LT3975
APPLICATIONS INFORMATION
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 LT3975 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT3975 (see the PCB Layout section).
Asecondprecautionregardingtheceramicinputcapacitor
concernsthemaximuminputvoltageratingoftheLT3975.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3975 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, pos-
sibly exceeding the LT3975’s voltage rating. If the input
supply is poorly controlled or the user will be plugging
the LT3975 into an energized supply, the input network
should be designed to prevent this overshoot. See Linear
TechnologyApplicationNote88foracompletediscussion.
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
AVX
URL
www.avxcorp.com
www.murata.com
www.t-yuden.com
www.vishay.com
www.tdk.com
Murata
Taiyo Yuden
Vishay Siliconix
TDK
Ceramic Capacitors
When in dropout, the LT3975 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.
Output Capacitor and Output Ripple
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3975. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped)tankcircuit. IftheLT3975circuitispluggedintoa
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3975’s rating. If the input
supply is poorly controlled or the user will be plugging
the LT3975 into an energized supply, the input network
should be designed to prevent this overshoot. See Linear
TechnologyApplicationNote88foracompletediscussion.
The output capacitor has two essential functions. Along
withtheinductor,itfiltersthesquarewavegeneratedbythe
LT3975toproducetheDCoutput. Inthisroleitdetermines
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
LT3975’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
200
COUT
=
VOUT •fSW
Catch Diode Selection
wheref isinMHz, andC
istherecommendedoutput
OUT
SW
The catch diode (D1 from the Block Diagram) conducts
current only during the switch off time. Average forward
current in normal operation can be calculated from:
capacitance in μF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
Transientperformancecanbeimprovedwithahighervalue
capacitorifcombinedwithaphaseleadcapacitor(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.
V – VOUT
IN
ID(AVG) = IOUT
V
IN
where I
is the output load current. The current rating of
OUT
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
the diode should be selected to be greater than or equal to
the application’s output load current, so that the diode is
3975f
16
Linear [ Linear ]
