lt3680 Linear Technology Corporation, lt3680 Datasheet - Page 11

• Manufacturer Part Number: lt3680
• Description: 36v, 3.5a, 2.4mhz Step-down Switching Regulator With 75?a Quiescent Current
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Of course, such a simple design guide will not always re-
sult in the optimum inductor for your application. A larger
value inductor provides a slightly higher maximum load
current and will reduce the output voltage ripple. If your
load is lower than 3.5A, then you can decrease the value
of the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one
with a lower DCR resulting in higher effi ciency. There are
several graphs in the Typical Performance Characteristics
section of this data sheet that show the maximum load
current as a function of input voltage and inductor value
for several popular output voltages. Low inductance may
result in discontinuous mode operation, which is okay
but further reduces maximum load current. For details of
maximum output current and discontinuous mode opera-
tion, see Linear Technology Application Note 44. Finally,
for duty cycles greater than 50% (V
is a minimum inductance required to avoid subharmonic
oscillations. See AN19.
Input Capacitor
Bypass the input of the LT3680 circuit with a ceramic
capacitor of X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 10μF to 22μF ceramic capacitor is
adequate to bypass the LT3680 and will easily handle 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 signifi cant
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 LT3680 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 LT3680 and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT3680. A ceramic input capacitor
combined with trace or cable inductance forms a high
OUT
/V
IN
> 0.5), there
quality (under damped) tank circuit. If the LT3680 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3680’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
For space sensitive applications, a 4.7μF ceramic capaci-
tor can be used for local bypassing of the LT3680 input.
However, the lower input capacitance will result in in-
creased input current ripple and input voltage ripple, and
may couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltage
of the LT3680 to ~3.7V.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it fi lters the square wave generated by the
LT3680 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
LT3680’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
where f
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 the compensation network is
also adjusted to maintain the loop bandwidth. A lower
value of output capacitor can be used to save space and
cost but transient performance will suffer. See the Fre-
quency Compensation section to choose an appropriate
compensation network.
When choosing a capacitor, look carefully through the
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 volt-
age rating, may be required. High performance tantalum
or electrolytic capacitors can be used for the output
C
OUT
SW
=
V
is in MHz, and C
OUT SW
100
f
OUT
is the recommended
LT3680
11
3680f