Datasheet

LT3690
15
3690fa
applicaTions inForMaTion
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 LT3690 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 ΔI
L
/2.
Input Capacitor
Bypass the input of the LT3690 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 10µF ceramic capacitor is adequate to bypass
the LT3690, and easily handles 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 lower 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 LT3690 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 10µF capacitor is capable of this task, but only if it is
placed close to the LT3690 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3690.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3690 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, pos-
sibly exceeding the LT3690’s maximum voltage rating.
See Application Note 88 for more details.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by the
LT3690 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
LT3690’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
C
OUT
=
150
V
OUT
• ƒ
SW
where ƒ
SW
is in MHz, and C
OUT
is the recommended out-
put capacitance in µF. Use X5R or X7R types, which will
provide low output ripple and good transient response.
Using a high value capacitor on the output can improve
transient performance, but a phase lead capacitor across
the feedback resistor R1 may be required to get the full
benefit (see the Frequency Compensation section).
High performance electrolytic capacitors can be used for
the output capacitor. If using an electrolytic capacitor,
choose one intended for use in switching regulators, and
with a specified ESR of 0.03Ω 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
Ceramic Capacitors
Ceramic capacitors are small, robust and have very
low ESR. However, ceramic capacitors can sometimes
cause problems when used with the LT3690 due to their
piezoelectric nature. When in Burst Mode operation, the
LT3690’s switching frequency depends on the load current,
and at very light loads the LT3690 can excite the ceramic
capacitor at audio frequencies, generating audible noise.