Datasheet

LT3976
17
3976f
For more information www.linear.com/3976
Input Capacitor
Bypass the input of the LT3976 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 LT3976 and will easily handle the ripple cur-
rent. Note that larger input capacitance is required when
a lower switching frequency is used (due to longer on
times). 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 LT3976 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 LT3976 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating
of the LT3976.
A
ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3976 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, pos-
sibly exceeding the LT3976’s voltage rating. If the input
supply is poorly controlled or the user will be plugging
the LT3976 into an energized supply, the input network
should be designed to prevent this overshoot. See Linear
Technology Application Note 88 for a complete discussion.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3976 to produce the DC output. In this role it deter-
mines the output ripple, so low impedance (at the switching
frequency) is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3976’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
=
300
V
OUT
• f
SW
where f
SW
is in MHz, and C
OUT
is the recommended output
capacitance 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 higher value
capacitor if combined with a phase lead capacitor (typically
10pF) between the output and the feedback pin. A lower
value of output capacitor can be used to save space and
cost but transient performance will suffer.
When choosing a capacitor, look carefully through the
data sheet to find 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. Table 3 lists several capacitor
vendors.
Table 3. Recommended Ceramic Capacitor Vendors
MANUFACTURER URL
AVX www.avxcorp.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Vishay Siliconix www.vishay.com
TDK www.tdk.com
Ceramic Capacitors
When in dropout, the LT3976 can excite ceramic ca-
pacitors at audio frequencies. At high load, this could be
unacceptable. Simply adding bulk input capacitance to
the input and output will significantly reduce the voltage
ripple and the audible noise generated at these nodes to
acceptable levels.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3976. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3976 circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3976’s rating. If the input
supply is poorly controlled or the user will be plugging
the LT3976 into an energized supply, the input network
should be designed to prevent this overshoot. See Linear
Technology Application Note 88 for a complete discussion.
applicaTions inForMaTion