LT3504
15
3504fa
For more information www.linear.com/LT3504
applicaTions inForMaTion
twice its nominal value, possibly exceeding the LT3504’s
voltage rating. This situation can be easily avoided by add-
ing an electrolytic capacitor in parallel with the ceramic
input capacitors. See Application Note 88.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor
, it filters the square wave generated by
the L
T3504 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
LT3504’s control loop.
Ceramic capacitors have very low equivalent series re
-
sistance (ESR) and provide the best ripple performance.
A good value is:
C
OUT
= 33/(V
OUT
• f
SW
)
where C
OUT
is in µF and f
SW
is in MHz. Use X5R or X7R
types and keep in mind that a ceramic capacitor biased
with V
OUT
will have less than its nominal capacitance. This
choice will provide low output ripple and good transient
response. Transient performance can be improved with a
high value capacitor, if the compensation network is also
adjusted to maintain the loop bandwidth.
A lower value of output capacitor can be used, but transient
performance will suffer.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose one
that is intended for use in switching regulators. The ESR
should be specified by the supplier and should be 0.1Ω
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.
Figure 6 shows the transient response of the LT3504 with
several output capacitor choices. The output is 3.3V. The
load current is stepped from 500mA to 1A and back to
500mA and the oscilloscope traces show the output volt
-
age. The upper photo shows the recommended value. The
second photo shows the improved response (less voltage
drop) resulting from a larger output capacitor and a larger
phase lead capacitor. The last photo shows the response
to a high performance electrolytic capacitor. Transient per
-
formance is improved due to the large output capacitance.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces
-
sively, an LT3504 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
L
T3504 is absent. This may occur in batter
y charging ap
-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3504’
s
output. If the V
IN
pin is allowed to float and the EN/UVLO
pin is held high (either by a logic signal or because it is
tied to V
IN
), then the LT3504’s internal circuitry will pull
its quiescent current through its SW pin. This is fine if
your system can tolerate a few mA in this state. If you
ground the EN/UVLO pin, the SW pin current will drop to
essentially zero. However, if the V
IN
pin is grounded while
the output is held high, then parasitic diodes inside the
LT3504 can pull large currents from the output through
the SW pin and the V
IN
pin. Figure 7 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
