements. The input capacitor is likely to see high
surge currents when the input source is applied. Tanta-
lum capacitors can fail due to an over-surge of current.
Only use tantalum capacitors with the appropriate surge
current rating. The manufacturer may also recommend
operation below the rated voltage of the capacitor.
ΔI /2.
L
Input Capacitor Selection
Bypass the input of the LT3474 circuit with a 2.2μF 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 sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
3474fd
12
Linear [ Linear ]
