LT3502 LINER [Linear Technology], LT3502 Datasheet - Page 17

• Manufacturer Part Number: LT3502
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
internal circuitry will pull its quiescent current through
its SW pin. This is fi ne 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
parasitic diodes inside the LT3502/LT3502A can pull large
currents from the output through the SW pin and the V
pin. Figure 8b shows a circuit that will run only when the
input voltage is present and that protects against a shorted
or reversed input.
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3502/LT3502A circuits. However,
these capacitors can cause problems if the LT3502/LT3502A
are 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
can ring to twice the nominal input voltage, possibly ex-
ceeding the LT3502/LT3502A’s rating and damaging the
part. If the input supply is poorly controlled or the user
will be plugging the LT3502/LT3502A into an energized
supply, the input network should be designed to prevent
this overshoot. Figure 9 shows the waveforms that result
when an LT3502/LT3502A circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The fi rst
plot is the response with a 2.2μF ceramic capacitor at the
Figure 8b. 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 LT3502/LT3502A Runs Only When
the Input is Present
IN
pin is grounded while the output is held high, then
V
IN
D4
V
SHDN
IN
LT3502A
GND
BD
BOOST
SW
DA
FB
IN
pin of the LT3502/LT3502A
+
3502 F08b
V
OUT
IN
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 alternative 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 LT3502/LT3502A use current mode control to regulate
the output. This simplifi es loop compensation. In particular,
the LT3502/LT3502A does not require the ESR of the output
capacitor 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 LT3502/
LT3502A 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 current proportional to the voltage at the V
Note that the output capacitor integrates this current,
and that the capacitor on the V
error amplifi er output current, resulting in two poles in the
loop. R
capacitor, the loop crossover occurs above the R
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. With a larger
ceramic capacitor (very low ESR), crossover may be lower
and a phase lead capacitor (C
divider may improve the phase margin and transient
response. Large electrolytic capacitors may have an ESR
large enough to create an additional zero, and the phase
lead may not be necessary.
C
provides a zero. With the recommended output
LT3502/LT3502A
C
PL
node (C
) across the feedback
C
) integrates the
C
C
17
C
C
node.
zero.
3502fc