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LT3502/LT3502A

3502fc

APPLICATIONS INFORMATION

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

VIN

3502 F08b

SHDN

BOOST

LT3502A

GND

VIN

VOUT

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

VIN pin is grounded while the output is held high, then

parasitic diodes inside the LT3502/LT3502A can pull large

currents from the output through the SW pin and the VIN

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 VINpinoftheLT3502/LT3502A

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 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 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 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 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 ﬁltering. This

solution is smaller and less expensive than the electrolytic

capacitor. 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.

Frequency Compensation

The LT3502/LT3502A use current mode control to regulate

the output. This simpliﬁes 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

ampliﬁer with ﬁnite output impedance. The power section,

consisting of the modulator, power switch and inductor,

is modeled as a transconductance ampliﬁer generating an

output current proportional to the voltage at the VC node.

Note that the output capacitor integrates this current,

and that the capacitor on the VC node (CC) integrates the

error ampliﬁer output current, resulting in two poles in the

loop. RC provides a zero. With the recommended output

capacitor, the loop crossover occurs above the RCCC zero.

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 (CPL) across the feedback

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.

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