LT3493-3 Datasheet, PDF (14/20 Pages) Linear Technology – 1.2A, 750kHz Step-Down Switching Regulator in 2mm × 3mm DFN

LT3493-3
APPLICATIO S I FOR ATIO
choosing a large RC time constant, the peak start up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply 20µA when the SHDN
pin reaches 2.3V.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3493-3 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
LT3493-3 is absent. This may occur in battery charging
applications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3493-3’s
output. If the V_IN pin is allowed to float and the SHDN pin
is held high (either by a logic signal or because it is tied to
V_IN), then the LT3493-3’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 SHDN 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 LT3493-3 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.
D4
V_IN
V_IN
BOOST
LT3493-3
SHDN
SW
GND
FB
V_OUT
BACKUP
3493-3 F08
Figure 7. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output; It Also Protects the Circuit
from a Reversed Input. The LT3493-3 Runs Only When the Input
is Present
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3493-3 circuits. However, these
capacitors can cause problems if the LT3493-3 is plugged
into a live supply (see Linear Technology Application Note
88 for a complete discussion). The low loss ceramic
capacitor combined with stray inductance in series with
the power source forms an underdamped tank circuit, and
the voltage at the V_IN pin of the LT3493-3 can ring to twice
the nominal input voltage, possibly exceeding the LT3493-
3’s rating and damaging the part. If the input supply is
poorly controlled or the user will be plugging the LT3493-
3 into an energized supply, the input network should be
designed to prevent this overshoot.
Figure 8 shows the waveforms that result when an LT3493-
3 circuit is connected to a 24V supply through six feet of
24-gauge twisted pair. The first plot is the response with
a 2.2µF ceramic capacitor at the input. The input voltage
rings as high as 35V and the input current peaks at 20A.
One method of damping the tank circuit is to add another
capacitor with a series resistor to the circuit. In Figure 8b
an aluminum electrolytic capacitor has been added. This
capacitor’s high equivalent series resistance damps the
circuit and eliminates the voltage overshoot. The extra
capacitor improves low frequency ripple filtering and can
slightly improve the efficiency of the circuit, though it is
likely to be the largest component in the circuit. An
alternative solution is shown in Figure 8c. A 1Ω resistor is
added in series with the input to eliminate the voltage
overshoot (it also reduces the peak input current). A 0.1µF
capacitor improves high frequency filtering. This solution
is smaller and less expensive than the electrolytic capaci-
tor. For high input voltages its impact on efficiency is
minor, reducing efficiency less than one half percent for a
5V output at full load operating from 24V.
3493-3f
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