Datasheet

LT3508
12
3508fd
the 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
load current from a single channel is ~1.4A, RMS ripple
current will always be less than 0.7A.
The high frequency of the LT3508 reduces the energy stor-
age requirements of the input capacitor. 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.
An alternative to a high value ceramic capacitor is a lower
value ceramic along with a larger electrolytic capacitor. 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. Tantalum capacitors
can fail due to an oversurge 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.
A fi nal caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit. If power is applied quickly (for example by plugging
the circuit into a live power source), this tank can ring,
doubling the input voltage and damaging the LT3508. The
solution is to either clamp the input voltage or dampen
the tank circuit by adding a lossy capacitor in parallel with
the ceramic capacitor. For details see Application Note 88.
Output Capacitor Selection
The output capacitor has two essential functions. Along
with the inductor, it fi lters the square wave generated by the
LT3508 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3508’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good value is:
C
OUT
=
50V
V
OUT
•
1MHz
f
where C
OUT
is in µF. Use X5R or X7R types. This choice
will provide low output ripple and good transient response.
Transient performance can be improved with a high value
capacitor if the compensation network is also adjusted to
maintain the loop bandwidth. A lower value of output capaci-
tor can be used, but transient performance will suffer. With
an external compensation network, the loop gain can be
lowered to compensate for the lower capacitor value. Look
carefully at the capacitor’s data sheet to fi nd out what the
actual capacitance is under operating conditions (applied
voltage and temperature). A physically larger capacitor, or
one with a higher voltage rating, may be required. High
performance electrolytic capacitors can be used for the
output capacitor. Low ESR is important, so choose one
that is intended for use in switching regulators. The ESR
should be specifi ed by the supplier, and should be 0.05Ω
or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR. Table 3 lists
several capacitor vendors.
Table 3. Capacitor Vendors
VENDOR PART SERIES COMMENTS
Panasonic Ceramic
Polymer
Tantalum
EEF Series
Kemet Ceramic
Tantalum T494, T495
Sanyo Ceramic
Polymer
Tantalum
POSCAP
Murata Ceramic
AVX Ceramic
Tantalum TPS Series
Taiyo Yuden Ceramic
TDK Ceramic
APPLICATIONS INFORMATION