LT3973/LT3973-3.3/LT3973-5
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3973fb
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APPLICATIONS INFORMATION
sheet that show the maximum load current as a function
of input voltage for several popular output voltages. Low
inductance may result in discontinuous mode operation,
which is acceptable but reduces maximum load current.
For details of maximum output current and discontinu
-
ous mode
operation, see Application Note
44. Finally, for
duty cycles greater than 50% (V
OUT
/V
IN
> 0.5), there is
a minimum inductance required to avoid subharmonic
oscillations. See Application Note 19.
Input Capacitor
Bypass the input of the LT3973 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 ceramic capacitor is adequate to bypass the
LT3973 and will easily handle the ripple current. 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 LT3973 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 LT3973 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3973.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3973 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, pos
-
sibly exceeding the LT3973’s voltage rating. This situation
is easily avoided (see the Hot Plugging Safely section).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. It stores
energy in order to satisfy transient loads and stabilize the
LT3973’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
=
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
15pF) 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.
The second function is that the output capacitor, along
with the inductor, filters the square wave generated by the
LT3973 to produce the DC output. In this role it determines
the output ripple, so low impedance (at the switching
frequency) is important. The output ripple decreases with
increasing output capacitance, down to approximately
1mV. See Figure 1. Note that a larger phase lead capacitor
should be used with a large output capacitor.
Figure 1. Worst-Case Output Ripple Across Full Load Range
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.
C
OUT
(µF)
0
0
WORST-CASE OUTPUT RIPPLE (mV)
2
6
8
10
40
80
100
16
3973 F01
4
20 60
12
14
FRONT PAGE APPLICATION
V
IN
= 24V
V
IN
= 12V