Datasheet

LT3493
15
3493fb
APPLICATIONS INFORMATION
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 9b
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 fi ltering and can
slightly improve the effi ciency of the circuit, though it is
likely to be the largest component in the circuit. An alterna-
tive solution is shown in Figure 9c. 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 fi ltering. This solution is smaller
and less expensive than the electrolytic capacitor. For high
input voltages its impact on effi ciency is minor, reducing
effi ciency less than one half percent for a 5V output at full
load operating from 24V.
Frequency Compensation
The LT3493 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular,
the LT3493 does not require the ESR of the output capaci-
tor for stability allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size.
Figure 10 shows an equivalent circuit for the LT3493 control
loop. The error amp is a transconductance amplifi er with
fi nite output impedance. The power section, consisting of
the modulator, power switch and inductor, is modeled as
a transconductance amplifi er generating an output cur-
rent proportional to the voltage at the V
C
node. Note that
the output capacitor integrates this current, and that the
capacitor on the V
C
node (C
C
) integrates the error ampli-
fi er output current, resulting in two poles in the loop. R
C
provides a zero. With the recommended output capacitor,
the loop crossover occurs above the R
C
C
C
zero. This simple
+
+
LT3493
2.2μF
V
IN
20V/DIV
I
IN
5A/DIV
20μs/DIV
V
IN
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
(9a)
(9b)
(9c)
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
+
LT3493
2.2μF
10μF
35V
AI.EI.
LT3493
2.2μF0.1μF
1Ω
3493 F09
V
IN
20V/DIV
I
IN
5A/DIV
20μs/DIV
V
IN
20V/DIV
I
IN
5A/DIV
20μs/DIV
DANGER!
RINGING V
IN
MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT3493
Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3493 is Connected to a Live Supply
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